Weight Week By Week Calculator

Background.

Most of the weight gained in pregnancy is not fat. The baby, placenta and amniotic fluid account for a large share, and much of the rest is the mother's expanded blood volume, a larger uterus and breasts, and extra tissue fluid, with some stored fat on top. How much gain is healthy depends heavily on where a woman started: someone who began underweight needs more in reserve than someone who began with obesity.

The ranges used most widely come from the US Institute of Medicine's 2009 report, Weight Gain During Pregnancy. For a singleton pregnancy it set total gains by pre-pregnancy BMI — 12.5 to 18 kg for underweight, 11.5 to 16 kg for normal weight, 7 to 11.5 kg for overweight and 5 to 9 kg for obesity — and, more useful for checking progress, a weekly rate for the second and third trimesters, on the assumption of 0.5 to 2 kg in the first.

The weekly rate turns one end-of-pregnancy figure into a path that can be checked at any appointment. Here the first 13 weeks rise evenly from zero to the 0.5–2 kg first-trimester allowance; after that, the weekly range for the BMI category is added for each further week. Because the path is built from those rates rather than stretched to fit the totals, at 40 weeks it lands close to the totals but not exactly on them — for normal BMI it reaches 9.95 to 15.5 kg.

Real pregnancies are rarely so tidy. Nausea can mean early weight loss, fluid retention can add weight late on, and twins, diabetes or worries about the baby's growth call for a plan set by the midwife or doctor. Treat the band as a population reference to discuss at antenatal visits, not a target to diet or eat toward.

What is weight week by week calculator?

A week-by-week weight gain range is the cumulative gain above pre-pregnancy weight that falls within the recommended band at a given week of a singleton pregnancy. It has a lower and an upper edge in kilograms and depends on two things only: the completed week of gestation, counted from the first day of the last menstrual period, and the pre-pregnancy BMI category — underweight below 18.5, normal from 18.5 to under 25, overweight from 25 to under 30, and obesity at 30 or above.

How to use this calculator.

1. 01Work out BMI from your weight before pregnancy and your height: kilograms divided by height in metres squared. At 65 kg and 1.65 m that is about 23.9.
2. 02Enter completed weeks of pregnancy, as dated by your scan or last period — 20 weeks and 5 days counts as 20.
3. 03Subtract your pre-pregnancy weight from today's weight to get your gain so far, and compare it with the band. If you weigh yourself in pounds, divide the gain by 2.205 to convert it to kilograms.
4. 04Check the BMI category shown with the result. If your BMI sits just either side of a boundary such as 25, look at the bands for both categories.
5. 05If you are expecting twins, or your clinician has set a personal weight plan, follow that instead; the band describes singleton pregnancies only.

The formula.

Up to week 13, the lower edge is week ÷ 13 × 0.5 kg and the upper edge is week ÷ 13 × 2 kg, an even spread of the first-trimester allowance the IOM assumed. From week 13 onward, lower = 0.5 + r(low) × (week − 13) and upper = 2 + r(high) × (week − 13), where r is the IOM weekly rate for the BMI category in kilograms per week: 0.44–0.58 for underweight, 0.35–0.50 for normal weight, 0.23–0.33 for overweight and 0.17–0.27 for obesity. The category has by far the biggest effect. At week 30 the normal-weight band is 6.45 to 10.5 kg, while the overweight band is 4.41 to 7.61 kg, because the weekly rates differ by roughly a third. The calculation is linear throughout, so it does not model the faster gain some women see mid-pregnancy or the plateau near term.

A worked example.

Frequently asked questions.

References& sources.

1. [1]National Academies / Institute of Medicine. Weight Gain During Pregnancy, 2009. Table S-1 BMI-specific singleton pregnancy ranges. Retrieved 2026-08-06.
2. [2]U.S. National Library of Medicine. PubMed record current 2026-08-06. Indexed derivation and validation literature. Retrieved 2026-08-06.
3. [3]World Health Organization. Patient safety resources current 2026. Clinical decision-support safety context. Retrieved 2026-08-06.