Sweat Rate Calculator
Liters per hour lost from weigh ins and fluids drunk.
What it does
Sweat rate hides in two weigh ins: the scale drop plus fluids drunk equals fluid lost, divided by hours. Sports dietitians run this test across heat, pace, and kit to build personal drink plans.
The tool above takes pre and post weights with fluids in either system. It returns sweat rate, total loss, and an 80% hourly drink plan.
Protocol decides accuracy more than arithmetic. Same scale, same nude towel dry state, no food or bathroom breaks between, and honest fluid logs keep errors near a cup.
Rates span widely: half a liter per hour for small cool runners past two and a half for big hot ones. Personal testing beats population averages for race day.
How to use
- Switch the unit system first: metric for kg, imperial for lb.
- Weigh nude and towel dry before.
- Log every fluid ounce or liter during.
- Weigh identically after and read the rate.
Formula
- pre - post
- Body mass lost, mostly water.
- + fluids
- Drinks already replaced mid session.
- x 0.8
- Replacement share avoiding overdrinking.
Glycogen and fuel mass confuse tiny sessions under an hour. Test across ninety plus minutes for clean numbers.
Worked example
Example: 70 to 69.5 kg, 0.5 L drunk, 1 hour
Pre 70 kg, post 69.5, half liter drunk, one hour run.
- Loss: 70 - 69.5 + 0.5 = 1.00 L.
- Rate: 1.00 / 1 = 1.00 L/hr.
- Drink plan: 1.00 x 0.8 = 0.80 L/hr.
- A two hour twin session loses about 2 liters.
Reference table
| Runner | Conditions | Rate |
|---|---|---|
| Small, cool | Easy pace | 0.4-0.6 L/hr |
| Average, mild | Steady pace | 0.8-1.2 L/hr |
| Large, hot | Hard pace | 1.5-2.0 L/hr |
| Extreme heat | Racing | 2.0-2.5 L/hr |
Limits
- Single session snapshot. Heat and fitness shift it.
- Food and bathroom breaks corrupt short tests.
- Salt loss needs separate sodium planning.
- Illness and dehydration skew test days.
Common mistakes
- Weighing clothed or on different scales.
- Forgetting fluids drunk mid session.
- Testing twenty minutes and scaling up.
- Drinking the full loss rate and sloshing.
- Ignoring salt on long hot days.
Try it yourself
Quick checks. Answers sit tucked under each question.
71 to 70 kg, 1 L drunk, 2 hours?
Loss 1 + 1 = 2 L. Rate 1.00 L/hr.
Drink plan for that rate?
1.00 x 0.8 = 0.80 L/hr.
Questions
How much should I drink running?
Near 80% of sweat rate, tuned by thirst. For 1 L/hr sweat, about 0.8 L/hr.
Can I drink too much?
Yes. Overdrinking dilutes sodium dangerously. Drink to thirst plus plan, not max.
Does heat change sweat rate?
A lot. The same runner doubles output from cool to hot humid days.
What about electrolytes?
Long salty sessions need sodium with water. White jersey streaks signal heavy salt loss.
Why weigh nude?
Sweaty kit weighs hundreds of grams, corrupting small losses.
How often should I retest?
Each season and heat wave. Fitness and acclimation move the number.