Water Weight Fluctuation Explainer
Break an overnight scale swing into its causes — sodium, glycogen, alcohol, training, cycle and travel — plus the fat-equivalent it would need.
About this tool
Water Weight Fluctuation Explainer
Break an overnight scale swing into its causes — sodium, glycogen, alcohol, training, cycle and travel — plus the fat-equivalent it would need.
Water weight is the fluid your body holds or sheds from day to day, and it moves far faster than fat ever can. This tool splits an overnight scale swing into measurable parts: extra sodium must be carried in isotonic fluid at about 0.31 litres per gram of sodium, each gram of new glycogen binds roughly 3 g of water, and ethanol drives about 10 mL of extra urine per gram. It then converts the total into the calorie imbalance the same figure would require if it were genuinely fat.
How to use Water Weight Fluctuation Explainer
- 1
Open Water Weight Fluctuation Explainer on AltFTool — it loads instantly in your browser.
- 2
Enter Body weight (kg), Sodium yesterday (mg), Carbohydrate yesterday (g), Alcoholic drinks and the optional Scale change this morning (kg).
- 3
Tick anything else that applies — hard training, a long flight, sauna or a low-fluid day — and set the Creatine and Menstrual cycle phase selectors.
- 4
Read the Estimated fluid swing in kg, its fat-equivalent kcal and the 'Where the swing came from' driver table, then press Copy result.
Why use Water Weight Fluctuation Explainer
Physics, not vibes
Sodium, glycogen and alcohol effects are computed from molar mass, water-binding and diuresis figures rather than guessed.
Fat-equivalent reality check
Converts the swing into the kcal imbalance it would need as fat — usually a number that makes the fat explanation obviously impossible.
Reconciles with your actual scale
Enter what the scale really did and it reports the unexplained remainder instead of pretending the model is complete.
Common use cases
- Explain a 1.5 kg jump the morning after a restaurant meal instead of assuming the diet failed.
- Separate the scale drop after a night of drinking from real fat loss before drawing conclusions.
- Understand the premenstrual rise that shows up in the same week every cycle.
- Account for the 1 kg gain during a creatine loading phase so training progress is not misread.
Frequently asked questions
How much water weight can you gain in a day?
Two to three kilograms is entirely possible from a single high-sodium, high-carbohydrate day. Each extra gram of sodium is carried in about 0.31 litres of fluid and each gram of stored glycogen holds around 3 g of water, so a large meal out plus a carbohydrate refeed adds up quickly.
Why did I gain 1 kg overnight when I ate at a deficit?
Because 1 kg of fat stores roughly 7,700 kcal — you would need that much surplus in one day to gain it. Overnight changes of that size are fluid, glycogen and gut contents. Weigh at the same time each morning and judge progress on a seven-day average.
Does salt make you gain weight?
Salt makes you gain water, not fat. Sodium is held in extracellular fluid at about 140 mmol per litre, so retained sodium pulls water with it until the kidneys clear it, usually over one to two days. Table salt is 39% sodium by weight, so 5 g of salt is about 1,970 mg of sodium.
Is water retention ever a warning sign?
Usually it is harmless day-to-day variation, but rapid gain of several kilograms over a few days, swelling that pits when pressed, breathlessness when lying flat, or swelling in only one leg can indicate heart, kidney, liver or clotting problems. Those need prompt medical assessment rather than a calculator.