Free Water Deficit (L) = TBW × [(Current Na ÷ Target Na) − 1] TBW (L) = Weight (kg) × TBW factor
| Patient category | Standard factor | Depleted preset |
|---|---|---|
| Child | 0.60 | 0.50 |
| Adult male | 0.60 | 0.50 |
| Adult female | 0.50 | 0.40 |
| Older male | 0.50 | 0.40 |
| Older female | 0.45 | 0.35 |
These factors are broad estimates. Individual body composition changes total body water. Clinical assessment remains essential for every patient calculation.
- Select the patient category and sex.
- Enter body weight and choose its unit.
- Enter current and target serum sodium values.
- Choose a hydration preset or custom TBW factor.
- Add an allocation period and optional ongoing losses.
- Select the required decimal precision.
- Review warnings before using any displayed estimate.
- Use exports for documentation, not automatic prescribing.
| Patient | Weight | Current Na | Target Na | TBW factor |
|---|---|---|---|---|
| Adult male | 70 kg | 155 | 140 | 0.60 |
| Adult female | 60 kg | 160 | 140 | 0.50 |
| Older male | 75 kg | 150 | 140 | 0.50 |
| Older female | 55 kg | 154 | 140 | 0.45 |
Example values demonstrate input formatting. They are not patient-specific treatment recommendations. Always verify every value using current clinical information.
No calculations are stored yet. This session keeps ten recent results. Closing the session may remove stored history.
The equation estimates water relative to a sodium target. It cannot measure actual bedside water balance. Ongoing clinical reassessment remains necessary throughout patient management.
A positive value represents a calculated water deficit. A negative value suggests water excess relative to target. Neither result independently determines appropriate fluid therapy choices.
The schedule divides estimated water across selected time. It excludes complex electrolyte and renal responses. Treat the displayed rate as arithmetic documentation only.
1. What does free water deficit mean?
It estimates water needed for a chosen sodium target. The value depends on estimated total body water. It remains an approximation requiring clinical interpretation and monitoring.
2. Why does the calculator use a TBW factor?
The factor estimates water as a weight fraction. Age, sex, body composition, and hydration affect it. A custom override supports documented clinical judgement when appropriate.
3. Can weight be entered in pounds?
Yes, choose pounds beside the weight field. The calculator converts pounds into kilograms automatically. The converted value appears within the calculation steps clearly.
4. What happens when target sodium exceeds current sodium?
The formula may return zero or negative deficit. A warning asks you to verify the target. Negative results should never trigger automatic treatment decisions alone.
5. Are ongoing losses included automatically?
No, they default to zero for safety. Enter an independently estimated hourly or daily loss. The tool adds losses across the chosen period mathematically.
6. Is the hourly allocation a prescribed infusion rate?
No, it is simple arithmetic for documentation. It ignores fluid composition and changing patient physiology. A clinician must choose treatment and monitoring independently afterward.
7. Can this calculator be used for children?
A child option is provided for estimation. Paediatric physiology and management can differ significantly. Specialist review is required before applying any calculation clinically.
8. Why can the result differ from other calculators?
Different tools may select different TBW factors. Rounding and target sodium choices also matter. Compare assumptions before comparing final calculated deficit values directly.
9. What should be documented with the result?
Record inputs, factor choice, target, and calculation time. Note assumptions and any separately estimated ongoing losses. Document clinical review rather than relying on exports alone.