Hydration Deficit Calculator
Estimate the fluid deficit in litres from body weight and an estimated percentage of dehydration, using the standard clinical formula.
How it works
Fluid deficit (L) = body weight(kg) × (percent dehydration ÷ 100) · 1 kg lost ≈ 1 L of water
The classic estimate of a fluid deficit multiplies body weight by the estimated percentage of dehydration, because roughly one kilogram of acute weight loss equals one litre of body water. Percentages are tied to clinical severity — around 3% is mild, 5% moderate and 9% or more severe in adults. This gives a starting figure for how much fluid has been lost, but the actual percentage is a clinical judgement, and how quickly and with what fluids to rehydrate depends on the cause and the person.
Worked example
A 70 kg adult judged to be about 5% dehydrated has a deficit of 70 × 0.05 = 3.5 kg of water — approximately 3.5 litres (3,500 mL) to be replaced, alongside ongoing losses, under clinical guidance.
Frequently asked questions
How is the percentage of dehydration estimated?
Clinically it is judged from signs such as thirst, skin turgor, dry mucous membranes, heart rate, blood pressure and — most accurately — a recent known weight compared with the current weight. It is an estimate, so the deficit figure inherits that uncertainty.
Why does 1 kg equal about 1 litre?
Water has a density close to 1 kg per litre, so acute body-weight loss over hours to days is mostly lost water. This makes a recent pre-illness weight the best basis for the calculation when it is available.
How fast should the deficit be replaced?
That is a clinical decision and varies with the cause, the person’s age and heart and kidney function, and whether replacement is oral or intravenous. Correcting too quickly can be harmful, so this figure is only the estimated volume — not a rehydration plan.
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Educational estimate only — not medical advice, and not a substitute for a clinician, pharmacist or product label. Dosing, fluid and kidney decisions must be confirmed by a qualified professional using the actual drug, patient and situation. Do not use this tool to self-treat.