Weight Loss Percentage Calculator

Background.

This calculator works out what percentage of your starting weight you have lost, and then does three things a generic percentage calculator cannot: it expresses the loss as a share of the excess weight you were carrying above BMI 25, it converts it to a rate per month, and it tells you which published guideline band the total falls in. Enter your starting weight, your current weight, your height and how long the two weigh-ins were apart.

Before any of the numbers: the single most important question about a weight change is whether you meant it. Intentional and unintentional weight loss of exactly the same size mean opposite things. The 2013 American Heart Association, American College of Cardiology and Obesity Society guideline says that intentional sustained loss of 3 % to 5 % produces clinically meaningful benefit. The European Society for Clinical Nutrition and Metabolism uses more than 5 % over three months, or more than 10 % over any period, as a criterion for the unintentional weight loss that contributes to a diagnosis of malnutrition. Same arithmetic, opposite conclusions. If you did not intend to lose this weight, the number on this page is a reason to talk to a doctor rather than a result to be pleased about.

The headline figure is percent total weight loss, usually written %TWL: weight lost divided by starting weight. That is deliberate. Until 2015 the bariatric surgery literature mostly reported percent excess weight loss instead, and in that year the American Society for Metabolic and Bariatric Surgery published standardised reporting guidance that made mean initial BMI, change in BMI and %TWL the recommended primary outcomes, with excess weight loss and excess BMI loss demoted to a fourth measure. The field followed: an audit published in 2022 found %TWL reporting rose from 17 % to 61 % between 2015 and 2021. The reason is that %EWL flatters. Because it divides by excess weight rather than total weight, it always produces a larger number for the same kilograms lost — 34.1 % against 8.0 % in the worked example on this page — and it depends heavily on how heavy you were to start with, which makes two people's figures hard to compare.

Both are shown here, with the total-loss figure leading. One thing you will not find is a separate percent excess BMI loss output, and that is not an oversight. Under the modern convention where 'ideal weight' means the weight at BMI 25, excess BMI loss and excess weight loss are algebraically the same number: divide out the height-squared term and the two expressions are identical, which the calculator's own figures confirm to ten decimal places. Older papers that used Metropolitan Life ideal-weight tables instead of BMI 25 are the ones where they differ. Showing both would present a duplicate as if it were a second opinion.

One consequence of anchoring excess weight to BMI 25 deserves flagging because it affects a lot of people. If your starting BMI was 25 or below, you had no excess weight by that definition, the denominator of the excess-loss calculation is zero or negative, and the metric is simply undefined. Rather than reject you — your total weight loss percentage is perfectly well defined and is the number that matters anyway — the calculator returns zero for both excess figures and says so in the band label. It is a limitation of the metric, not of your progress.

The AHA/ACC/TOS statement is worth quoting properly because it is more specific than the usual paraphrase. Recommendation 2 of that guideline, graded Class I with Level of Evidence A, says that sustained weight loss of 3 % to 5 % 'is likely to result in clinically meaningful reductions in triglycerides, blood glucose, hemoglobin A1c, and the risk of developing type 2 diabetes', and that greater amounts of weight loss 'will reduce BP, improve LDL-C and HDL-C, and reduce the need for medications to control BP, blood glucose, and lipids'. Note what that does and does not say: 3 to 5 % buys you glycaemic and triglyceride benefit, and you need to go further for blood pressure and cholesterol. It is one of the most useful and most consistently under-quoted findings in obesity medicine, because it sets a first target that is genuinely achievable.

Finally, the rate. Percentage per month is included because that is the form the malnutrition criterion is written in, and because rate matters independently of total: five percent lost over three months and five percent lost over three years are clinically different events even though the calculator's headline number is identical. Use the elapsed-time field honestly — the date of the weigh-in, not the date you started thinking about it.

What is weight loss percentage calculator?

Weight loss percentage expresses a change in body weight as a proportion rather than an absolute amount, so that losses can be compared between people of different sizes. Two definitions are in common use. Percent total weight loss (%TWL) is weight lost divided by starting weight, multiplied by 100 — the measure the American Society for Metabolic and Bariatric Surgery made a recommended primary outcome in its 2015 standardised reporting guidance, alongside initial BMI and change in BMI. Percent excess weight loss (%EWL) is weight lost divided by the weight the person was carrying above a reference 'ideal' weight, conventionally the weight corresponding to BMI 25. The two answer different questions: %TWL asks how much of you has gone, %EWL asks how much of the surplus has gone. %EWL is always the larger figure for a given loss and is more sensitive to the starting point, which is why the ASMBS demoted it and why reporting practice shifted sharply towards %TWL after 2015. A third figure, percent excess BMI loss, is algebraically identical to %EWL whenever the ideal weight is defined by BMI 25. Weight loss percentage is not by itself a measure of health or of body composition — it says nothing about whether the weight lost was fat or lean tissue, which is why it is usually read alongside a body-composition measure — and its meaning depends entirely on whether the loss was intentional.

How to use this calculator.

1. 01Enter your starting weight — the weight at whatever baseline you want to measure from, not necessarily your highest ever weight.
2. 02Enter your current weight in the same unit. A higher current weight is allowed and gives a negative percentage.
3. 03Enter your height. It is used only for the BMI and excess-weight figures and has no effect on the total weight loss percentage.
4. 04Enter the number of months between the two weigh-ins. Be honest about this — it drives the rate figure, which is what the unintentional-weight-loss criteria are written in terms of.
5. 05Read the total weight loss percentage as the headline. This is the measure current reporting standards recommend.
6. 06Check the band label beside it, which names the AHA/ACC/TOS band your loss falls in for intentional weight loss.
7. 07Compare the excess weight loss figure if you are tracking against bariatric literature, remembering that it is always the larger number.
8. 08If the loss was not intentional, take the rate figure to a doctor. More than 5 % in three months, or more than 10 % over any period, is a criterion ESPEN uses in assessing malnutrition.

The formula.

Percent total weight loss is (starting weight − current weight) ÷ starting weight × 100. It is unit-independent: pounds and kilograms give the same percentage. Percent excess weight loss is (starting weight − current weight) ÷ (starting weight − weight at BMI 25) × 100, where the weight at BMI 25 is 25 × height in metres squared. Percent excess BMI loss is (starting BMI − current BMI) ÷ (starting BMI − 25) × 100, and IT IS THE SAME NUMBER: dividing numerator and denominator through by height squared turns the second expression into the first exactly, which is why only one is shown. Working the shipped example: someone starting at 100 kg and now at 92 kg has lost 8 kg, so %TWL is 8 ÷ 100 × 100 = 8.0000000000 %. Over 6 months that is 1.3333333333 percentage points per month. At 175 cm their height squared is 3.0625 m², so their starting BMI is 100 ÷ 3.0625 = 32.6530612245 and their current BMI is 92 ÷ 3.0625 = 30.0408163265. The weight at BMI 25 is 25 × 3.0625 = 76.5625 kg, so their baseline excess was 100 − 76.5625 = 23.4375 kg, and %EWL is 8 ÷ 23.4375 × 100 = 34.1333333333 %. Checking the identity: (32.6530612245 − 30.0408163265) ÷ (32.6530612245 − 25) × 100 = 2.6122448980 ÷ 7.6530612245 × 100 = 34.1333333333 %, identical as the algebra requires. DIRECTIONAL FACTS: %EWL always exceeds %TWL for anyone above BMI 25, by exactly the ratio of starting weight to baseline excess — here 100 ÷ 23.4375 = 4.27 times. And %TWL goes negative when the current weight exceeds the starting weight. ROUNDING STAGE: everything is carried at full decimal precision and rounded once at the return, and the guideline band is decided on the unrounded percentage, so 2.9999999 % does not display as 3 % and pick up the 3–5 % band. DOMAIN: weights and height must be positive and the elapsed period at least 0.1 months, since a rate needs something to divide by. When the starting BMI is 25 or below the excess-weight denominator is zero or negative, so both excess outputs return zero and the band text says the metric does not apply.

A worked example.

Frequently asked questions.

References& sources.

1. [1]Jensen MD, Ryan DH, Apovian CM, et al. "2013 AHA/ACC/TOS Guideline for the Management of Overweight and Obesity in Adults: A Report of the American College of Cardiology/American Heart Association Task Force on Practice Guidelines and The Obesity Society." Circulation. 2014;129(25 Suppl 2):S102–S138. PMC5819889. Source of the 3–5 % benefit band quoted on this page — Recommendation 2, Class I, Level of Evidence A: "Sustained weight loss of 3%–5% is likely to result in clinically meaningful reductions in triglycerides, blood glucose, hemoglobin A1c, and the risk of developing type 2 diabetes; Greater amounts of weight loss will reduce BP, improve LDL–C and HDL–C…". Open access.
2. [2]Cederholm T, Bosaeus I, Barazzoni R, et al. "Diagnostic criteria for malnutrition — An ESPEN Consensus Statement." Clin Nutr. 2015;34(3):335–340. PMID 25799486. Source of the unintentional weight loss criterion: ">10% of habitual weight indefinite of time, or >5% over 3 months". Abstract open access; full text paywalled.
3. [3]Brethauer SA, Kim W, el Chaar M, et al. (ASMBS Clinical Issues Committee). "Standardized outcomes reporting in metabolic and bariatric surgery." Surg Obes Relat Dis. 2015;11(3):489–506. PMID 26093765. The reporting standard that made percent total weight loss a recommended primary outcome and demoted percent excess weight loss to a fourth measure. Note on provenance: PubMed carries no abstract for this record and the full text is paywalled, so its content is attributed on this page via the 2022 audit below.
4. [4]"Reporting of weight loss outcomes in bariatric surgery following introduction of 2015 ASMBS guidelines." Surgery for Obesity and Related Diseases, 2022. The readable attribution point for the 2015 standard, stating that the recommended primary outcomes are "the mean initial body mass index (BMI), change in the BMI (ΔBMI), and percent total WL (%TWL), with percent excess BMI loss (ΔBMIL) and/or percent excess WL (%EWL) as a fourth outcome", that %EWL and ΔBMIL are "potentially error-sensitive", and that %TWL reporting rose from 17 % to 61 % between 2015 and 2021.
5. [5]National Institute of Standards and Technology — "Guide for the Use of the International System of Units (SI)," NIST Special Publication 811, 2008 edition, Appendix B.9. Source of the exact conversions 1 lb = 0.45359237 kg and 1 in = 2.54 cm.