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Free health tool

Weight loss percentage calculator

Calculate how much a current weight differs from a starting weight as a percentage of the starting value, with the direction shown separately.

  • Free
  • Instant result
  • No account

Calculate your result

Enter the earlier value as a number. Kilograms, pounds or decimal stone all work, but both fields must use the same unit. Do not use a thousands separator.

Accepted range: 0.01 to 1000000

Enter the later value in exactly the same unit and on the same scale basis as the starting value. Enter the number only.

Accepted range: 0.01 to 1000000

Your result will appear here

Nothing is saved or added to the URL.

What this result means

Enter a starting weight and a current weight in the same unit. The calculator divides the absolute difference by the starting weight, multiplies by 100 and states whether the current value is lower, higher or unchanged. [1]

What the percentage means

The starting weight is the baseline. A result of 10.0% lower means the current value is one tenth below that baseline. A result of 10.0% higher means it is one tenth above. The tool shows the magnitude and direction separately to avoid conflicting sign conventions. [1]

Use the same unit in both fields

Because one weight is divided by the other, kilograms cancel with kilograms and pounds cancel with pounds. Mixing units produces a meaningless result. You do not need to choose a unit when both entries already match. [1]

Weight change is not body-fat change

This calculation compares two total scale readings. It cannot identify how much of the difference came from fat, muscle, fluid, food, clothing or measurement variation, and it must not be read as a body-fat percentage.

Make the two measurements comparable

NHS England advises using the same scales in the same location, weighing at a consistent time of day and wearing similar clothing. Those steps improve comparability, but the calculator cannot correct an inconsistent or inaccurate source reading. [2]

No target or healthy range is applied

The result is deliberately not placed into good, bad, safe, rapid or clinically significant bands. Meaning depends on timing, measurement conditions, personal context and why the change occurred; two numbers alone cannot answer those questions.

Practical next steps

Check that both entries use the same unit and that the earlier value is in the starting field. Keep the dates and original records beside the percentage. If the change was unintended, rapid, concerning or part of clinical care, use the original measurements and discuss the context with an appropriately qualified professional rather than relying on this arithmetic result.

Formula and assumptions

Formula
Change magnitude (%) = |current weight − starting weight| ÷ starting weight × 100. Direction is reported separately as lower, higher or no change. [1]
Version
Baseline-relative percentage change in weight, baseline-weight-percentage-change-v1
Rounding
Use exact decimal arithmetic, then round once to one decimal place. A tie exactly halfway between two displayed values rounds upward in magnitude: 0.05% becomes 0.1% and 1.05% becomes 1.1%. A positive change below 0.05% is displayed as less than 0.1%, never as zero.
Worked example
A starting weight of 90 kg and a current weight of 81 kg differ by 9 kg. The calculation is 9 ÷ 90 × 100 = 10.0%, and the result is labelled lower. The same percentage appears after converting both values consistently to pounds. [1]

Who this is for — and when not to use it

Intended use

Neutral percentage arithmetic for two existing body-weight records entered in the same unit, with the difference expressed relative to the starting value.

Not intended for

This tool does not set a goal, judge whether a change is healthy or safe, classify body size, estimate body fat, explain why weight changed, predict future change, diagnose a condition, guide diet or exercise, or support medication, dehydration or clinical-nutrition decisions.

Exclusions

  • Children and adolescents
  • Pregnancy or postpartum weight assessment
  • Eating-disorder care or any situation where weight tracking is distressing
  • Unintentional, rapid or otherwise concerning weight change
  • Clinical nutrition, hospital, dehydration, dialysis or fluid-balance monitoring
  • Medication, supplement or treatment dosing
  • Weight goals, predictions, competitions or eligibility decisions
  • Body-fat, muscle or other body-composition interpretation
  • Mixed units, including kilograms in one field and pounds in the other
  • Compound stone-and-pound notation; convert it to one decimal unit first

Evidence sources

  1. 1. NIST Guide to the SI, Chapter 7: Rules and Style Conventions for Expressing Values of Quantities — National Institute of Standards and Technology. Accessed Aug 10, 2026.
  2. 2. How to record your weight — NHS England. Accessed Aug 10, 2026.