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Free Water Deficit Calculator

Hypernatremia Rehydration

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1 What is the Free Water Deficit Calculator?

The Free Water Deficit Calculator estimates the volume of free water needed to correct hypernatraemia in a child, based on body weight and measured sodium. It applies FWD = 0.6 × weight × ([Na]/140 − 1), then spreads that volume over 48 hours to achieve a gradual, safe correction. The tool pairs the deficit calculation with explicit safety limits on how fast sodium should be lowered.

2 When to use it

  • Calculating the free water deficit in a child with hypernatraemia (the tool requires sodium above 140 mEq/L as input)
  • Planning a 48-hour rehydration schedule using D5W or D5 quarter-normal saline
  • Guiding safe, gradual sodium correction to avoid dropping sodium too quickly
  • Managing hypernatraemia from suspected diabetes insipidus (central or nephrogenic), alongside desmopressin or salt-restriction/thiazide therapy
  • Determining how much additional fluid is needed on top of standard maintenance fluids during hypernatraemia correction

3 Formula & method

FWD = 0.6 × wt × ([Na]/140 − 1), with the app instructing to replace the deficit 'over 48 hours' using 'D5W or D5 ¼NS.' The app states a goal of '↓ Na 10–15 mEq/L per 24 hr' and notes that '4 mL free water per kg ≈ ↓ Na by 1 mEq/L.'

4 Frequently asked questions

What formula calculates the free water deficit?

FWD = 0.6 × weight (kg) × ([measured Na]/140 − 1), per the app's result card.

How quickly should the free water deficit be replaced?

The app recommends replacing it over 48 hours, with a goal of lowering sodium by 10–15 mEq/L per 24 hours.

How fast is too fast when correcting hypernatraemia?

The app's safety warning states sodium should never be dropped faster than 0.5 mEq/L/hr (15 mEq/L/day) because of the risk of cerebral oedema.

What fluid is used for free water replacement?

The app suggests D5W or D5 quarter-normal saline (D5 ¼NS), given in addition to the patient's usual maintenance fluids.

What if the hypernatraemia is due to diabetes insipidus?

The app notes that central diabetes insipidus should have desmopressin added, while nephrogenic DI is managed with salt restriction and a thiazide diuretic.

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ESPGHAN 2022
For qualified clinicians. This page and the PediAid app are a clinical aid only. Calculations and reference data must be verified against the patient's clinical context, the source guideline and your local protocols before any treatment decision is made.