Water Intake Calculator

How It Works

This calculator builds a daily fluid target from body weight using a common sports-nutrition baseline, then adds fixed allowances for hot climates and for pregnancy or breastfeeding. Everything updates live in your browser as you type.

The 30–45 mL per kg baseline

Water needs scale roughly with body mass, so the baseline is expressed per kilogram: about 30 mL/kg on mostly seated days, 35 mL/kg with moderate activity, 40 mL/kg with hard training and 45 mL/kg for very intense or all-day physical work. A 70 kg person at moderate activity gets 70 × 35 mL = 2,450 mL, roughly 2.5 liters before adjustments. The 8-cup rule you may have heard is a simple memo, not a measurement — the per-kilogram baseline adapts to body size instead.

Why a target, not an exact number

Individual needs genuinely vary — body size, muscle mass, sweat rate, humidity, diet and acclimatization all shift the real number by half a liter or more. That is why this tool gives you a reasonable starting target rather than a precise prescription, and why the honest advice is: aim for the target, then let pale-yellow urine and steady energy tell you whether to nudge up or down.

Thirst, sweat and steady sipping

Thirst is a real signal but a slightly late one — by the time you feel it you are already mildly dehydrated, and even mild dehydration is known to affect concentration, mood and physical performance. That is the case for sipping across the day (the pace line spreads your target over about 16 waking hours) rather than gulping it all at once. Keeping steadily hydrated also simply gives your kidneys the fluid they use to clear everyday waste.

Frequently Asked Questions