Water Weight and the Scale
Two kilos overnight is not two kilos of fat. It takes a 14,000 calorie surplus to gain that, and you did not eat one.
Last updated: 30 September 2026
Overview
You weighed 74.2kg on Tuesday and 76.1kg on Wednesday. Nothing you did on Tuesday can explain that, and the panic it causes is the reason a lot of people abandon a diet that was working.
Quick Answer
Day-to-day weight changes are almost entirely water, food in transit and glycogen. Gaining a kilogram of fat requires roughly a 7,700 calorie surplus, so an overnight rise of one or two kilograms cannot be fat.
Key Points
- A kilo of fat needs about a 7,700 calorie surplus. Overnight rises are not fat.
- Salt, carbohydrates, training and hormones all move water.
- Swings of 1 to 2kg within a week are entirely normal.
- Weigh daily, judge on the weekly average.
- A leg that is suddenly swollen and painful is a different thing. See below.
What actually moved
| Cause | Typical swing | Clears in |
|---|---|---|
| A salty meal | 0.5 to 2kg | 1 to 3 days |
| A high carbohydrate day | 0.5 to 1.5kg | 1 to 2 days |
| Hard or unfamiliar training | 0.5 to 1.5kg | 2 to 4 days |
| Food still in transit | 0.5 to 1kg | 1 to 2 days |
| Menstrual cycle | 1 to 2kg | Cyclical |
| Starting creatine | 0.5 to 1.5kg | Stays, and is not fat |
| Alcohol | Up or down | 1 to 3 days |
| Poor sleep or high stress | 0.5 to 1kg | With the stressor |
Salt. Sodium holds water. A takeaway on Friday shows on Saturday's scale and is gone by Monday. It is not a reason to avoid salt.
Carbohydrates. Each gram of stored glycogen holds roughly three grams of water with it. This is why the first week of a low carbohydrate diet drops several kilos — and why they return the moment you eat carbohydrates again. It is the whole mechanism behind both the rapid "success" and the "I regained it all in a weekend".
Training. Unfamiliar or hard training causes inflammation and fluid shift into the muscle. Which is why your legs can be up a kilo the day after a hard leg session, and why that is a sign the session worked.
The menstrual cycle. Retention of one to two kilos, typically in the late luteal phase, is normal. See training and your cycle.
How to read the scale properly
1. Weigh daily, same time, after the toilet, before food, minimal clothing.
2. Record all of it, including the days you do not like.
3. Use the weekly average, not any single day.
4. Compare averages to averages — this week's mean against last week's.
5. Give it three to four weeks before concluding anything.
A single weigh-in carries so much noise it is close to useless. A weekly average is a genuinely good measurement, and it is free.
| What you see | What it means |
|---|---|
| +1.5kg overnight | Salt, carbohydrate, food, water |
| Flat for 5 days then a 1kg drop | Normal. The trend was there all along |
| Weekly average flat for 3 weeks | Worth investigating. See plateaus |
| Down 3kg in week one of a diet | Mostly glycogen and water |
That last row is worth expecting. The first week of any diet drops fast, then slows dramatically, and people read the slowdown as failure when it is just the water having already gone.
The "why do my legs look bloated" question
Usually one of: a salty day, a lot of standing, heat, a long flight or drive, a hard lower body session, or where you are in your cycle. All of it settles.
Stop and get medical advice if swelling is in one leg rather than both, especially with pain, warmth or redness, or if it comes with breathlessness or chest pain. That is not a water-weight question and it should be looked at the same day. Persistent swelling in both legs that does not settle is worth a GP appointment too.
Common Mistakes
| Mistake | Why it causes problems | Better approach |
|---|---|---|
| Panicking at an overnight rise | It is arithmetically impossible as fat | Weekly averages |
| Cutting calories after a bad weigh-in | Reacting to noise | Wait three weeks |
| Only logging the good days | Corrupts the average you rely on | Record everything |
| Cutting carbohydrates to "lose" quickly | Loses water, returns instantly | Judge by the trend |
| Weighing at different times | Adds noise you could have avoided | Same time, same conditions |
| Avoiding salt entirely | Unnecessary, and salt matters for training | Expect the swing |
Frequently Asked Questions
Why did I gain 2kg overnight?
You did not gain 2kg of fat — that would need about a 15,000 calorie surplus. It is water, food in transit and glycogen.
How do I get rid of water retention?
Usually nothing: it clears in one to three days. Normal salt, normal fluid, sleep and movement all help it settle.
Why does the scale go up after a hard session?
Inflammation and fluid moving into the trained muscle. It settles in a couple of days and is a sign the session did something.
How often should I weigh myself?
Daily, and judge only the weekly average. Daily weighing gives you a good average; daily reacting gives you nothing.
Is losing 3kg in the first week real?
Mostly water and glycogen rather than fat. It is normal, and so is the slowdown that follows.
Sources
NHS healthy weight guidance - https://www.nhs.uk/live-well/healthy-weight/
NHS guidance on salt in your diet - https://www.nhs.uk/live-well/eat-well/food-types/salt-nutrition/
British Dietetic Association - https://www.bda.uk.com/