Weight loss percentage calculator

The formula

Percentage of weight lost is one subtraction and one division:

The units cancel out, which is why the calculator above gives the same answer whether you work in pounds or kilograms. A 200 lb start and a 185 lb weight now is 7.5%. So is 90.7 kg to 83.9 kg. It is the same body.

The mistake: dividing by the wrong weight

The denominator is the weight you started at, never the weight you are now. Divide 15 lb by your current 185 lb instead of your starting 200 lb and you get 8.1% rather than 7.5%. The error always flatters you, it always grows as you lose more, and it makes your number incomparable with every clinical figure you will read — trials report change from baseline, and baseline is the weight at the start.

The calculator above prints both figures side by side so the gap is visible rather than arguable.

Why 5% is the number everybody quotes

It is not a marketing round number. It comes from the FDA’s guidance for companies developing weight-reduction drugs, which states that in patients with obesity or overweight, long-term weight reduction greater than or equal to 5% of baseline body weight or BMI — whether from diet, exercise or some drug therapies — is associated with improvement in various metabolic and cardiovascular risk factors.

The same document sets the regulatory bar: a drug is generally considered effective for weight reduction if, after one year of treatment at the maintenance dose, the difference in mean percentage weight reduction between the drug and the control group is at least 5% and is statistically significant. That is the whole test. Not a pound count, not a BMI category — a percentage difference against placebo.

What quietly changed in January 2025

Older versions of that guidance offered a second, categorical route to approval: a drug could qualify if the proportion of people losing at least 5% of baseline body weight was at least 35% in the treated group and roughly double the control group. That alternative is gone from the current draft. The revision issued in January 2025 keeps only the mean criterion.

It goes further, and says why. Responder analyses — counting how many people crossed a 5%, 10% or 15% threshold — “may inappropriately exaggerate treatment effects”, and the guidance works through a case where two groups differing by only 2% in mean weight reduction produce a 68-point difference in the proportion crossing a 5% line. Its conclusion is blunt: responder analyses for the evaluation of weight reduction are generally not recommended.

That is worth knowing before you read the next “X% of patients lost 15% of their body weight” headline. The regulator that has to sign the label now treats that framing as the one most likely to mislead.

Percentage or pounds?

Percentage, if you want to compare yourself to anything published. Twenty pounds off a 300 lb frame is a different event from twenty pounds off a 140 lb one, and the trials know it: the STEP 1 trial of semaglutide reported a mean change in body weight of −14.9% against −2.4% on placebo at week 68, not a pound figure, precisely so the result means the same thing for every participant.

Pounds are still the number you feel. Use both — the calculator shows both — and use the percentage whenever you are comparing yourself to a study, a label or somebody else.

Next

• Weight loss timeline calculator — the same weights, projected forward onto real calendar dates.
• Macro calculator — the calorie target underneath the percentage.
• How long semaglutide takes to work — when the published percentages were actually measured.