Sick Day Fluid Intake Calculator

Being ill costs you fluid faster than usual: sweating through a fever, vomiting, diarrhoea, and simply breathing harder and faster. This works out a daily target that covers the baseline plus what the illness is taking.

Baseline daily need

Base intake (mL) = body weight (kg) × 32.5

The commonly quoted range is 30 to 35 mL per kg for an adult, and this page uses the midpoint. It is a rule of thumb, not a physiological constant, and it already includes the water you get from food.

What the illness adds

The fever figures come from the usual clinical estimate that water lost through skin and breath rises by roughly 10 to 15% for each degree above 37°C, which for an average adult is a few hundred millilitres a degree. They already include the sweating and the faster breathing a fever brings.
The diarrhoea figure is the middle of the 200 to 400 mL of oral rehydration solution the British National Formulary gives an adult after each loose stool. The vomiting one is rougher, because how much comes up varies so much.

Total = baseline + fever + vomiting + diarrhoea

An earlier version also added a flat 500 mL “illness allowance” to everyone, fever or not. It had no source, and with a fever it counted the same sweating twice, so it is gone.

Worked example

A 70 kg adult, high fever at 39.5°C, 3 vomiting episodes, 4 episodes of diarrhoea:

• Baseline: 70 × 32.5 = 2,275 mL
• High fever: +1,000 mL
• Vomiting: 3 × 200 = +600 mL
• Diarrhoea: 4 × 300 = +1,200 mL
• Total: 5,075 mL, about 5.1 litres

That is a large number, and it is meant to be. Someone losing that much to a GI (gastrointestinal) illness is usually not able to drink it, which is the real reason people end up on a drip.

What to drink

Oral rehydration solution is the one that actually matters for a GI illness, because plain water replaces the volume but not the sodium and potassium going out with it. Sachets are cheap and every pharmacy has them. Clear broth is a decent stand-in for the salt. Diluted sports drinks work, undiluted ones are mostly sugar. Plain water is fine alongside the others but should not be the only thing you drink through a day of vomiting.

When to stop calculating and get help

No urination for 8 hours or more, urine that has gone dark, sunken eyes, severe dizziness on standing, confusion, or being unable to keep any fluid down for 12 hours. In an older adult, act on those sooner, and for a child see the signs below. Dehydration moves faster at both ends of life, and a target on a screen is no substitute for someone examining the patient.

This is a guide for an otherwise healthy adult. Anyone on fluid restriction for heart or kidney disease should not use it: for them, drinking to these numbers is the dangerous option, and their own limit comes from their doctor.

Children are worked out differently. A flat 500 mL for a fever is a third of a small child’s whole day, so below 40 kg the page switches to the Holliday-Segar rule doctors use for a child’s daily maintenance fluid: 100 mL per kg for the first 10 kg, 50 mL per kg for the next 10, and 20 mL per kg above that. A 12 kg toddler needs about 1,100 mL a day. On top of that, the WHO (World Health Organization) gives 50 to 100 mL of oral rehydration solution after each loose stool under the age of 2, and 100 to 200 mL from 2 to 10.
The Mayo Clinic’s signs that a baby or young child needs to be seen: no wet nappy for three hours, a dry mouth, crying without tears, sunken eyes or a sunken soft spot on the head, and unusual sleepiness or irritability.