Weight Loss Percentage Calculator

Background.

Eight kilograms is a very different change for someone who started at 70 kg than for someone who started at 140 kg: about 11.4% of body weight in the first case, about 5.7% in the second. That is why doctors, trials and weight-management programmes talk about weight loss in percentages. Divide the weight lost by the starting weight, multiply by 100, and people of any size can be compared on the same scale.

The percentage carries real clinical meaning. The 2013 obesity guideline from the American Heart Association, American College of Cardiology and The Obesity Society found that keeping off 3–5% of body weight is likely to bring clinically meaningful falls in triglycerides, blood glucose and HbA1c and a lower risk of type 2 diabetes, and that larger losses also improve blood pressure and cholesterol. For a 90 kg adult, 3–5% is just 2.7 to 4.5 kg.

The same number reads quite differently when the loss was not intended. The European Society for Clinical Nutrition and Metabolism (ESPEN) counts unintentional loss of more than 5% of habitual weight within three months, or more than 10% over any period, as one criterion in diagnosing malnutrition. If you have been losing weight without trying, the result is something to take to a doctor, not a milestone.

Alongside the headline percentage you also get the monthly rate, your BMI at both weigh-ins, and percent excess weight loss, the share of your weight above BMI 25 that has gone. That last figure was long the standard in weight-loss surgery research but has largely given way to the simpler total percentage.

What is weight loss percentage calculator?

Percent total weight loss (%TWL) is weight lost as a share of starting weight. It is the measure the American Society for Metabolic and Bariatric Surgery made a recommended primary outcome in its 2015 reporting standards, alongside starting BMI and change in BMI. Percent excess weight loss (%EWL) divides the same loss by only the weight above a reference, here the weight that would give a BMI of 25, so it is always the larger number. Neither says whether the weight lost was fat, muscle or water; a scale cannot tell, and a body-composition measurement is needed for that.

How to use this calculator.

1. 01Enter your starting weight and current weight in the same unit, choosing kilograms or pounds; the percentage comes out the same either way.
2. 02Pick a starting point that matches your question: the start of a programme, the date of surgery, or your usual weight before an illness.
3. 03Enter your height for the BMI and excess-weight figures. It does not change the total percentage.
4. 04Enter the months between the two weigh-ins, from the actual dates on the scale, since it sets the monthly rate.
5. 05If your current weight is higher than your starting weight, the percentage shows as negative, which measures regain.

The formula.

%TWL = (W₀ − W₁) ÷ W₀ × 100, where W₀ is the starting weight and W₁ the current weight. Because the units cancel, kilograms and pounds give the same answer; pounds are converted at exactly 0.45359237 kg only for the kilogram and BMI outputs. The monthly rate is %TWL divided by the months elapsed, an average that says nothing about whether the loss came early or late.

BMI = weight in kg ÷ height in metres squared, worked out for both weigh-ins. The weight at BMI 25 is 25 × h², and excess weight is W₀ minus that figure. %EWL = (W₀ − W₁) ÷ (W₀ − 25 × h²) × 100. Dividing top and bottom by h² shows that excess BMI loss, (BMI₀ − BMI₁) ÷ (BMI₀ − 25) × 100, is the same number, so only one is shown. Older papers that took ideal weight from life-insurance tables instead of BMI 25 are where the two differ.

If your starting BMI was 25 or below there is no excess weight to divide by, so excess weight and %EWL show as zero and the band label adds "(no excess weight at baseline)" at every loss size; the total percentage still applies to everyone.

The band label compares %TWL with the guideline's 3% and 5% marks and also flags 10%. The 10% line is a reporting milestone rather than a guideline threshold, and the bands describe intentional loss only.

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.