Weight gain percentage

See your weight gain as a percentage and, if you add the weeks, how quickly it has come.

Your details

Weight gained

+5.7%

7 lb gained in 8 weeks

Start8 st 10 lb
Now9 st 3 lb
Per week0.9 lb (0.72% of your start weight)
Per month3.8 lb (3.1% a month)
5% gainedreached in 8 weeks or less
10% gainedat this pace, in about 6 more weeks (1.4 months)
If you are building back upinside a gentle 0.25 to 0.5 kg a week
BMI then and nowadd your height

Gaining weight is often the goal, after illness, an injury or a spell of being underweight. Slow and steady keeps it comfortable. If you were not trying to gain, a 5% rise inside a year is worth a look at sleep, medicines and routine, and a chat with your GP if it keeps going.

The working

Gain divided by start

From 8 st 10 lb (55.3 kg) to 9 st 3 lb (58.5 kg) is a gain of 3.2 kg. 3.18 ÷ 55.3 × 100 = 5.7%. Over 8 weeks that is 0.4 kg a week, or 1.7 kg (3.1%) a month, inside a gentle 0.25 to 0.5 kg a week. At that pace 10% gained is about 6 weeks away.

On purpose

Gaining after illness or underweight

If you are building back up, a steady 0.25 to 0.5 kg a week is comfortable for most people. Little and often helps: three meals plus two or three snacks, with protein at each. A dietitian can help if eating enough feels like hard work.

Building muscle

A lean pace

For people training to build muscle, a review of the research suggests gaining about 0.25 to 0.5% of body weight a week (Iraki and colleagues, 2019); faster than that, more of the gain tends to be fat. At 55.3 kg, 0.4 kg a week is 0.72%, so a little quick for lean gain, though fine when building back after illness. Add your height to see your BMI then and now and how far the next marker is: at 1.60 m, 58.5 kg is a BMI of 22.8, and a BMI of 25 would be 64.0 kg.

Not on purpose

When quick gain needs a check

Gaining 2 kg or more in a week or two, especially with swollen ankles or breathlessness, can be fluid rather than fat. It can be linked to the heart, kidneys or some medicines, so it is worth contacting your GP or calling NHS 111.

Questions

Common questions

How do I calculate weight gain percentage?

Weight gained divided by starting weight, times 100.

How fast should I gain weight if I am underweight?

About 0.25 to 0.5 kg a week is a gentle pace. Your GP or a dietitian can tailor it, especially after illness.

Is sudden weight gain dangerous?

Usually it is water from salt, hormones or a big weekend. But fast gain with swelling or breathlessness should be checked by a doctor the same day.

Sources

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

If your weight is on your mind

Talk to your GP if your weight has changed a lot without trying, or if it is worrying you. If food or your body feels hard to think about, Beat has a free helpline on 0808 801 0677 (England, weekdays 3pm to 8pm).