Free Water Deficit Calculator

Written by Great Medico

Updated September 24, 2026 •2 min read

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When sodium, bicarbonate, osmolality, or an infusion rate is changing, clean arithmetic matters. Free Water Deficit Calculator focuses on the exact variables shown in the tool and returns the estimate its engine supports.

What this calculator does

Free Water Deficit Calculator uses Age / sex category, Weight, Serum sodium, and Desired sodium to calculate free water deficit. Where supported, it also shows Estimated total body water so the primary result is easier to check. It follows the exact field flow and engine behavior in this plugin rather than describing a different calculator with the same name. Fluid and electrolyte formulas are sensitive to units, timing, ongoing losses, kidney function, and rapidly changing physiology. They are most useful as structured estimates alongside repeat measurements and bedside assessment.

How to use it

Enter or select Age / sex category, Weight, Serum sodium, and Desired sodium. Use the units offered by the calculator and keep paired laboratory or treatment values from the same clinical episode whenever the formula assumes that. If an entry is unknown, do not substitute a guess simply to obtain a number.

How the calculation works

The calculator first estimates total body water from body weight and the selected age/sex factor. Free water deficit is then TBW × (measured sodium ÷ desired sodium − 1).

Example

Using the sample values prefilled in this calculator, the primary result is 4.5 L (Free water deficit) and Estimated total body water is 42 L. This example demonstrates the plugin’s own calculation path; replace the sample inputs with verified values before interpreting a real result.

How to interpret the result

Read the output as free water deficit from the implemented formula. Compare it with measured values, volume status, ongoing inputs/losses, and serial trends; treatment decisions should not rely on the calculator in isolation.

Limitations and notes

Free-water deficit estimates the amount needed to move sodium toward a target under simplified assumptions; it does not specify a safe correction rate. Ongoing losses/gains, hyperglycemia, renal function, and acute versus chronic hypernatremia change treatment.