Free Water Deficit Table: Free water deficit by Body weight
Reference table of free water deficit for Free Water Deficit across a range of body weight values — exact, engine-computed figures you can read off at a glance.
Free Water Deficit Calculator
Verified formula Updated Jun 2026 Private — runs on your device
Verified formula
Private
Free water deficit
4.20
For general information only — not financial, tax, legal or medical advice. Verify before you rely on it.
How to use the Free Water Deficit Calculator
The Free Water Deficit Calculator works out your free water deficit in an instant. Enter body weight, current sodium and body water fraction (0.6 men, 0.5 women) and the result updates as you type — it is free, needs no sign-up, and runs entirely in your browser so your figures stay private.
- Enter the body weight.
- Enter the current sodium.
- Enter the body water fraction (0.6 men, 0.5 women).
- Read off your free water deficit — the calculator updates automatically, with no button to press.
Formula
The Free Water Deficit Calculator uses the formula:
Free water deficit = Body water fraction (0.6 men, 0.5 women) × Body weight × (Current sodium ÷ 140 - 1)
Worked example
For example, 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 is 4.20.
| 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 |
|---|
Results are estimates for educational use, not professional advice.
Frequently asked questions
It estimates how much water the body is short by when sodium is high, guiding correction of hypernatremia.
Deficit = body water fraction × weight × (current sodium ÷ 140 − 1), giving litres of water needed.
About 0.6 for men and 0.5 for women is common, with lower values used for elderly patients.
Usually slowly, often no more than about 10 mEq/L per day, to avoid complications. A clinician must supervise.
Enter the body weight. Enter the current sodium. Enter the body water fraction (0.6 men, 0.5 women). Read off your free water deficit — the calculator updates automatically, with no button to press.
Related guides
How to Calculate Free Water Deficit: Formula, Steps & Examples
Learn how to calculate Free Water Deficit — the formula explained step by step, with worked examples and a free calculator to check your answer.