Sweat rate calculator
Weigh before, weigh after, add what you drank. Indoors most riders lose roughly twice what they do on the road, and almost nobody realises how much that is until they measure it.
Your sweat rate
Weigh yourself before and after the same session, with similar clothing, having been to the loo first. Add whatever you drank during it.
Subtract your weight after from your weight before, add the fluid you drank, and divide by the hours. A kilogram lost in one session is a litre of fluid. Indoor rates commonly run 0.8 to 2 litres an hour, often double the same rider’s outdoor figure.
## Why indoor riders need this and outdoor riders mostly do not
On the road you are moving through air at twenty or thirty kilometres an hour, and that air carries your heat away almost as fast as you make it. On a turbo you are stationary inside a closed room, producing several hundred watts of waste heat with nowhere for it to go.
The result is that sweat rates indoors are routinely double what the same rider produces outdoors at the same effort. People who have only ridden outside are genuinely shocked the first time they see a towel after an hour on the trainer.
Which means the drinking habits you built on the road will leave you short indoors, and the only way to know by how much is to measure it once.
## What the number is actually measuring
Weigh yourself, ride, weigh yourself again. The difference is fluid you have lost as sweat and in your breath. Add back whatever you drank during the session, because that fluid went in and then came out again, and divide by the time.
A kilogram of weight change across a single session is a litre of fluid. That is all it is. Nothing else in the body moves that fast, and it comes straight back as soon as you drink.
Typical rates indoors run from around 0.8 litres an hour for a smaller rider in a cool room to over 2 litres for a large one working hard in a warm one. There is no good or bad figure here — it is a property of your body, your size and your room, and knowing it is the entire point.
## What to do with it
Replacing all of it while you ride is usually unrealistic. Most people can absorb somewhere around 0.8 to 1.2 litres an hour, and drinking much beyond that tends to sit in the stomach and slosh. If your sweat rate is two litres an hour, you will finish a long session down on fluid and make it up afterwards. That is normal.
Aim to drink most of it, and spread it out. Small amounts regularly beat one large bottle at the end. The table shows roughly what your own rate implies for sessions of different lengths.
For anything over about an hour, or if you sweat heavily, put something in the bottle. You lose salt as well as water, and replacing a lot of fluid with plain water alone can leave you feeling worse rather than better. An electrolyte tab is the simple version.
Measure it again in summer. Your sweat rate is not a constant. A warm room changes it substantially, so the figure you take in January will understate what you need in July.
## The fan is the real answer
A sweat rate calculator can tell you how much you are losing. A fan reduces how much you lose in the first place, and it does more for indoor training than almost any other piece of equipment.
Moving air does two things: it carries heat away by convection, and it lets your sweat actually evaporate, which is where the cooling comes from. Sweat that drips off you has done nothing useful at all. A decent fan aimed at your chest and face will typically cut your sweat rate noticeably and let you hold more power for longer, for the price of the cheapest piece of kit in the room.
## When to take it seriously
If you finish sessions with a headache, feel unusually flat the next day, or find your heart rate is consistently higher than normal for the same power, fluid is one of the first things worth checking.
This is general information about replacing what you sweat out, not medical advice. If you have a condition that affects fluid or salt balance, or you are on medication that does, talk to a doctor about what is right for you rather than following a calculator.
Questions people ask
How much should I drink on a turbo trainer?
Measure your own rate once and work from that. Indoors it is commonly between 0.8 and 2 litres an hour, which is often double what the same rider needs outdoors, because there is no airflow to carry the heat away.
Why do I sweat so much more indoors?
Because you are not moving through air. At 200 watts on the pedals your body is shedding roughly 700 watts of heat, and outdoors the wind removes it as fast as you make it. Indoors it stays in the room and in you, so you sweat harder to shed it.
Is the weight I lose during a ride fat?
No. Over a single session, weight change is fluid — sweat and breath — and it returns as soon as you drink. That is precisely why this calculation works as a measure of sweat rate.
Do I need electrolytes or is water fine?
For sessions under an hour at a modest sweat rate, water is usually fine. Beyond that, or if you sweat heavily, replacing a lot of fluid with plain water alone can leave you feeling worse, because you lose salt as well. An electrolyte tab is the simple fix.
Will a fan really make a difference?
Yes, and more than most equipment costing ten times as much. Moving air lets sweat evaporate, which is where the cooling actually happens, and sweat that drips off you has done nothing useful. Expect to hold more power for longer.
Still bored of the turbo?
That is the problem we are actually trying to solve. Turbo Trainer Games turns your watts into the controls of a browser game — your power flies the aircraft, climbs the mountain, pulls the sledge. Nothing to install, and it works with a smart trainer you already own.
It is in closed testing at the moment, so there is nothing to play yet. But you can put your name down.
Ask for early access