Why the scale lies day to day
Daily weight fluctuates by 1–2 kg from water, glycogen, salt, gut contents, training and hormones — while a 500 kcal daily deficit produces only about 450 g of actual fat loss per week. The noise is larger than the signal, so weigh daily and compare weekly averages.
The noise is bigger than the signal
This is the whole problem in one sentence, and it explains almost every episode of diet-related despair.
A well-run deficit loses roughly 450–700 g of fat per week. Meanwhile your body weight can move 1–2 kg in a single day for reasons entirely unrelated to fat. So on any given morning, the fluctuation is two to four times larger than a whole week's progress.
Which means a single weigh-in carries almost no information about whether your diet is working. Two weigh-ins a week apart carry only slightly more.
| Cause | Typical scale effect | How long it lasts |
|---|---|---|
| A high-salt meal | +0.5 to +1.5 kg | 1–3 days |
| A high-carbohydrate day | +0.5 to +2 kg | 1–3 days |
| A hard or novel training session | +0.5 to +1 kg | 1–3 days |
| Not having been to the toilet | +0.3 to +1 kg | Hours |
| Alcohol | −0.5 to +1 kg, either way | 1–2 days |
| Menstrual cycle (luteal phase) | +0.5 to +2 kg | Up to a week |
| Starting creatine | +0.5 to +1.5 kg | Permanent while supplementing |
| Poor sleep or high stress | +0.3 to +1 kg | Days |
| Actual fat loss at 500 kcal/day | −0.45 kg per week | Permanent |
Look at the bottom row against everything above it. That is why people conclude a working diet has failed.
What is actually moving
Water and sodium
Your body holds water in proportion to sodium intake, hormones and carbohydrate. A salty meal shifts fluid balance for a day or two. This is transient and has no relationship to fat.
Glycogen
Each gram of stored carbohydrate holds roughly three grams of water with it. Full glycogen stores versus depleted ones is a swing of 1–2 kg — which is why low-carbohydrate diets appear to work miraculously in week one and why a high-carbohydrate day looks like a disaster.
Gut contents
Food in transit weighs what it weighs. A high-fibre day, or simply not having been to the toilet, moves the number.
Training
Hard or unfamiliar training causes inflammation and fluid retention in the muscle. Starting a new programme frequently produces a scale rise in week one — see delayed onset muscle soreness.
Hormonal cycle
For women, fluid retention in the luteal phase can add 0.5–2 kg and persist for up to a week. This is the single largest source of confusion for anyone tracking weight while menstruating, and the practical solution is to compare the same phase across months rather than week to week.
Creatine
Draws water into muscle cells. Starting supplementation adds 0.5–1.5 kg within a fortnight, permanently while you continue. Not fat, and not a problem.
How to weigh yourself properly
- Daily, not weekly. Counterintuitive but correct — more data points mean a more reliable average. Weighing once a week means one reading that might be a fluctuation.
- Same conditions. Morning, after the toilet, before eating or drinking, minimal clothing.
- Record every reading and ignore individual numbers entirely.
- Compare weekly averages. This week's seven-day mean against last week's. This is the only comparison that means anything.
- Give it three weeks before concluding anything. Two weeks of flat average can still be noise, particularly across a menstrual cycle.
MyFitsy stores weight alongside your measurements and progress photos, so the trend is visible rather than something you have to reconstruct from memory.
If daily weighing makes you anxious, that is a good reason not to do it. Weigh weekly at the same point in the week, or drop the scale entirely and use waist measurements and photos. Those are slower signals but they are honest ones — and no measurement is worth a bad relationship with food.
Better measurements than weight
Weight is convenient and crude. These tell you more about what you actually care about:
- Waist circumference at the navel. The best single fat-loss measure. Less noisy than weight and directly relevant. Measure fortnightly, same time of day, relaxed.
- Progress photos. Every four weeks, same lighting, same position, same time of day. Change is invisible day to day and obvious across eight weeks.
- Your logbook. If your lifts hold or improve during a deficit, the weight coming off is mostly fat. This is the muscle-retention check and it is free.
- How clothes fit. Unscientific, genuinely informative.
- Other circumferences. Hips, thigh, chest, arm — useful for seeing where change is happening.
What to be sceptical about: bioimpedance body-fat scales. The number they give is heavily affected by hydration and has a large error margin. Directionally useful over months at best; not something to read weekly.
Reading the trend correctly
Some patterns and what they actually mean:
- Up 1.5 kg overnight. Salt, carbohydrate, or gut contents. It is arithmetically impossible to gain 1.5 kg of fat in a day without eating around 13,500 surplus calories.
- Flat for two weeks, then a sudden drop. Very common, especially for women. Fat was being lost while water masked it — the "whoosh" people describe.
- Down fast in week one, then nothing. Glycogen and water first, then fat loss at its real rate. Expected.
- Up in week one of a new programme. Muscle inflammation and fluid retention.
- Genuinely flat for three-plus weeks. Now investigate — tracking accuracy, step count, and whether maintenance has fallen. See building a calorie deficit you can sustain.
Common questions
Should I weigh myself every day?
Yes, if it does not distress you — more data gives a better average. If it does distress you, weekly or not at all is fine.
Why am I heavier after a workout?
Fluid retention from inflammation and repair, plus whatever you drank.
Why did I gain weight overnight?
Salt, carbohydrate, or food in transit. Not fat.
How much should I weigh myself around my period?
Keep weighing, and compare the same phase across cycles rather than consecutive weeks.
Are body-fat scales accurate?
Not very. Large error margins and heavily hydration-dependent.
My weight is up but my waist is down. What is happening?
Almost certainly water, and possibly some recomposition. Trust the tape.
How long before I should worry about a stall?
Three weeks of flat weekly averages, with honest tracking.
The practical version
- Daily fluctuation of 1–2 kg dwarfs a week's actual fat loss of 450–700 g.
- Water, glycogen, salt, gut contents, training and hormones all move the number.
- Weigh daily under the same conditions and compare weekly averages only.
- Give it three weeks before drawing any conclusion.
- Waist circumference, photos and your logbook tell you more than weight does.
- Body-fat scales have large error margins and depend heavily on hydration.
Key takeaways
- Daily weight swings of 1-2 kg exceed a full week's real fat loss of 450-700 g.
- Glycogen holds about three grams of water per gram, causing 1-2 kg swings.
- Luteal-phase fluid retention adds 0.5-2 kg — compare the same phase across cycles.
- Weigh daily under identical conditions and compare weekly averages, never single days.
- Waist circumference, progress photos and your logbook are more informative than weight.
- Bioimpedance body-fat scales have large error margins and depend on hydration.
Related reading
General information, not medical or individualised advice. Speak to a doctor before starting a new programme, especially if you have a medical condition, are pregnant, or are returning from injury. If you train with a coach, their guidance takes precedence.
Track it, or none of this applies
MyFitsy pre-fills the weight you used last time, so progression is a decision instead of a guess.
