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 work out to roughly 10 to 15% above baseline per degree above 37°C, which is the usual clinical estimate. The last row is applied whenever you are ill at all, fever or not, because a raised respiratory rate loses water even without sweating.

Total = baseline + fever + vomiting + diarrhoea + the standing illness allowance

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
• Illness allowance: +500 mL
• Vomiting: 3 × 200 = +600 mL
• Diarrhoea: 4 × 300 = +1,200 mL
• Total: 5,575 mL, about 5.6 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 a small child or an older adult, halve those thresholds. 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.