Free Water Deficit Calculator
Estimate free water deficit using total body water and sodium goals. Add optional ongoing losses and choose a correction window. Review warnings and monitor labs. This tool supports quick CSV and PDF summaries.
Developed by: Nohman Habib
Example Data
| Sex | Age group | Weight | Na⁺ current | Na⁺ target | Window | Ongoing losses |
|---|---|---|---|---|---|---|
| Male | Adult | 70 kg | 160 mEq/L | 145 mEq/L | 24 h | 800 mL/day |
In this example, the calculator estimates TBW, free water deficit, adds the 24-hour portion of ongoing losses, and shows a practical hourly replacement rate.
Formula
- Total Body Water (TBW): TBW = coefficient × weight(kg)
- Free Water Deficit (FWD): FWD = TBW × ((Nacurrent / Natarget) − 1)
- Total planned water: Total = FWD + ongoing_losses × (hours / 24)
- Rate: mL/hr = (Total(L) × 1000) / hours
How to Use This Calculator
- Select sex and age group, then enter weight.
- Enter current sodium and your target sodium.
- Choose a correction window (24–72 hours).
- If applicable, enter estimated ongoing water losses (mL/day).
- Click Calculate to view deficit and replacement rate above.
- Use Download CSV or Download PDF to export the summary.
FAQs
1) What is free water deficit?
Free water deficit estimates how much water is missing relative to body sodium and total body water, commonly used when managing hypernatremia. It provides a starting estimate, not a complete treatment plan.
2) Which target sodium should I use?
Many clinicians choose a gradual target such as 145 mEq/L or a stepwise reduction rather than normalizing immediately. The target depends on chronicity, symptoms, and clinical context.
3) Why does the calculator ask for a correction window?
The correction window helps translate the estimated deficit into an hourly or daily replacement rate. Slower correction is typically preferred when hypernatremia is chronic or duration is uncertain.
4) What are “ongoing losses” and why add them?
Ongoing losses are continuing water losses from fever, diarrhea, polyuria, or other causes. Adding them helps avoid under-replacement when the underlying water loss continues during correction.
5) Can I override the TBW coefficient?
Yes. The coefficient varies by sex, age, and body composition. If you prefer a specific coefficient based on your protocol or patient context, enter it to override the automatic selection.
6) Does this calculator choose the best fluid?
No. It estimates the free water amount. The appropriate route and fluid (oral/enteral water, IV D5W, etc.) and monitoring plan should be determined clinically with repeat sodium checks.