For healthcare professionals
Free Water Deficit Calculator
When the body loses more water than salt, the blood sodium rises (hypernatremia). The free water deficit estimates how much water is missing. This tool uses the formula printed in two open-access reviews of arginine vasopressin (AVP) deficiency. It is written for clinicians and is not a treatment plan or a diagnosis.
Weight and sodium
Use the usual weight before the illness and a current serum sodium. Nothing you enter leaves this page.
What you will see
The estimated free water deficit in litres, with the working and half of it as the first-24-hour amount.
Educational only, not medical advice. Not a diagnosis. Decisions belong to the treating team.
Source
The formula and the half-replacement advice as printed by Refardt J, Atila C and Christ-Crain M, Reviews in Endocrine and Metabolic Disorders 2024, and the correction rates as printed by Atila C, Refardt J and Christ-Crain M, Journal of Clinical Endocrinology and Metabolism 2026. Both are open-access (CC BY) reviews. The tests check the formula at exact values, the half amount, that a sodium of 140 mmol/L or less gives no deficit, and that a small deficit is never shown as zero.
Validated in
Both reviews give the formula for dehydration in people with arginine vasopressin (AVP) deficiency. They are reviews, not validation studies, and report no accuracy figures for the formula.
Limits
The formula uses a fixed total body water of 0.6 x weight for everyone; it has no adjustment for age, sex or body build in these sources. It estimates the deficit at one point in time. The correction rates come from the 2026 review and are general; follow your local protocol.
The formula
Total body water deficit (L) = 0.6 x premorbid weight (kg) x (1 - 140 / measured sodium in mmol/L) (Refardt 2024) (Atila 2026).
Worked example: a usual weight of 70 kg and a sodium of 160 mmol/L give 0.6 x 70 x (1 - 140 / 160) = 5.25 L. At a sodium of 140 mmol/L or less, the formula gives no deficit.
How fast to replace
Refardt 2024 recommends replacing about 50% of the calculated free water deficit over the first 24 hours, with hypotonic fluids by mouth or tube or, if needed, 5% dextrose intravenously (Refardt 2024). Atila 2026 states that in acute hypernatremia that developed over a few hours, correction of up to 1 mmol/L per hour is safe; in chronic or unknown-duration hypernatremia, correction should be no faster than 0.5 mmol/L per hour to prevent cerebral edema and seizures; and overall a fall of about 10 mmol/L per day, aiming for a sodium of 145 mmol/L, is recommended (Atila 2026).
Related tools
To correct sodium for high glucose first, see the corrected sodium calculator. For the blood's overall concentration, see the serum osmolality calculator. To estimate how much one litre of a chosen fluid changes the sodium, see the sodium infusate calculator (Adrogue-Madias). Other bedside scores are listed under For healthcare professionals.
Sources
- Refardt J, Atila C, Christ-Crain M. New insights on diagnosis and treatment of AVP deficiency. Rev Endocr Metab Disord. 2024;25(3):639. doi:10.1007/s11154-023-09862-w. PMID 38087160. Open access (CC BY). Management section (total body water deficit formula; replace about 50% over the first 24 hours). pmc.ncbi.nlm.nih.gov/articles/PMC11162367/
- Atila C, Refardt J, Christ-Crain M. Arginine vasopressin deficiency and oxytocin deficiency in the endocrine clinic. J Clin Endocrinol Metab. 2026;111(4):922-937. doi:10.1210/clinem/dgaf651. PMID 41363127. Open access (CC BY). Management section (water deficit formula; correction rates in acute and chronic hypernatremia). pmc.ncbi.nlm.nih.gov/articles/PMC13017940/
Written and maintained by the uncovermyrisk.com editorial team. We do not have a medical reviewer; every claim links to its primary source above. Last reviewed: 2026-11-02. Spotted an error? See About.