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Free Water Deficit Calculator

Work out free water deficit instantly with clear inputs, formula shown and shareable results.

About Free Water Deficit Calculator

Estimates the water needed to bring sodium back to 140 mmol/L, using total body water scaled from body weight.

Formula

Free water deficit

Deficit (L) = total body water x (measured Na / 140 - 1); TBW = weight x factor

Worked example

With the default values below, Free Water Deficit Calculator returns Free water deficit of 4.50. Change any input above and every figure here is recalculated in your browser.

Inputs used

Body weightaccepts 2–250 kg
70 kg
Current sodiumaccepts 135–200 mmol/L
155 mmol/L
Total body water factorone of Adult male 0.6, Adult female 0.5, Elderly female 0.45
Adult male 0.6

Results

Free water deficit
4.50 L
Current total body water
42.0 L
Even replacement over 24 hours
188 mL/h

Step by step

  1. 1Values used
  2. 2Free water deficit
  3. 3Free water deficit= 4.50 L
  4. 4Current total body water= 42.0 L
  5. 5Even replacement over 24 hours= 188 mL/h

Assumptions and limits

  • The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
  • Educational estimate only. Not a substitute for professional clinical judgement, and must not be used to make treatment or dosing decisions.
  • This is an estimate for general information and not professional medical advice. See the full disclaimer for the limits that apply to this kind of calculator.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Educational estimate only. Not a substitute for professional clinical judgement, and must not be used to make treatment or dosing decisions.

Frequently Asked Questions

Does this include ongoing losses?

No. Insensible and urinary losses must be added to the calculated deficit.

How quickly should hypernatraemia be corrected?

Usually no faster than 0.5 mmol/L per hour, or about 10 to 12 mmol/L in 24 hours.

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Free Water Deficit Calculator – Calculate Free water deficit