Free Water Deficit Calculator
What is Free Water Deficit Calculator
The free water deficit calculator estimates how much electrolyte-free water a patient with hypernatraemia needs to bring a raised sodium down to target. It multiplies total body water, taken as a fraction of body weight for the patient group, by the proportional gap between the current and target sodium. The result is a volume in litres that guides replacement, on top of maintenance and ongoing losses.
How to use
- Enter the body weight in kilograms.
- Enter the current sodium and the target sodium in mmol/L.
- Choose the patient group, which sets the body water fraction.
- Read the estimated free water deficit in litres.
When to use it
A 70 kg man comes in with a sodium of 160 and you want a rough replacement volume to reach 140? Enter those values with the adult male group and the deficit comes out at 6.0 litres. It gives you a starting point for the water replacement, which you then spread out slowly and combine with maintenance fluids and whatever the patient is still losing.
Frequently asked questions
What formula is used?
Free water deficit equals total body water times (current sodium divided by target sodium, minus one). Total body water is body weight times 0.6 for adult men, 0.5 for adult women and older men, and 0.45 for older women.
How fast should I correct hypernatraemia?
Slowly. A common ceiling is a fall of 10 to 12 mmol/L per day to avoid cerebral oedema. This tool only sizes the deficit, it does not set the rate, so plan the replacement over an appropriate number of hours.
Does this include ongoing losses?
No. The deficit is the water already missing. You still need to add maintenance requirements and replace continuing urinary, gastrointestinal and insensible losses on top of it.
Can I share this tool with my inputs pre-filled?
Yes. The URL updates automatically as you type. Copy it from the address bar or use the Copy link button, and anyone who opens the link will see your exact inputs ready to go.
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