Free Water Deficit

What is this formula?

The Free Water Deficit formula estimates the volume of water required to reduce an elevated serum sodium concentration back to a normal value, commonly 140 mEq/L.

When to use it

Use this formula when assessing patients with hypernatremia, dehydration, excessive free-water loss, diabetes insipidus, or inadequate fluid intake.

Example

Data:

TBW = 42 L

Na = 160 mEq/L

Formula:

FWD = TBW × ((Na / 140) − 1)

Substitution:

FWD = 42 × ((160 / 140) − 1)

FWD = 42 × 0.1429

Result:

FWD = 6.0 L

Approximately 6 liters of free water are required to correct the water deficit.

Applications

- Hypernatremia management

- Critical care medicine

- Nephrology

- Dehydration assessment

- Fluid replacement planning

Note

This formula is a clinical approximation that assumes a target serum sodium concentration of 140 mEq/L. Total body water must be estimated separately using patient characteristics such as sex, age, and body composition. Alternative correction targets and calculation methods may be used in specific clinical settings.