Free Water Deficit calculator
Result
4.5L free water deficit
Correct over ≥48h — max 10 mmol/L per 24h (replace ~0.1 L/day)
Never correct Na by > 10 mmol/L per 24h to avoid cerebral oedema.
How to use it
- 1
Free water deficit = TBW × (Na/target Na − 1). Total body water = weight × sex-specific fraction.
- 2
Correct over ≥48h. Max Na fall 10 mmol/L/24h (≈0.5 mmol/L/h). Faster correction causes cerebral oedema.
- 3
Add ongoing losses. Insensible losses ~500 mL/day; fever +200 mL/day. Total daily fluid = deficit/48h + maintenance + ongoing losses.
When to use it
- Hypernatraemia (Na > 145 mmol/L)
- Most common cause: free water loss (dehydration, diabetes insipidus). Correct sodium level determines management intensity.
- Diabetes insipidus
- Massive free water loss — high-volume hypotonic urine. FWD may be > 5 L.
- DKA and HHS
- Water deficit (not Na deficit) drives osmolality — FWD guides hypotonic fluid use.
A reference for clinicians and students, not medical advice. A score supports a decision; it does not make one. Check every result against the patient in front of you and your local protocol.
At a glance
- Specialty
- Fluids and electrolytes
- Inputs
- 4
- Type
- Formula
