Understanding This Calculator
The free water deficit equation estimates how much water an individual with hypernatremia (elevated serum sodium) has lost relative to their body’s normal fluid balance. It is a starting-point calculation used to help plan the pace and volume of rehydration, but actual fluid replacement decisions also weigh ongoing losses, correction-rate limits, and the underlying cause of the sodium imbalance.
Total body water (TBW) is estimated as a fraction of body weight because water content varies with age, sex, and body composition — younger males tend to have proportionally more lean tissue and therefore more total body water than elderly females of the same weight.
- TBW total body water = weight (kg) × factor
- Na serum sodium concentration, mEq/L
| Category | TBW Factor | Rationale |
|---|---|---|
| Adult Male | 0.6 | Higher lean mass, more total body water |
| Adult Female | 0.5 | Higher average body fat percentage |
| Elderly Male | 0.5 | Age-related decline in lean mass |
| Elderly Female | 0.45 | Lowest average total body water fraction |
Frequently Asked Questions
Does this number represent how fast fluid should be given?
No. The deficit is a total volume estimate only. Sodium correction rates are typically limited to avoid complications, so replacement is normally spread over many hours to days, not given all at once.
Why do the TBW factors differ by sex and age?
Muscle holds more water than fat tissue. Because average lean-to-fat ratio differs by sex and declines with age, the standard TBW fraction used in the formula is adjusted for each group.