Free Water Deficit Calculator

Free water deficit is the water a hypernatraemic patient has already lost, calculated as total body water multiplied by the measured sodium over the target sodium, minus one. Enter the weight, the age and sex group that sets the total body water fraction, and the measured sodium, and the result gives the deficit in litres and millilitres alongside the working. It also applies a maximum fall in sodium over twenty-four hours, defaulting to 10 mmol/L, and reports the volume and hourly rate that limit allows on the first day. Ongoing urinary and insensible losses are not included and must be added separately. This is a starting estimate for a clinician, not a treatment plan or a fluid order.