Audited —·Last updated 27 Jul 2026·8 citations·Tier 1·0 uses

Ovulation Calculator

Free ovulation calculator. Find your most likely ovulation day, fertile window (5 days before through 1 day after), next period, and current cycle day from LMP.

Ovulation Calculator

The first day of bleeding in your most recent menstrual period — not the last day. This is the anchor for every other date the calculator produces.
Average length of your menstrual cycle (21–45 days). Count from day one of one period to day one of the next. The textbook value is 28 but anything from 21 to 45 is biologically normal.
Days from ovulation to the next period (10–16, default 14). The luteal phase is the most stable part of the cycle in a given person; most cycle-length variation lives in the follicular phase before ovulation, not after.
ISO 8601 date (YYYY-MM-DD). Used only to compute the 'cycle day' output — what day of the current cycle you are on right now. Leave blank if you only want the ovulation and fertile-window dates.
Ovulation day (from LMP)
14
Days from the first day of your last period to predicted ovulation, calculated as cycleLength − lutealPhase. Add this number to your LMP date to land on the day the egg is most likely released.
Fertile window starts
9 days
Fertile window ends
15 days
Next period starts
28 days
Current cycle day
517

Background.

This ovulation calculator predicts the single day in your menstrual cycle on which an egg is most likely released, the six-day fertile window around it, the start of your next period, and where you are in the current cycle today — all from three numbers: the first day of your last menstrual period (LMP), your average cycle length, and your luteal phase length. The math is the calendar method codified by the American College of Obstetricians and Gynecologists in Practice Bulletin No. 197 (2018) on fertility-awareness-based methods: in any menstrual cycle, ovulation occurs (cycleLength − lutealPhase) days after the first day of bleeding, because the luteal phase — the interval between ovulation and the next period — is the most stable phase of the cycle in any given person, typically 12 to 14 days with a strict biological ceiling around 16. Everything else — the follicular phase from period to ovulation — is the variable part, which is why two healthy cyclers with 26-day and 35-day cycles ovulate on very different days but both have luteal phases near two weeks.

The fertile window itself, marked here as five days before ovulation through one day after, is the empirical six-day interval established by Wilcox, Weinberg and Baird in their landmark 1995 New England Journal of Medicine study (NEJM 333:1517) on 625 healthy planning-pregnancy couples: virtually no conceptions occurred outside this window, and within it conception probability peaked the day before ovulation rather than on ovulation itself, because sperm survive in the reproductive tract for up to five days while the unfertilised egg lives only 12 to 24 hours. The CDC and WHO reproductive health pages use the same window and the same biological reasoning.

The calculator works in whole-day offsets from your LMP — so the primary output 'ovulation day' really means 'days after LMP', and you read your ovulation date by adding that integer to your LMP. Everything is anchored at UTC midnight so daylight-saving and timezone shifts cannot move the answer by even an hour.

A word about accuracy: the calendar method is the least accurate way to detect ovulation. It tells you where the fertile window is supposed to be in an average cycle, given your inputs — but in any one real cycle, ovulation can drift by several days due to stress, illness, travel, weight change, or simple biological noise. The 2013 meta-analysis by Pallone and Bergus in the Journal of the American Board of Family Medicine put calendar-method timing accuracy at roughly ±4 days for women with regular cycles, and far worse for women with cycles that vary by more than seven days month to month.

For higher precision: urinary luteinising hormone (LH) strips detect the pre-ovulatory LH surge 24–36 hours before egg release and are accurate to within a day in the vast majority of cycles, with sensitivity above 95 percent in head-to-head comparisons against ultrasound follicle tracking; basal body temperature (BBT) charted to 0.1 °C resolution shows a sustained 0.3–0.5 °C rise the morning after ovulation, which confirms ovulation has already happened but does not predict it; and transvaginal ultrasound follicular monitoring, the clinical gold standard, visualises the dominant follicle and its collapse directly. Use this calculator as a starting frame — a sane prior on where the fertile window probably sits — and pair it with LH strips or BBT for the cycle in which you are actively trying to conceive or actively trying to avoid pregnancy through fertility awareness.

One important counterexample the calendar method completely misses: anovulatory cycles. Roughly 1 in 10 cycles in healthy reproductive-age women is anovulatory (no egg is released, but a period-like bleed still occurs from estrogen withdrawal), and the rate climbs substantially in adolescents, perimenopausal women, women with PCOS, and athletes in negative energy balance. The calculator cannot detect these and will report a fertile window that, biologically, does not exist. If you have been tracking for several cycles and never see an LH surge on strips or a sustained temperature shift on BBT, the cycles in question were probably anovulatory and a reproductive endocrinology workup is warranted.

What is ovulation calculator?

Ovulation is the moment in the menstrual cycle when a mature ovarian follicle ruptures and releases its egg into the fallopian tube. It is triggered by a sharp surge of luteinising hormone (LH) from the anterior pituitary, occurring roughly 24 to 36 hours before follicular rupture. The egg itself is viable for 12 to 24 hours after release; if no sperm reach it in that interval it disintegrates and is reabsorbed. The 'fertile window' — the days during a cycle on which intercourse can result in conception — is therefore not a single day but a six-day interval, because sperm deposited in the reproductive tract up to five days before ovulation can survive in the cervical crypts (the cervical mucus reservoir) and remain capable of fertilising the egg when it appears. The window runs from approximately five days before ovulation through the day after. The most stable part of the cycle is the luteal phase: the interval from ovulation to the next menses. In any given person it usually varies by only 1–2 days from cycle to cycle, anchored by the lifespan of the corpus luteum (the post-ovulatory hormonal structure that maintains the uterine lining until either pregnancy implants or the corpus luteum involutes and menstruation begins). A luteal phase shorter than 10 days is called a 'luteal phase defect' and is associated with infertility because the uterine lining is not maintained long enough for implantation. Cycle length is the interval from the first day of one period to the first day of the next; the textbook value is 28 days but normal ranges anywhere from 21 to 45 days in healthy reproductive-age women. The follicular phase (period to ovulation) absorbs essentially all the cycle-length variation: a person with consistent 35-day cycles has a 21-day follicular phase and a 14-day luteal phase, not a 17.5/17.5 split.

How to use this calculator.

  1. Find the first day of your last period — the first day of actual bleeding, not spotting — and enter it as the LMP. If your last period started on the evening of one day, use that day, not the next.
  2. Enter your average cycle length in days. Count from day one of one period to day one of the very next period; do not count the period itself separately. If you do not know your cycle length, leave it at the textbook 28 and treat the result with extra uncertainty. If your cycles vary by more than 7 days month to month, the calendar method becomes unreliable and you should use LH strips or BBT instead.
  3. Leave the luteal phase at 14 days unless you have specifically charted your own luteal phase from BBT or LH strip data. The 14-day default fits the majority of healthy cycles; only adjust if you have measured your own number.
  4. Optionally enter today's date so the calculator can tell you what cycle day you are on right now. If you only want the predicted ovulation and fertile-window dates, you can leave this blank.
  5. Read the primary 'Ovulation day' output as days after LMP. Add that integer to your LMP date to land on the calendar date of predicted ovulation. The fertile-window start and end fields are similarly days from LMP — add them to your LMP to get the calendar dates of the fertile window.
  6. If you are trying to conceive, plan intercourse every 1–2 days from the fertile-start day through the day after ovulation. The Wilcox study found per-cycle conception probability peaks at intercourse the day before ovulation. If you are using fertility awareness to avoid pregnancy, treat the entire fertile window plus a 2-day safety buffer on each side as 'unsafe' — and recognise that calendar method failure rates for contraception are 12–15 percent per year, much higher than for hormonal methods.

The formula.

O = C − L ; fertile window = O − 5 … O + 1
O · ovulation day offset from period startC · cycle length (days)L · luteal phase length (days)

Calendar (rhythm) method per ACOG Practice Bulletin 197:

ovulationDayOffset = cycleLengthDays − lutealPhaseDays

This is the foundational identity. The luteal phase is held fixed in any given person; cycle length variation lives in the follicular phase before ovulation. So in a 28-day cycle with a 14-day luteal phase, ovulation falls on day 14. In a 35-day cycle with the same 14-day luteal phase, ovulation falls on day 21. In a 24-day cycle with a 13-day luteal phase, ovulation falls on day 11.

Fertile window (per CDC and Wilcox et al., NEJM 1995):

fertileStartOffset = ovulationDayOffset − 5 fertileEndOffset = ovulationDayOffset + 1

The five-day pre-ovulatory window is set by sperm viability in cervical mucus (median ~3 days, maximum ~5 days). The one-day post-ovulatory window is set by the egg's own viability (~12–24 hours). Outside this six-day window, the per-cycle conception probability collapses to essentially zero.

Next period and cycle day:

nextPeriodOffset = cycleLengthDays cycleDay = today − LMP (signed integer days, 0-indexed at LMP)

All dates are anchored at UTC midnight and arithmetic is done in whole-day deltas, so daylight-saving transitions and timezone shifts cannot perturb the answer. The calculator does not attempt to predict ovulation in anovulatory cycles, does not adjust for PCOS or other endocrine disorders, and does not use BBT or LH-strip data; for those refinements, layer this calendar prediction on top of an LH-strip or BBT chart for the specific cycle of interest.

A worked example.

Example

Take an LMP of January 1, 2025 with a textbook 28-day cycle and a 14-day luteal phase. Ovulation day offset = 28 − 14 = 14, so ovulation is predicted for January 15, 2025 (14 days after Jan 1). Fertile window start = 14 − 5 = 9, so the window opens on January 10. Fertile window end = 14 + 1 = 15, so it closes on January 16. Next period offset = 28, so the next period is predicted for January 29, 2025. With today set to January 12, cycle day = 11 days since LMP — meaning we are currently inside the fertile window (which runs days 9 through 15) and two days before predicted ovulation, the highest-probability conception day in the Wilcox 1995 data. If this couple is trying to conceive, they would plan intercourse today, tomorrow, the day after, and the day after that to cover the peak window; if they are using fertility awareness to avoid pregnancy, they would have abstained from approximately day 7 (a two-day safety buffer before fertile-start) through day 17 (a two-day buffer after fertile-end).

cycle Length Days28
luteal Phase Days14
lmp Date2025-01-01
today2025-01-12

Frequently asked questions.

How accurate is the calendar (rhythm) method for predicting ovulation?
For a woman with regular cycles (month-to-month variation under 7 days), the calendar method places the predicted ovulation day within ±4 days of the true ovulation day in most cycles, per the Pallone and Bergus 2009 review in the Journal of the American Board of Family Medicine. For women with irregular cycles, accuracy degrades sharply and the method is no longer clinically useful. As a contraceptive method, the calendar method (Standard Days Method) has a typical-use failure rate of about 12–15 percent per year, which is why the CDC and WHO categorise it as 'less effective' compared with hormonal or barrier methods. Use it as a starting frame, not as the sole basis for trying-to-conceive or trying-to-avoid timing.
How do LH ovulation predictor strips work, and are they more accurate than this calculator?
Yes, substantially. Luteinising hormone (LH) surges sharply 24 to 36 hours before ovulation, and modern over-the-counter urinary LH strips detect that surge with 95+ percent sensitivity. The strips do not detect ovulation itself; they detect the upstream pituitary signal that triggers it. The standard protocol is to test once daily starting a few days before your calendar-predicted ovulation, switching to twice-daily once the test line starts to darken; the first 'positive' (test line as dark as or darker than the control line) means ovulation is expected in 12–36 hours. Pair the strips with this calculator: the calendar prediction tells you when to start testing, the strips give you the specific day. Note that some women — particularly those with PCOS — have chronically elevated LH or multiple LH surges per cycle, which can produce false positives; in those cases, BBT or follicular ultrasound is the better confirmation.
What is basal body temperature (BBT) charting, and when should I use it?
BBT is your resting temperature, measured at the same time every morning before getting out of bed, using a thermometer with at least 0.1 °C resolution. Progesterone released from the corpus luteum after ovulation causes a sustained rise of 0.3 to 0.5 °C in BBT; the morning after ovulation, your temperature jumps up and stays elevated until the next period (when progesterone collapses) or, if conception occurred, throughout early pregnancy. BBT confirms ovulation has already happened — it cannot predict ovulation prospectively. Its value is in (a) confirming whether a given cycle was ovulatory at all, (b) measuring your personal luteal phase length so you can refine the input to this calculator, and (c) detecting early pregnancy via a luteal phase that runs longer than usual without a temperature drop. Combine it with LH strips for the most complete picture of any one cycle.
When should I take a pregnancy test if I think I conceived?
Home pregnancy tests detect human chorionic gonadotropin (hCG) produced by the implanting embryo. Implantation typically occurs 6 to 12 days after ovulation, and hCG doubles every 48 hours from there. The earliest reliable test is 10 to 12 days after ovulation, which in a 28-day cycle is 1 to 3 days before the missed period; many over-the-counter tests advertise as 'early detection' from this point but have a higher false-negative rate than waiting. A test taken the day of, or the day after, the missed period has >99 percent sensitivity in most modern brands. If the calculator says your next period offset is, say, day 28 of the cycle, the safe-side rule is: test on cycle day 29 with first-morning urine, and if negative but no period, retest 48 hours later before assuming you are not pregnant.
What are anovulatory cycles and can this calculator detect them?
An anovulatory cycle is one in which no egg is released despite a period-like bleed (which is actually estrogen-withdrawal bleeding, not a true menstrual bleed following ovulation). Roughly 10 percent of cycles in healthy reproductive-age women are anovulatory; the rate is much higher in adolescents (up to 50 percent in the first two post-menarche years), perimenopausal women, women with PCOS, women with hypothalamic amenorrhea from low energy availability, and women with significant thyroid dysfunction. The calculator cannot detect anovulation — it assumes every cycle is ovulatory and reports the fertile window that would exist if it were. The diagnostic signs of anovulation are: no LH surge on strips, no BBT shift, no luteal-phase progesterone rise on a day-21 serum draw, and persistent infertility despite well-timed intercourse. If you suspect anovulation, see a reproductive endocrinologist; common treatable causes include thyroid disease, hyperprolactinemia, PCOS, and underweight or overtraining.
Why does the calculator ask for luteal phase length — what difference does it make?
The luteal phase is the constant in the cycle equation; cycle-length variation lives in the follicular phase before ovulation. Holding cycle length at 28 days but changing luteal phase from 14 to 12 days shifts predicted ovulation from day 14 to day 16 of the cycle — a meaningful difference if you are timing intercourse or interpreting an LH strip. Most women have a luteal phase between 12 and 14 days; the strict biological range is 10 to 16. If you have never charted your own luteal phase, leave it at 14. If you have measured it via BBT charts (count from the temperature shift to the next period) or LH strips (count from the surge to the next period), enter your measured value for a more accurate ovulation date. A luteal phase under 10 days is a 'luteal phase defect' and warrants medical workup because the corpus luteum is not maintaining the uterine lining long enough to support implantation.
Can I get pregnant outside the fertile window?
Almost never, but not literally zero. The Wilcox 1995 NEJM study followed 625 healthy planning-pregnancy couples and found that essentially every observed conception traced back to intercourse within the six-day window ending on the day of ovulation. Conceptions reported from intercourse outside that window are typically explained by inaccurate self-reporting of intercourse timing or unrecognised ovulation drift — that is, the woman actually ovulated several days earlier or later than expected, so the intercourse was within the true window even though it appeared to be outside the predicted one. The practical implication for trying-to-avoid is that the calendar method's 12–15 percent annual failure rate is driven mostly by this ovulation drift: a person whose cycles are normally 28 days has one cycle of 24 days, ovulation shifts five days earlier than predicted, and intercourse that 'should' have been safe lands inside the actual fertile window. Hormonal or barrier methods are far safer if pregnancy avoidance is the goal.
How does PCOS or irregular cycles affect ovulation predictions?
Polycystic ovary syndrome (PCOS) is the most common cause of irregular cycles in reproductive-age women, affecting 8–13 percent globally per the 2023 International PCOS Guideline. Hallmarks include cycles longer than 35 days, fewer than 9 cycles per year, and frequent anovulation. For PCOS cycles, the calendar method is essentially unusable: the day of ovulation in any given cycle is genuinely unpredictable, and many cycles do not ovulate at all. The clinical workaround is letrozole or clomiphene to induce ovulation in a controlled way, combined with ultrasound follicle tracking. If you have suspected PCOS and irregular cycles, do not rely on this calculator for either conception timing or contraception; the ovulation prediction will be off by an unpredictable amount.
Does breastfeeding or recent contraception affect when I ovulate?
Yes, substantially. Exclusive breastfeeding suppresses ovulation through elevated prolactin (the lactational amenorrhea method, LAM, is roughly 98 percent effective as contraception in the first 6 months postpartum if the infant is exclusively breastfeeding day and night and the mother has not yet resumed menses) — but the first post-partum ovulation can occur before the first post-partum period, so calendar prediction is unreliable until cycles re-establish. After stopping hormonal contraception, ovulation resumes within a few weeks for most methods (combined oral contraceptive, patch, ring, mini-pill, implant, hormonal IUD) but can take 3–6 months for depot medroxyprogesterone acetate (DMPA, the 'shot'). During this resumption window, cycle length is unpredictable and the calendar method does not give reliable results.
How does this calculator differ from a pregnancy due date calculator?
An ovulation calculator predicts the fertile window inside a single menstrual cycle before conception. A pregnancy due date calculator (also called an EDD calculator) takes a known conception or LMP date and projects 280 days forward to estimate when delivery will occur. They share the same LMP input and the same '40 weeks from LMP' / '266 days from conception' framework, but they answer opposite-direction questions: ovulation asks 'when am I most likely to conceive', due date asks 'given I have conceived, when will I deliver'. If you are trying to conceive, use this calculator first; once you have a positive pregnancy test, switch to a due date calculator using either your LMP or your known ovulation date from this tool.

References& sources.

  1. [1]American College of Obstetricians and Gynecologists. Practice Bulletin No. 197: Fertility Awareness-Based Methods of Family Planning. Obstet Gynecol. 2018;132(4):e144–e158. The definitive U.S. clinical guideline on calendar (rhythm), Standard Days, TwoDay, ovulation, BBT, and symptothermal methods, including the cycleLength − lutealPhase identity used here.
  2. [2]Wilcox AJ, Weinberg CR, Baird DD. Timing of sexual intercourse in relation to ovulation — effects on the probability of conception, survival of the pregnancy, and sex of the baby. N Engl J Med. 1995;333(23):1517–1521. The landmark prospective cohort study (n=625 planning-pregnancy couples) that established the six-day fertile window ending on the day of ovulation.
  3. [3]Centers for Disease Control and Prevention. Reproductive Health: Infertility FAQs and Trying to Conceive. CDC, 2024. National-authority lay-language summary of the fertile-window concept and recommendations for couples trying to conceive.
  4. [4]Eunice Kennedy Shriver National Institute of Child Health and Human Development (NIH NICHD). Trying to Conceive — Information for Couples. NIH, 2023. Public-facing summary of ovulation timing, fertile window, and conception probability — based on the Wilcox cohort and subsequent NIH-funded reproductive epidemiology.
  5. [5]World Health Organization. Family planning / contraception methods: Fertility Awareness-Based Methods (FAB). WHO, 2023. International guidance on calendar, Standard Days, and symptothermal methods, including typical-use failure rates (12–15 percent annual) used as benchmarks here.
  6. [6]Pallone SR, Bergus GR. Fertility awareness-based methods: another option for family planning. J Am Board Fam Med. 2009;22(2):147–157. The systematic review that quantified calendar-method timing accuracy (±4 days for regular cyclers) and per-cycle pregnancy-avoidance failure rates.
  7. [7]Fehring RJ, Schneider M, Raviele K. Variability in the phases of the menstrual cycle. JOGNN. 2006;35(3):376–384. Cohort data on follicular and luteal phase variability, supporting the calculator's assumption that the luteal phase is the more stable of the two.
  8. [8]Teede HJ, Tay CT, Laven JJE, et al. Recommendations from the 2023 International Evidence-based Guideline for the Assessment and Management of Polycystic Ovary Syndrome. Fertil Steril. 2023;120(4):767–793. Current authoritative guidance on PCOS prevalence (8–13 percent of reproductive-age women) and its impact on ovulation predictability.
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