Sweat rate and hydration calculator
Weighing yourself before and after a session is the only accurate way to know your sweat rate, and it varies enormously between people.
minutes
Sweat rate
1433mL per hour
- Total sweat lost
- 2150
- Body mass lost
- 1.87 % Mild dehydration
- Fluid to drink per hour
- 1433
- Fluid to replace afterwards About 1.5 times the deficit, because some is lost as urine.
- 2100
- Approximate sodium lost At about 1 g of sodium per litre of sweat; salty sweaters lose twice that.
- 2150 mg
Show the working
- Weight before = 75
- Weight after = 73.6
- Fluid drunk during = 750
- Duration = 90
- Temperature = 22
- weight_lost = (75 - 73.6) * 1000 = 1400
- Total sweat lost = 1400 + 750 = 2150
- Sweat rate = 2150 / 90 * 60 = 1433.33
- Body mass lost = 75 > 0 ? (75 - 73.6)/75*100 : 0 = 1.866 67
- Fluid to drink per hour = 1433.33 = 1433.33
- Approximate sodium lost = 2150 / 1000 * 1000 = 2150
- Fluid to replace afterwards = 1400 * 1.5 = 2100
Values are shown to six significant figures here; the arithmetic itself runs on exact fractions and is only rounded where a number is printed.
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The formula
- sweat loss = weight lost + fluid drunk
- sweat rate = sweat loss / duration
Sweat rates range from under 0.5 to over 2.5 litres an hour. Generic hydration advice is nearly useless without measuring your own.
Losing more than about 2% of body mass measurably impairs endurance performance, more so in heat.