Weight change percentage

See your weight change as a percentage. Useful for tracking, for forms from your doctor and for spotting a change you did not plan that is worth raising with your GP.

Your details

Your weight has changed by

-4.3%

Down 7 lb

Before11 st 7 lb
Now11 st 0 lb
A 5% loss, the first NICE milestoneat 10 st 13 lb
Over timeadd the months this took to see it per month
MUST step 1: BMIadd your height to score this step
MUST step 2: unplanned loss0 (only unplanned loss counts)
MUST step 3: ill and not eating0
MUST total so far0: low risk of malnutrition, add your height for the full score

Percentages make changes comparable between people of different sizes, which is why doctors use them.

The working

Change divided by start

Percentage change = (now − before) ÷ before × 100. From 11 st 7 lb (73.0 kg) to 11 st 0 lb (69.9 kg): −3.2 ÷ 73.0 = −4.3%.

When it matters

Unplanned loss

Dietitians and nurses use the Malnutrition Universal Screening Tool, called MUST, to spot people at risk. It flags an unplanned loss of 5% to 10% in 3 to 6 months, and treats more than 10% as high risk. If you are losing weight without trying, that is a good reason to see your GP, even if you feel fine.

The MUST check

Three steps BAPEN uses

MUST adds three scores. BMI: over 20 scores 0, 18.5 to 20 scores 1, under 18.5 scores 2. Unplanned loss in the last 3 to 6 months: under 5% scores 0, 5 to 10% scores 1, over 10% scores 2. Being unwell with little or nothing to eat for more than 5 days scores 2. A total of 0 is low risk, 1 medium and 2 or more high. From 70 kg to 62 kg without trying is an 11.4% loss (2 points); at 1.75 m the BMI now is 20.2 (0 points), so the total is 2, high risk, and worth a GP visit.

Why percentages

Fair at any size

Losing 3 kg is a much bigger deal for someone who weighs 50 kg than for someone who weighs 120 kg. Percentages put everyone on the same footing, which is why research and clinics use them.

Questions

Common questions

How do I calculate weight change percentage?

Take your weight now away from your starting weight, divide by your starting weight and multiply by 100.

How much unexplained weight loss is a concern?

Losing 5% of your weight over 6 to 12 months without trying is a common trigger for a GP check. Faster or bigger losses should be checked sooner.

Does it matter if I use stone or kilos?

No. A percentage is the same in any unit.

Sources

Last checked 21 October 2026. Spotted something wrong? Tell us and we will fix it.

Losing weight without trying

Unplanned weight loss can have lots of causes, most of them treatable. Book a GP appointment and take your numbers. If you also have night sweats, a lump, blood in your poo or ongoing tummy pain, ask to be seen soon.