What this calculator does
Free water deficit works out estimated water needed to correct a high serum sodium (hypernatraemia). Enter your own figures above and the answer updates as you type: nothing is fixed in the code, so the result reflects exactly the numbers you supply.
The formula this calculator evaluates is printed under the tool and explained below, so you can check the working by hand or reuse it in a spreadsheet.
The formula
FormulaTBW = weight × body-water factor; Deficit = TBW × (measured Na / target Na − 1)
The inputs explained
| Field | What to enter |
|---|---|
| Body weight (kg) | A number, measured in kg. Starts at 75. |
| Measured sodium (mEq/L) | A number, measured in mEq/L. Starts at 155. |
| Target sodium (mEq/L) | A number, measured in mEq/L. Starts at 140. |
| Body-water factor | Choose from Adult male (0.6), Adult female or elderly male (0.5), Elderly female (0.45), Child (0.6). |
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How the answer changes with body weight
Every other input is held at the calculator’s starting values while body weight varies. Select any row to load that scenario into the calculator.
How the answer changes with body weight
| Body weight (kg) | Estimated free water deficit | Total body water used | Note |
|---|---|---|---|
| 37.5 | 2.41 L | 22.5 L | A starting estimate only: correction should proceed gradually under clinical guidance, typically no faster than about 10 mEq/L per 24 hours. |
| 56.25 | 3.62 L | 33.8 L | A starting estimate only: correction should proceed gradually under clinical guidance, typically no faster than about 10 mEq/L per 24 hours. |
| 75 | 4.82 L | 45.0 L | A starting estimate only: correction should proceed gradually under clinical guidance, typically no faster than about 10 mEq/L per 24 hours. |
| 113 | 7.26 L | 67.8 L | A starting estimate only: correction should proceed gradually under clinical guidance, typically no faster than about 10 mEq/L per 24 hours. |
| 150 | 9.64 L | 90.0 L | A starting estimate only: correction should proceed gradually under clinical guidance, typically no faster than about 10 mEq/L per 24 hours. |
| 225 | 14.46 L | 135.0 L | A starting estimate only: correction should proceed gradually under clinical guidance, typically no faster than about 10 mEq/L per 24 hours. |