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Two riders finish the same hot four-hour ride. One has lost 1.2 kg and feels fine. The other has lost 4 kg, has salt crusted on their face, cramped on the last climb and will spend the evening with a headache. They rode side by side the whole way.
Sweat rate and sweat sodium concentration vary enormously between people — far more than most general hydration advice acknowledges. Which is why "drink 500 ml an hour" is useless guidance for a large share of riders, and why an hour of simple arithmetic tells you more about your own hydration than any article can.
The two numbers that matter
Sweat rate is how much fluid you lose per hour. It depends on you, the intensity, the temperature and the humidity, and it can range from well under half a litre an hour to well over two.
Sweat sodium concentration is how salty that sweat is. This is much more individual and much less influenced by conditions — it is largely a property of you. The range across people is wide enough that two riders drinking identically can end a long hot ride in completely different states.
The first you can measure at home this weekend. The second needs a lab test to measure properly, though there are useful clues.
Measuring your sweat rate: the towel-and-scales method
Weigh yourself naked, immediately before
Empty your bladder first. Digital scales, same scales every time.
Ride for exactly one hour
At an intensity and temperature that matches what you actually want to know about. A cool endurance ride tells you nothing about a hot race.
Record every drop you drink
Weigh the bottles before and after, or simply note how many known volumes you got through.
Towel dry and weigh again, naked
Do not urinate in between. Towel off properly — sweat left on skin is weight you are about to miscount.
Do the arithmetic
Weight lost in kilograms, times 1,000, gives grams — roughly millilitres of sweat. Add whatever you drank. That total is your sweat rate per hour for those conditions.
You start at 74.8 kg and finish at 73.6 kg, having drunk 750 ml. Weight lost: 1.2 kg, or about 1,200 ml. Add the 750 ml you drank and did not retain as weight, and your sweat rate was roughly 1.95 litres per hour in those conditions. Now do the same session on a hot day and watch that number move.
The single most useful thing this gives you is a sense of scale. A rider who discovers they sweat two litres an hour and habitually carries one 750 ml bottle for a three-hour summer ride has just found the explanation for every bad August afternoon they have had.
The indoor test is the easy one
Doing this outdoors introduces variables — wind, terrain, changing effort. On a trainer you can hold a fixed intensity in a known room temperature and get a genuinely repeatable number, then repeat it with the fan on and off to see how much difference airflow makes. The answer is usually startling, and it is the strongest argument for the big fan that pain cave setup keeps insisting on.
Moveee Indoor will hold a steady target for exactly an hour in a browser tab, which is all this protocol asks of a trainer — and the saved ride gives you the intensity to go with the number, so the measurement means something when you compare it later.
Are you a salty sweater?
The classic salty-sweater tell. Visible salt residue after a hot ride suggests you are losing sodium at the higher end.
Not proof of anything — cramp has several causes and the science is less settled than the sports-drink industry suggests — but a pattern that only appears in heat is worth noticing.
Taking on lots of plain water while losing a lot of salt can leave you worse off, not better. If more water is not helping, more water may not be the answer.
Riders who repeatedly struggle in hot events more than their fitness predicts often have a fluid and sodium problem rather than a heat-tolerance one.
None of these are diagnostic. A proper sweat test — available at many sports labs and through some commercial services — gives you an actual concentration, and for a rider doing long hot events it is one of the more genuinely actionable tests available. For everyone else, the signs above are enough to justify experimenting with more sodium on hot days and seeing whether things improve.
What to do with the numbers
- Aim to replace most, not all, of what you lose. Trying to drink litre-for-litre while riding hard tends to end in a sloshing stomach. Keeping losses modest is the realistic target, not zero.
- Put sodium in the bottle on hot and long days. Plain water for a ninety-minute spin is fine. Four hours in 30°C is a different problem, and the electrolyte side of it is covered in electrolytes, sodium and cramp.
- Drink to a schedule, not to thirst, on long hot rides. Thirst is a reasonable guide at moderate intensity and a lagging one when it is hot and hard.
- Carry enough, or plan where to refill. This is a route problem as much as a nutrition one — knowing where the taps and shops are on a 200 km day is part of the planning, as how to plan a 200 km ride covers.
- Do not overdo plain water. Drinking very large volumes of water while losing substantial sodium can dilute blood sodium dangerously. It is rare in cycling, but it is serious when it happens, and it is another reason salt belongs in the bottle on the biggest days.
The half-hour that is worth it
Most riders spend more time choosing a gel flavour than they have ever spent finding out how much they sweat. One hour on the trainer with a set of scales and a towel gives you a number that makes every hot ride for the rest of your life easier to plan — and unlike almost everything else in this sport, it is free.
Sources 6
Where this article summarises a study, the study itself is linked — not a write-up of it.
- 1 Baker LB Sweating Rate and Sweat Sodium Concentration in Athletes: A Review of Methodology and Intra/Interindividual Variability · Sports Medicine · 2017
- 2 Sawka MN, Burke LM, Eichner ER, et al. Exercise and Fluid Replacement (ACSM Position Stand) · Medicine & Science in Sports & Exercise · 2007
- 3 Hew-Butler T, Rosner MH, Fowkes-Godek S, et al. Statement of the Third International Exercise-Associated Hyponatremia Consensus Development Conference · Clinical Journal of Sport Medicine · 2015
- 4 Schwellnus MP Cause of Exercise Associated Muscle Cramps (EAMC) — altered neuromuscular control, dehydration or electrolyte depletion? · British Journal of Sports Medicine · 2009
- 5 Kenefick RW Drinking Strategies: Planned Drinking Versus Drinking to Thirst · Sports Medicine · 2018
- 6 Saunders AG, Dugas JP, Tucker R, Lambert MI, Noakes TD The effects of different air velocities on heat storage and body temperature in humans cycling in a hot, humid environment · Acta Physiologica Scandinavica · 2005
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