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Water Weight Fluctuation Calculator

Work out water weight fluctuation instantly with clear inputs, formula shown and shareable results.

About Water Weight Fluctuation Calculator

Scale weight swings mostly through water, not fat. Each gram of stored glycogen binds about three grams of water, and each extra gram of sodium holds roughly 150 ml. A 200 g carbohydrate increase plus 2 g more sodium can add about 0.9 kg overnight - which is why daily weigh-ins need a weekly average to be meaningful.

Formulas

Glycogen water

water = carbohydrate stored x 3 g of water per gram

Sodium water

about 150 ml retained per extra gram of sodium

Worked example

With the default values below, Water Weight Fluctuation Calculator returns Expected Water Weight Change of 0.90. Change any input above and every figure here is recalculated in your browser.

Inputs used

Carbohydrate intake change
200 g
Sodium intake change
2,000 mg
Body weightminimum 0 kg
75 kg

Results

Expected Water Weight Change
0.90 kg
From Glycogen Storage
0.600 kg
From Sodium Retention
0.300 kg
As A Share Of Body Weight
1.20%

Step by step

  1. 1Values used
  2. 2Glycogen water
  3. 3Sodium water
  4. 4Expected Water Weight Change= 0.90 kg
  5. 5From Glycogen Storage= 0.600 kg
  6. 6From Sodium Retention= 0.300 kg
  7. 7As A Share Of Body Weight= 1.20

Assumptions and limits

  • The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
  • These figures are population-average estimates, not a medical assessment or diagnosis. Body composition equations carry errors of several percentage points, and healthy ranges vary with age, ethnicity, muscularity and medical history. Discuss weight or body composition goals with a qualified clinician or dietitian.
  • This is an estimate for general information and not professional medical advice. See the full disclaimer for the limits that apply to this kind of calculator.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

These figures are population-average estimates, not a medical assessment or diagnosis. Body composition equations carry errors of several percentage points, and healthy ranges vary with age, ethnicity, muscularity and medical history. Discuss weight or body composition goals with a qualified clinician or dietitian.

Frequently Asked Questions

Why did I gain 1 kg overnight?

Almost certainly water and gut contents. Gaining a kilogram of fat requires a 7700 kcal surplus, which is not achievable in one meal.

How should I weigh myself?

Same time each morning after the toilet, and judge the seven-day rolling average rather than day-to-day changes.

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