How to Use the Dehydration Calculator
Enter the person's usual, well-hydrated weight and their weight right now, and the dehydration calculator shows the percentage of body weight lost, the estimated fluid deficit and a severity level. It works for adults and children, with rehydration guidance tailored to each.
- Who is this for: choose adult/teen or child. The severity bands are the same, but the rehydration advice differs.
- Weight unit: kilograms or pounds. Use the same scale for both weights if you can.
- Usual weight: a recent weight when the person was healthy and well hydrated, such as a morning weigh-in from the past week or a recent clinic visit.
- Current weight: today's weight, taken in similar clothing and at a similar time of day.
Body weight lost is the dehydration percentage. The badge shows the severity band, and fluid deficit converts the weight loss into liters, since 1 kg of acute weight loss is about 1 liter of water. Fluid to drink is the suggested rehydration volume. Always weigh the result against symptoms: thirst, dark urine, dizziness and fatigue are just as important.
How the Dehydration Calculator Works
The calculator uses acute body-weight change, the standard bedside and sports-medicine measure of dehydration. Sudden weight loss over hours or a few days is almost entirely water, so the drop on the scale approximates the fluid deficit.
Dehydration formula
Dehydration (%) = (Usual weight − Current weight) ÷ Usual weight × 100Fluid deficit (L) = Usual weight kg − Current weight kgAdult rehydration (L) = Fluid deficit × 1.25 to 1.5Child oral rehydration (ml) = 50 to 100 × Current weight kg, over 3–4 hours- Severity bands: under 3% minimal, 3–9% mild to moderate, over 9% severe. These follow the CDC's clinical classification for gastroenteritis-related dehydration. In active adults, performance drops from about 2%.
- 1.25–1.5× replacement: covers the urine you'll produce while rehydrating, per sports-medicine guidance.
- 50–100 ml/kg: the oral rehydration solution range recommended for children with mild-to-moderate dehydration.
Worked example: An adult who usually weighs 75 kg weighs 72 kg after two days of a stomach bug. Dehydration = (75 − 72) ÷ 75 × 100 = 4.0%, which falls in the mild-to-moderate band. Fluid deficit = 3 L, so the rehydration target is 3 × 1.25 to 3 × 1.5 = 3.75–4.5 L (shown as 3.8–4.5 L after rounding) over the next several hours, ideally including oral rehydration solution. For a 12 kg child in the same band, the target would be 600–1,200 ml of ORS over 3–4 hours.
The calculation assumes the weight change is recent. Gradual weight loss over weeks is mostly fat and muscle, not water, and will overstate dehydration. Results are rounded to one decimal place for percentages and the nearest 50 ml for volumes.
Dehydration Calculator FAQs
How do you calculate the percentage of dehydration?
Subtract your current weight from your usual, well-hydrated weight, divide by the usual weight and multiply by 100. Because short-term weight loss is almost all water, a 70 kg person who drops to 68.6 kg is about 2% dehydrated.
What percentage of dehydration is dangerous?
Losses above about 2% of body weight start to impair physical and mental performance, 3–9% is classed as mild-to-moderate dehydration, and more than 9% is severe and a medical emergency. Infants, older adults and people with heart or kidney conditions are at risk at lower levels.
How accurate is a weight-based dehydration calculator?
It is the most practical at-home method, but it is only as good as your "usual weight" figure. Normal day-to-day swings of 0.5–1% from food, bowel movements and salt intake mean small results are hard to read, and the calculation can't detect dehydration if you don't know your baseline weight.
How much fluid do I need to drink to rehydrate?
Drink about 1.25 to 1.5 liters for every kilogram of weight lost (20–24 fl oz per pound), spread over several hours rather than all at once. Fluids with sodium, such as oral rehydration solution, broth or a sports drink, are retained better than plain water.
How is dehydration treated in children?
For mild-to-moderate dehydration, pediatric guidelines recommend oral rehydration solution at 50–100 ml per kg of body weight over 3–4 hours, given in small, frequent sips. Babies and young children with signs of dehydration should be assessed by a doctor.
When should I seek medical help for dehydration?
Get urgent care for confusion, fainting, a rapid heartbeat, no urine for 8 hours or more, inability to keep fluids down, or a sunken soft spot, no tears or very few wet diapers in a baby. These signs matter more than any calculator result.