Water Intake Calculator

How it works

fl oz per day = weight (lb) × 0.5 + exercise minutes ÷ 30 × 8, 12 or 16 fl oz (light, moderate, hard) + 0.5 L (warm) or 1 L (hot) + 0.5 L at high altitude + 0.3 L (pregnant) or 1.1 L (breastfeeding)

A widely used practical rule: half your body weight in fluid ounces as a base (about 33 mL per kg), plus about 12 oz per half hour of moderate, sweaty activity — 8 oz for light activity and 16 oz for hard training. Warm or humid weather adds about half a liter and hot outdoor conditions about a liter; living or training above roughly 8,000 ft adds another half liter. Pregnancy and breastfeeding add 0.3 L and 1.1 L, the differences in the US National Academies adequate intakes (2.7 L of total water a day for women, 3.0 L in pregnancy and 3.8 L while breastfeeding; 3.7 L for men). The result is shown in fluid ounces, 8-oz cups, liters and bottles of the size you choose, with a schedule that spreads it evenly across your waking hours.

Worked example

At 170 lb with 45 minutes of moderate training: 85 oz base + 18 oz exercise ≈ 103 fl oz — about 13 cups, 3 liters or six 16.9-oz bottles. Over 16 waking hours that is about 6.4 oz an hour. On a hot day, add about 34 fl oz (1 L) for roughly 137 fl oz.

Sources: National Academies — Dietary Reference Intakes for water, potassium, sodium, chloride and sulfate (2005) · ACSM position stand (2007) — Exercise and fluid replacement · Mayo Clinic — Water: how much should you drink every day? · CDC — Water and healthier drinks

Frequently asked questions