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Hydration is the training variable riders most often get wrong in both directions. Some finish a four-hour summer ride having drunk one bottle and wonder why the last hour fell apart. Others drink so aggressively that they spend the ride sloshing and stopping. Both cost you.
The good news is that this is a solvable problem with a bit of arithmetic. Here's a practical strategy, plus the simple test that replaces all the guesswork.
- Start every ride properly hydrated — it's far easier than catching up mid-ride.
- Roughly 500–750 ml an hour in mild conditions; substantially more in heat or indoors.
- Add sodium once you're past about two hours, or any time it's hot.
- Do the sweat test once. Your own number beats every guideline on the internet.
How much to drink
Sweat rate varies enormously between individuals — a big rider working hard in the heat can lose several times what a small rider loses on a cool day. Treat this table as a starting point to be personalised, not a rule.
| Conditions | Fluid | Sodium | Notes |
|---|---|---|---|
| Cool (under 15 °C) | 400–600 ml / hr | Usually not needed | Easy to under-drink here because you don't feel thirsty. Keep sipping anyway. |
| Mild (15–22 °C) | 500–750 ml / hr | Useful over 2 hr | The typical spring/autumn ride. One bottle an hour is a reasonable default. |
| Hot (23–30 °C) | 750–1,000 ml / hr | Yes | Plan bottle stops before you leave. You will not carry enough for a long ride. |
| Very hot (30 °C+) | 1,000 ml+ / hr | Definitely | Absorption becomes the limit. Drink steadily, pre-hydrate, and slow down. |
| Indoors, no fan | 1,000 ml+ / hr | Definitely | Sweat rates indoors are far higher than most riders expect. Get a fan first. |
The sweat test — do it once
Twenty minutes of effort that replaces years of guessing. Repeat it once in summer and once in winter, because your rate changes with conditions.
Weigh yourself naked immediately before a ride.
Ride for exactly one hour at a typical intensity, in typical conditions.
Record everything you drink during that hour, in millilitres.
Towel off and weigh yourself naked again straight afterwards.
Sweat rate = (weight lost in kg × 1000) + millilitres drunk. That's your ml/hour.
Example: you lose 0.8 kg and drink 500 ml. Sweat rate = (0.8 × 1000) + 500 = 1,300 ml per hour. That's your real target in those conditions.
What about electrolytes?
Sodium is the one that genuinely matters for cyclists. It's the electrolyte lost in the largest quantity in sweat, it helps you retain the fluid you drink rather than passing it straight through, and it aids absorption in the gut. Potassium, magnesium and the rest get a lot of marketing attention and are mostly covered by eating normally.
Practical guidance: plain water is fine for rides under about 90 minutes in mild weather. Past that — or any time it's hot, or you're indoors without serious cooling — use a drink with sodium in it. If you're a heavy or notably salty sweater (white marks on your kit, stinging eyes), you'll want more than the standard dose.
Four hydration myths
"Thirst means you're already dehydrated — drink ahead of it."
Thirst is a decent guide for most rides. The old advice to drink aggressively ahead of thirst has been walked back; over-drinking carries its own risks.
"You must replace every gram you sweat out."
Losing a small percentage of body mass over a long, hot ride is normal and generally well tolerated. Chasing a perfect balance isn't necessary.
"Water is always enough."
For an hour, usually yes. Beyond two hours in the heat, sodium matters — for absorption as much as for replacement.
"More electrolytes are always better."
Very concentrated drinks slow gastric emptying and can leave you feeling sloshy. Match concentration to conditions.
Drinking large volumes of plain water over many hours can dilute blood sodium — a condition called hyponatraemia, which in serious cases is genuinely dangerous. It's rare, but it's most common in long events among slower, cautious participants who have been told to "drink plenty." If you're going long, include sodium and don't drink far beyond thirst. This is general information, not medical advice — if you have any concerns, talk to a doctor.
Indoors is its own category
Worth repeating because riders consistently underestimate it: without the airflow you get outdoors, indoor sweat rates are dramatically higher. Two bottles an hour is not unusual on a hard trainer session, and the sodium losses are correspondingly large. If you finish indoor rides with a headache or a puddle under the bike, this is why. Cooling comes first — see our pain cave setup guide — and then drink to match.
Build it into the ride, not the panic
The most common failure isn't ignorance, it's forgetting. Set a reminder, drink at fixed points (every 15 minutes, or at the top of each climb), and plan refill stops into long routes before you set off — our route-building rules cover designing around water and café stops.
For the indoor half of your training, Moveee Indoor at least removes the excuse of not knowing how long you'll be on: the session is written, the route is loaded, and you can see exactly how far there is to go — which makes it a lot easier to plan two bottles and actually finish them. It's free while in alpha and runs in the browser.
Do the sweat test once, carry the right number of bottles, add sodium when it's hot or long, and drink to a schedule rather than to panic. That's the whole strategy.
Sources 7
Where this article summarises a study, the study itself is linked — not a write-up of it.
- 1 Sawka MN, Burke LM, Eichner ER, Maughan RJ, Montain SJ, Stachenfeld NS American College of Sports Medicine Position Stand: Exercise and Fluid Replacement · Medicine & Science in Sports & Exercise · 2007
- 2 Hew-Butler T, Rosner MH, Fowkes-Godek S, et al Statement of the Third International Exercise-Associated Hyponatremia Consensus Development Conference, Carlsbad, California, 2015 · Clinical Journal of Sport Medicine · 2015
- 3 Baker LB, Barnes KA, Anderson ML, Passe DH, Stofan JR Normative data for regional sweat sodium concentration and whole-body sweating rate in athletes · Journal of Sports Sciences · 2016
- 4 Goulet EDB Effect of exercise-induced dehydration on time-trial exercise performance: a meta-analysis · British Journal of Sports Medicine · 2011
- 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
- 7 Vist GE, Maughan RJ The effect of osmolality and carbohydrate content on the rate of gastric emptying of liquids in man · The Journal of Physiology · 1995
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Moveee builds your week from your own rides and keeps it honest about duration and intensity — which is the number every fuelling decision on this page depends on. Free coaching, and it adapts when life gets in the way.
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