Weight fluctuation and water

Weight goes up and down from day to day. Put in a week of weigh-ins to see the average, which is the most useful number to track.

Your details

Day 1
Day 2
Day 3
Day 4
Day 5
Day 6 (optional)
Day 7 (optional)
Last week's average (optional)

Same scales, same time of day, ideally first thing after the loo.

Your average this week

11 st 4 lb

Swing of 4 lb over 5 weigh-ins

  • Lightest11 st 2 lb
  • Heaviest11 st 6 lb
  • Typical day to day wobbleabout 1.6 lb either side of your average

Day to day wobbles like this are normal and mostly water. Compare one week’s average with the next, not one morning with another.

The working

Average and swing

Add the readings and divide by how many there are. Five readings of 11 st 4, 11 st 6, 11 st 3, 11 st 2 and 11 st 5 average out at 11 st 4 lb, with a swing of 4 lb between the lightest and heaviest day.

The typical wobble is the standard deviation of your readings: here about 1.6 lb either side of the average. Add last week's average and we compare the two. The noise around a week's average is about 2 × the wobble ÷ √(number of readings), here 1.4 lb, so a change smaller than that could just be water. From last week's 11 st 6 lb, this week is down 2 lb: bigger than the noise, so likely a real change. A fall of more than 1 kg a week is quicker than the NHS 0.5 to 1 kg; early weeks often drop faster from water.

Why it jumps

Mostly water

  • Salt: a salty meal makes your body hold extra water for a day or two.
  • Carbohydrate: every gram of stored glycogen holds about 3 g of water.
  • Hormones: many women hold more water in the days before a period.
  • Exercise: a hard session causes a little swelling in the muscles while they repair.
  • Weekends: research from Finland found weight is usually highest on Monday and lowest on Friday.

What to watch

Compare averages

Compare this week’s average with last week’s, not one morning with the next. A difference of half a kilo either way is noise.

Questions

Common questions

How much does weight normally fluctuate in a day?

Up to 1 to 2 kg (2 to 4 lb) is common between morning and evening, mostly from food, drink and water.

Why do I weigh more after exercise?

Muscles hold extra water while they repair, and you may drink more. It goes within a few days.

When is fluid retention a worry?

If you have swollen ankles or legs that do not go down overnight, or swelling with breathlessness, see your GP or call NHS 111.

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).

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