Hot weather

Heat hydration calculator

Estimate extra water for hot weather, heat waves, outdoor work, and sweaty summer days.

Hot weather usually adds 250-500 ml of fluid per hour of heat exposure, depending on shade, sweat, and effort. Use this as an extra amount on top of your normal daily water target.

Hours in heatOutdoor or poorly cooled time

hours

Effort / sweatAdjusts the buffer

Already replacedOptional

Bottle sizeFor tracking

Extra fluid estimate

0.9litres

About 32 oz total, or 1.9 bottles over 3 hours.

Pace

325 ml

per hour

Left

0.9 L

1.9 bottles

Electrolytes

Optional

today

When to use electrolytes

Electrolytes are worth considering if you are sweating heavily for 2+ hours, working outdoors, or your clothes dry salty.

Safety note

Heat illness is not a hydration math problem. Confusion, fainting, chest pain, very high temperature, or stopped sweating need urgent care.

Start before the hottest part of the day.

Use shade, cooling, and breaks instead of only adding water.

Do not chug more than about 1 litre per hour.

Frequently asked

How much extra water do I need in hot weather?

Many adults need about 250-500 ml extra per hot hour, but sweat rate, shade, clothing, and effort matter more than the calendar temperature.

Do I need electrolytes in heat?

Sometimes. Plain water is fine for light heat exposure, but salty sweat, hard work, long outdoor time, or repeated hot days make sodium more useful.