Sweat Rate Sheet
Weigh in, weigh out, subtract. The result is a ceiling on what to drink, not a target.
BFCBrilliance · bfcbrilliance.com/tools/hydration-during-exercise-calculator
The measurement
- SWEAT RATE
- (weight lost + drunk − urine) ÷ hours
- Typical range
- 0.5 to 1.5 L/h — but individuals vary widely
- ACSM guide
- 0.4-0.8 L/h in intense endurance work
- Sodium, sessions over 1h
- 0.5-0.7 g per litre of fluid
- 1 kg of weight
- about 1 litre of fluid
- ⚠ SWEAT RATE IS A CEILING
- not a target — never drink beyond it
- ⚠ Finishing UP on weight
- means you drank more than you sweated
- ⚠ Hyponatremia
- almost always from drinking TOO MUCH
- ⚠ Confusion, headache, swelling
- urgent medical help — do NOT drink more
Sessions measured
| Date | Conditions | Duration | Weight lost | Drank | Sweat L/h |
|---|---|---|---|---|---|
This session
- Name
- Date
- Temperature and humidity
- Session type
- Weight BEFORE
- Weight AFTER
- Fluid drunk
- Urine passed
- Duration
- SWEAT RATE
- % body weight lost
- CEILING for next long session
- Sodium needed
Measuring it properly
- Weighed undressed and towelled dry, both times
- Every mouthful of fluid recorded
- Urine passed during the session noted
- Session between 60 and 120 minutes
- Conditions recorded — heat and humidity change everything
- Repeated in the conditions that actually matter to you
- Result treated as a CEILING, never as a quota
- Sodium planned for anything over an hour
The ceiling, not the target
Exercise-associated hyponatremia is almost always caused by drinking too much, and the standard advice for avoiding it is to ensure fluid intake does not exceed sweat loss. ACSM's 0.4-0.8 L/h for intense endurance work is often LESS than a measured sweat rate — finishing slightly down on fluid is normal and tolerated, while finishing up on fluid is the direction that causes harm.
It can look like dehydration
Confusion, headache, nausea or swelling of hands, feet or face during a long event needs urgent medical attention — and if the cause is hyponatremia, drinking more makes it substantially worse. Do not diagnose it from symptoms mid-event. Get help, and tell whoever assesses you how much you have been drinking.