Why does my weight fluctuate so much?

Reviewed 2 November 2026 · How these numbers are calculated

Daily swings of 1–2 kg (2–4 lb) are normal and almost never fat. They come from water (salt, carbs, hormones, exercise), food and drink still in your gut, and for women the menstrual cycle. A real fat loss of 0.3–0.5 kg a week is smaller than one day's noise, so compare weekly averages, not single weigh-ins.

Daily weight vs the weekly average

Illustration: losing 0.4 kg a week underneath normal daily swings
Day	Scale	7-day average
Day 1	79.7 kg	79.7 kg
Day 4	80.3 kg	80.0 kg
Day 7	80.3 kg	80.0 kg
Day 10	80.6 kg	80.1 kg
Day 14	80.1 kg	79.7 kg
Day 18	79.5 kg	79.1 kg
Day 21	78.8 kg	78.8 kg
Day 25	79.3 kg	78.9 kg
Day 28	78.8 kg	78.9 kg

Made-up but realistic numbers. The scale jumps by more than a kilo from day to day and can even look higher after a week; the 7-day average moves far less and shows the real downward trend after 2–3 weeks.

What moves the scale

Carbs and glycogen: your muscles and liver store carbohydrate as glycogen, and each gram holds about three grams of water. A high-carb day can add a kilo or more overnight; cutting carbs drops it just as fast, which is why the first week of a low-carb diet looks so good.
Salt: a salty meal makes your body hold water for a day or two.
Food in your gut: a large meal, or a day without a bowel movement, can weigh several hundred grams.
Your cycle: many women hold extra water in the days before their period and lose it once it starts.
Exercise: a hard or new workout makes muscles hold water while they repair, sometimes for days.
Alcohol, poor sleep, stress and travel all shift water balance.

How to weigh yourself

Same time, same conditions: in the morning, after the toilet, before eating or drinking.
Weigh several times a week, or daily if it doesn't stress you.
Compare weekly averages, or the same day of your cycle month to month.
Judge a plan after 3–4 weeks, not 3–4 days.

Your averages also tell you your real burn: put them in the box above. If the average hasn't moved for a month, see calorie deficit but not losing weight or weight-loss plateau.

For women

Cycle-related water can hide a week or two of fat loss. Compare your weight at the same point in each cycle, and don't change your plan because of the days before your period. More in how many calories a woman should eat.