Calculator

Free Water Deficit Calculator (Hypernatraemia) for Dogs & Cats

Updated 25 Oct 20261 source

Free water deficit in litres from the current and normal sodium, and the minimum time to correct at the rate you choose.

Results are calculated in your browser. Always double-check before administering and use your clinical judgement.

How it is calculated

Free water deficit (L) = ((current Na ÷ normal Na) − 1) × (0.6 × body weight in kg) ^[1].

The cited source advises lowering sodium no faster than 1.0 mmol/L/h ^[1].

Worked example

10 kg dog, Na 170 (normal 145): (170/145 − 1) × 6 = 1.03 L.

Frequently asked questions

How do you calculate free water deficit in a dog?

Free water deficit (L) = ((current Na ÷ normal Na) − 1) × (0.6 × body weight in kg) ^[1].

More electrolytes & acid–base calculators

All clinical calculators →

Embed this calculator on your website

Free for clinics, colleges and blogs — paste this code into your page.

<iframe src="https://wevet.in/calculators/freewater/embed/" title="Free water deficit — WeVet" width="100%" height="560" style="border:0;max-width:720px" loading="lazy"></iframe>

Have a case in front of you?Ask WeVet — the AI assistant built for veterinarians. Differentials, treatment plans with calculated doses, drug info from trusted sources and SOAP notes.

Ask WeVet about this

References

Hypernatraemia in a dogAnimal Emergency Service (Australia) — articles for referring vets

For veterinary professionals. Information supports — never replaces — clinical judgement; always check the current product label and local regulations.