How fast should sodium fall?

Typically aim for cautious correction (often ≤10–12 mEq/L/day unless acute and symptomatic — follow guidelines).

Free Water Deficit

Nephrology Critical Care Emergency Medicine

Medical context

Estimate free water deficit in hypernatremia for replacement planning.

Nephrology and medicine teams use this when interpreting renal function, electrolytes, or fluid plans.

This page explains the Free Water Deficit formula, how to interpret the output, known limitations, and peer-reviewed references so you can verify results against primary sources and local protocols.

It is grouped under Nephrology because kidney function, electrolytes, and renal dosing helpers.

How to use this calculator

Enter the values requested by the Free Water Deficit calculator using the units shown beside each field.
Review the calculated result and read the interpretation section before acting on the output.
Check limitations and references if the result will change management, dosing, or disposition.
Use the related calculators section when you need supporting values (for example electrolytes, weight, or complementary risk scores).

Calculator

Sex

Weight (kg)Current Na (mEq/L)Desired Na (mEq/L)

Result

6 L

Free water deficit

TBW × (Na/desired Na − 1). Replace slowly; watch sodium change rate.

Educational use only. Confirm results with clinical judgment, primary sources, and local protocols. Not a substitute for professional medical advice. Full disclaimer.

Formula

Deficit = TBW × (Na/desired Na − 1); TBW ≈ 0.6×wt (men) or 0.5×wt (women).

Interpretation

Guides total free water need; correct sodium slowly to avoid cerebral edema.

Limitations

TBW fraction varies with age and habitus; ongoing losses must be replaced separately.

Frequently asked questions

How fast should sodium fall?: Typically aim for cautious correction (often ≤10–12 mEq/L/day unless acute and symptomatic — follow guidelines).
Is the Free Water Deficit result enough to make a treatment decision?: No. Calculators support medical decision-making but do not replace examination, shared decision-making, institutional protocols, or consultant input when stakes are high.
Does Calcmedical store the values I type in?: Inputs are processed in your browser for this session. We do not ask for patient names or medical record numbers. See our privacy policy for hosting and analytics details.

References

Standard hypernatremia free water deficit formula