Sweat rate calculator for cyclists: stop guessing, weigh yourself twice
Road & indoor cycling · the protocol · honest bands · the trainer twist · your bottle plan
Hydration advice is drowning in averages — “drink 500 to 750 ml per hour” covers riders who lose half a litre and riders who lose two. Your sweat rate is personal, it's stable enough to plan with, and it's measurable with a bathroom scale and one honest hour on the bike. Two weigh-ins, one division — done. This page is the protocol, the calculator, and what to do with your number.
How do you calculate sweat rate?
Weigh yourself nude before and after exactly one hour of riding, don't use the bathroom during the test, and track what you drink. Sweat rate = (weight before − weight after + fluid drunk) per hour. Example: −0.6 kg plus 0.5 L drunk = 1.1 L/hour.
The protocol (do it once, properly)
- Pick a representative hour — race-like effort, race-like weather. A January base spin tells you nothing about a July gran fondo.
- Weigh nude, before — kit soaks up sweat and lies by hundreds of grams.
- Ride 60 minutes, drink normally, but MEASURE it (start with full bottles, weigh or eyeball what's gone).
- No bathroom stops during the test — that's part of the arithmetic.
- Towel off, weigh nude, after.
- The calculator does the math: every kilo lost is a litre sweated.
Repeat it twice a year — fitness, heat adaptation and season change the number. And repeat it INDOORS separately, because…
The indoor twist (the number nobody measures)
Indoor sweat rates are routinely the highest of the year: no 35 km/h headwind doing free evaporation, so your body pours out more and it evaporates less. Riders who measure 0.8 L/h outdoors regularly find 1.5+ in the pain cave. If you race on Zwift, your indoor number is the one that matters most — and it's also the easiest to measure: the scale is in the next room and there are no bathroom-stop excuses. (Fan strength changes the result; test with your race setup.)
Reading your number (bands, honestly)
Typical adult ranges on the bike: under ~0.8 L/h light · 0.8–1.4 moderate · 1.4–2.0 heavy · above 2.0 very heavy — genetics, heat and intensity move you within them. Two honest notes: you don't need to replace 100% of losses in rides under ~2 hours (modest deficit is normal and fine), and more is not safer — overdrinking plain water beyond losses is how hyponatremia happens. The goal is informed sipping, not maximal intake.
Sodium (role, not prescription)
Sweat carries sodium out with the water — that's why heavy sweaters cake white on black kit. Sodium keeps the fluid you drink actually inside the system and drives thirst sensibly; race drinks and our DIY mix carry 300–600 mg per litre as a working band. Actual per-litre loss varies person to person by a factor of five, so we don't prescribe milligrams: if you're a visibly salty, crampy, very heavy sweater, a proper sweat sodium test (lab or patch) is the measured route. Everyone else does fine salting bottles inside the band.
From number to bottle plan
Say the tool reads 1.2 L/h for a 3-hour race: that's ~3.6 L total, in practice five 750 ml bottles — two on the bike, refills planned at feed zones — and it slots straight into the Fuel Lab's hourly ration, where your carb concentration decides how much of that fluid is mix and how much is plain water (a concentrated race bottle wants water beside it).
FAQ
What is a normal sweat rate?
Most riders land between 0.8 and 1.4 L/h; heat, intensity and genetics stretch the range.
How much should I drink per hour cycling?
Whatever YOUR measured rate says, minus an acceptable deficit on shorter rides — not a universal number. That's the point of measuring.
How much sodium do I lose in sweat?
Between roughly 200 and 2,000 mg per litre depending on the person — which is exactly why this page gives a band and a measuring route instead of a dose.
Does sweat rate change?
Yes — heat adaptation, fitness and season all move it. Retest twice a year, and separately indoors.
Feed your number into your race plan
Your sweat rate belongs in the hourly ration: the Fuel Lab runs tank, intake and bottles in one place.
All fuel guides →Sources
- Standard fluid-balance protocol (GSSI/ACSM references)
Sources as named — title and year are enough to look them up.
Sport, not medicine: ranges, not prescriptions — especially for sodium. Measurement guidance for healthy athletes, no substitute for medical advice. Zwift is a trademark of Zwift, Inc.; this page is an independent project with no affiliation to Zwift. All facts from the sources listed — no AI-generated numbers.