Calculating your free water deficit is straightforward once you know the Free Water Deficit formula and what each input means. This guide explains the method in plain language, walks through a manual calculation, and gives worked examples you can follow — then you can do it instantly with the Free Water Deficit Calculator.
What is Free Water Deficit?
The Free Water Deficit calculation tells you your free water deficit from a few simple inputs. The figure you are solving for here is the free water deficit.
The Free Water Deficit formula
The core formula is:
Free water deficit = Body water fraction (0.6 men, 0.5 women) × Body weight × (Current sodium ÷ 140 - 1)Here is what each input means:
- Body weight — a value measured in kg. Example: 70 kg.
- Current sodium — a value measured in mEq/L. Example: 154 mEq/L.
- Body water fraction (0.6 men, 0.5 women) — a number. Example: 0.6.
How to calculate it step by step
- Write down the body weight (for example, 70 kg).
- Write down the current sodium (for example, 154 mEq/L).
- Write down the body water fraction (0.6 men, 0.5 women) (for example, 0.6).
- Apply the formula above to get your free water deficit.
- Double-check the result with the Free Water Deficit Calculator.
Worked examples
Example 1
| Input / Output | Value |
|---|---|
| Body weight | 70 kg |
| Current sodium | 154 mEq/L |
| Body water fraction (0.6 men, 0.5 women) | 0.6 |
| Free water deficit | 4.20 |
With body weight of 70 kg, current sodium of 154 mEq/L and body water fraction (0.6 men, 0.5 women) of 0.6, the free water deficit works out to 4.20.
Example 2
With body weight of 140 kg, current sodium of 154 mEq/L and body water fraction (0.6 men, 0.5 women) of 0.6, the free water deficit works out to 8.40.
| Result | Value |
|---|---|
| Free water deficit | 8.40 |
Example 3
With body weight of 35 kg, current sodium of 154 mEq/L and body water fraction (0.6 men, 0.5 women) of 0.6, the free water deficit works out to 2.10.
| Result | Value |
|---|---|
| Free water deficit | 2.10 |
Tips for an accurate result
- Keep your units consistent — mixing, say, months with years or grams with kilograms is the most common source of error.
- Round only at the very end. Rounding inputs early can shift the final answer noticeably.
- Re-run the numbers whenever an input changes, rather than estimating from an old result.
- These figures are general estimates, not medical advice — check with a qualified professional before acting on them.
Prefer not to do the maths by hand? — the Free Water Deficit Calculator does it instantly, for free, with the formula and a worked example built in.
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