Child Hydration Needs Calculator
Estimates your child's daily fluid intake in milliliters based on weight, age, activity level, climate, and health status. Use it when your child is active outdoors, living in a hot climate, or recovering from illness.
Last updated: September 2026
Formula below · 3 sources (medlineplus.gov, healthychildren.org, aap.org) · Updated Sep 2026
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About this calculator
This calculator estimates a child's daily fluid requirement starting from the Holliday-Segar maintenance volume, the standard pediatric reference: 100 mL/kg for the first 10 kg of body weight, 50 mL/kg for the next 10 kg, and 20 mL/kg for every kg above 20 kg. It then adds fixed allowances: 200 mL for moderate and 400 mL for very active children, 300 mL for hot/humid and 250 mL for hot/dry climates, and 200 mL for fever or 400 mL for vomiting/diarrhoea. Daily Fluid (mL) = Holliday-Segar(weight) + activity + climate + health adjustments. The total covers water from drinks and food. Babies under 6 months get all the fluid they need from breast milk or formula and should not be given extra water, so the calculator returns a message for them. For vomiting or diarrhoea, oral rehydration solution is better than plain water, and a dehydrated child may need more than this estimate.
How to use
Suppose your child weighs 20 kg, is 8 years old, is active, lives in a hot climate, and has no illness. Step 1 – Holliday-Segar base: 1,000 + (20 − 10) × 50 = 1,500 mL. Step 2 – Activity (very active): +400 mL. Step 3 – Climate (hot/humid): +300 mL. Step 4 – Health (healthy): +0 mL. Total = 1,500 + 400 + 300 = 2,200 mL per day, roughly 9 cups of fluid including water from food. Spread it across meals, snacks, and play breaks throughout the day.
Frequently asked questions
How much water should a child drink per day based on weight?
The standard pediatric starting point is the Holliday-Segar rule: 100 mL/kg for the first 10 kg, 50 mL/kg for the next 10 kg and 20 mL/kg above that, so a 20 kg child needs about 1,500 mL from that base alone. Age, activity level, climate, and health status can add several hundred milliliters on top of that baseline. This calculator combines all those factors into one personalized daily target. Always remember that fluids come from both drinks and water-rich foods like fruits and soups.
Why does a fever or vomiting increase a child's hydration needs?
Fever raises the body's metabolic rate and causes extra fluid loss through sweating and faster breathing, which is why this calculator adds 200 mL for a febrile child. Vomiting is even more dehydrating because the child is actively losing fluid and electrolytes, hence the larger 400 mL addition. Replacing these losses promptly helps prevent dehydration, which can progress quickly in young children. Oral rehydration solutions are often recommended during illness rather than plain water alone.
When should I be concerned that my child is not drinking enough water?
Warning signs of dehydration in children include a dry mouth, fewer than four wet diapers or bathroom trips per day, dark yellow urine, lethargy, and sunken eyes. Infants and toddlers are especially vulnerable because they cannot communicate thirst effectively. If your child shows these signs despite encouragement to drink, contact your pediatrician, particularly during hot weather or illness. This calculator can help you set a daily goal, but persistent refusal to drink always warrants professional advice.