Calculate free water deficit and maximum safe rehydration rate.

What it is and when to use it

The free water deficit is the volume of solute-free water needed to bring serum sodium down to a target value. This free water deficit calculator estimates total body water (TBW) as a fraction of body weight: 0.6 in adult men, 0.5 in adult women and in men aged 65 or over, and 0.45 in women of that age. It then applies: deficit (L) = TBW × (current Na / target Na − 1), with a default target of 140 mEq/L that can be set between 130 and 145. It cites the review by Adrogué and Madias (N Engl J Med, 2000), which defines hypernatraemia as a serum sodium above 145 mmol/L.

How to interpret it

The result is litres of free water, not of a particular intravenous fluid. An adult man weighing 70 kg with a sodium of 160 mEq/L and a target of 140 has a TBW of 42 L and a deficit of 6 L. The page also reminds users not to lower sodium by more than 8–10 mEq/L in 24 hours (about 0.5 mEq/L per hour) to avoid cerebral oedema, so the deficit is replaced gradually over time rather than all at once. A deficit above 4 L appears in the high band, a threshold set by the calculator rather than the article.

Limitations and when not to use it

This is only a starting estimate. It leaves out ongoing losses (urine, gastrointestinal losses, fever, insensible losses), which must be added to the replacement plan. The TBW fractions are population averages and fit poorly in obesity, oedema or malnutrition. The formula assumes that what was lost is pure water and ignores the composition of the infusate: Adrogué and Madias also propose estimating how much each litre of the chosen fluid will lower serum sodium, a calculation this page does not perform. None of this replaces serial measurement of serum sodium during correction.

Frequently asked questions

How do you calculate the free water deficit?

Multiply weight by 0.6 (adult man), 0.5 (adult woman, or man aged 65 or over) or 0.45 (woman aged 65 or over) to obtain total body water, then multiply that by (current sodium / target sodium − 1).

How fast should hypernatraemia be corrected?

The calculator reminds users not to lower serum sodium by more than 8–10 mEq/L in 24 hours, about 0.5 mEq/L per hour, because a rapid fall can cause cerebral oedema. That rate, not the size of the deficit, sets the pace of replacement, with frequent sodium checks.

What total body water fraction is used in older adults?

The calculator uses 0.5 of body weight in men aged 65 or over and 0.45 in women of that age, compared with 0.6 and 0.5 in younger adults. Because total body water falls with age, the young-adult fraction would overestimate the deficit.

Does the free water deficit include ongoing losses?

No. It only counts the water missing at the time of the blood test. Later losses (urine, stool, fever, insensible losses) must be added to the fluid plan; otherwise sodium will fall less than expected.