Reviewed for accuracy: This article draws on the sweat and sodium research popularised by Dr. Andy Blow and Precision Fuel & Hydration, the hydration work of Dr. Allen Lim, and established exercise-physiology data on sweat composition. The figures are performance guidance, not medical advice. Riders with heart, kidney or blood-pressure conditions should take sodium guidance from their doctor.
Here's something that should bother you about every hydration article you've ever read: they all give you one number. Drink this much, add this much sodium, done. But the single biggest variable in the whole equation — how much sodium you lose — varies more than fourfold from one rider to the next. Two people on the same ride, in the same heat, can lose 300mg and 1,300mg of sodium per litre of sweat. One generic dose can't be right for both.
So the useful version of this isn't a number. It's a method: work out your own sweat rate, get a feel for how salty your sweat is, and build a bottle that matches your physiology instead of an average you don't fit.
The short answer
Cyclists lose somewhere between roughly 200mg and 2,000mg of sodium per litre of sweat — a huge individual range, which is why generic advice fails. Find your sweat rate by weighing yourself naked before and after a one-hour ride with no drinking or toilet breaks: each kilogram lost is about a litre of sweat. For rides under 60-90 minutes in cool weather, plain water is fine. Beyond that, or in heat, target roughly 500-800mg of sodium per litre of fluid, and push higher if you're a salty sweater — white marks on your kit, gritty skin, salt stinging your eyes. Sodium matters because it pulls fluid out of your gut and into your blood and helps hold blood volume. And the real over-hydration danger, hyponatremia, comes from drinking too much plain water, not too little.
How to measure your sweat rate
This takes one ride and a set of bathroom scales, and it's the single most useful thing in this article.
- Weigh yourself naked right before the ride, after going to the toilet.
- Ride for exactly one hour at a typical effort. Ideally drink nothing — if you must drink, note exactly how much.
- Towel off and weigh yourself naked again.
Each kilogram lost equals roughly one litre of sweat. If you drank 500ml during the hour, add that 0.5kg back onto your loss — you sweated it out too. So a rider who's 1kg lighter and drank 500ml actually lost 1.5 litres of sweat in that hour.
Do this a few times across different conditions, because sweat rate isn't fixed. It climbs steeply with temperature and intensity. The same rider might lose 0.6 litres an hour on a cool Zone 2 spin and 1.8 litres on a hot threshold day. Most cyclists land between 0.5 and 1.5 litres per hour; heavy sweaters in real heat go past 2.
Once you know your sweat rate, you know your fluid target — you're aiming to replace most of what you lose, not all of it (a little deficit is fine and often unavoidable on hard efforts). The in-ride fuelling calculator takes ride duration, body weight and intensity and gives you fluid and sodium targets alongside your carbohydrate, which saves you doing the maths every time.
Why sodium, specifically
Of all the electrolytes in sweat — sodium, potassium, magnesium, chloride — sodium is the one that matters most for cyclists, for two reasons.
It's lost in the biggest quantities. Sweat sodium dwarfs the others. Potassium and magnesium losses are real but small, and a normal diet replaces them easily. Sodium is the one you can genuinely run short of on a long, hot ride.
It drives fluid absorption. This is the bit most riders miss. Sodium and glucose are co-transported across the gut wall, dragging water with them. A drink with some sodium and carbohydrate is absorbed into your blood faster than plain water. So sodium isn't just replacing what you lose — it's helping you actually use the fluid you're drinking instead of having it slosh around your stomach. Dr. Andy Blow, the physiologist behind Precision Fuel & Hydration, has built much of his sweat-testing work on exactly this point: matching sodium intake to individual sweat sodium concentration is what makes hydration actually work, rather than chasing a volume target alone.
Dr. Allen Lim — the physiologist who founded Skratch Labs after years mixing bottles for World Tour teams — came at the same problem from the practical end. His whole philosophy was that the drinks teams were using were too sweet, too concentrated and didn't match what riders actually lost, so he started building lighter, real-ingredient mixes with sodium dosed to physiology. Same conclusion from a different door: the bottle should match the rider.
Are you a salty sweater?
You don't always need a lab to know roughly where you sit. The signs of high sweat sodium are obvious once you look:
- White tide-marks crusting on your kit, cap or helmet straps after a ride.
- Gritty, crystalline skin — run a finger across your forehead and feel salt.
- Salt stinging your eyes or tasting strongly on your lips mid-ride.
- A history of cramping late in long or hot rides (sodium isn't the whole cramping story, but it's part of it for many riders).
If that's you, the standard "500mg per litre" advice is probably too low. Salty sweaters can lose 1,500mg per litre or more and need to dose accordingly — this is exactly the rider for whom a proper sweat test from a company like Precision Fuel & Hydration pays off, because it puts a real number on your concentration instead of a guess.
If your kit comes home clean and you've never felt salt on your skin, you're likely at the lower end and can run lighter.
When plain water becomes the problem
This is the part that flips most people's intuition. We're trained to think "drink more water" is always the safe choice. On a long, hot, sweaty ride, it isn't.
When you lose heavy sodium in sweat and replace it with large volumes of plain water, you dilute the sodium concentration in your blood. Push that far enough and you get hyponatremia — dangerously low blood sodium. The symptoms overlap confusingly with dehydration early on — nausea, bloating, headache, confusion — and they can even be mistaken for the bonk, though that's a blood-sugar crash with a different cause and fix entirely. With hyponatremia, the cause and the fix are opposite to dehydration. Dehydration means you need fluid. Hyponatremia means you've had too much fluid relative to sodium. In severe cases it causes seizures and is genuinely dangerous; it's killed endurance athletes who kept drinking plain water through long events.
The riders most at risk are the careful ones: people who've absorbed "stay hydrated" so thoroughly that they over-drink, particularly slower riders on long events who have more hours to keep pouring water in. The prevention is simple — drink to match your sweat losses rather than maximise intake, and put sodium in your fluids on anything long or hot. The hydration guide covers the fluid side of this in full; this article is the sodium half of the same equation.
Building a bottle that matches you
You don't need to overcomplicate this. A practical setup:
The two-bottle system. One bottle with your carbohydrate/electrolyte mix, one with plain water or a lighter electrolyte. This lets you adjust sodium and carbohydrate independently as the ride and the heat change — exactly the flexibility Allen Lim was after when he started building team drinks.
Match the dose to the data. Start at 500-800mg of sodium per litre on rides over 90 minutes. If you're a salty sweater or it's genuinely hot, go to 1,000mg or beyond. Average sweaters in mild conditions can sit at the low end.
Cheap works. The ingredient that matters is sodium, and you can get it from a high-sodium tablet, a measured pinch of table salt, or a homemade mix just as well as from a premium product. What you pay for in branded products is convenient, palatable, known dosing — not magic. For a lot of riders a measured DIY bottle is the honest answer; you're buying physiology, not packaging.
Practise it. Electrolyte and fluid intake is a skill like fuelling. Don't debut a new hydration strategy on event day. Rehearse it on long training rides — ideally the same rides you're using for gut training and fuelling practice, since it's all the same bottle.
Where this sits in the bigger picture
Hydration and electrolytes aren't a separate department from fuelling — they're the same bottle, the same gut, the same ride, and they carry on past the finish: settling the fluid and sodium deficit you've built up is a core part of post-ride recovery nutrition, not just an on-bike concern. Get the whole nutrition system working together: carbohydrate matched to the demand, fluid matched to your sweat rate, sodium matched to your sweat concentration. That's three numbers, all of them yours, none of them borrowed from an average.
Most riders never measure any of it and run a generic mix that's wrong for them in both directions — too little sodium on hot days, too much fluid on long ones. Twenty minutes with a set of scales fixes the first half. Paying attention to the salt on your kit fixes most of the rest.
Got a question about your own sweat rate, which mix to run for a hot target event, or whether you need a proper sweat test? Join the community and put it to riders solving the same problem, drawing on the conversations we've had with sports scientists and pro-team nutritionists — including Dr. Allen Lim and Dr. Andy Blow of Precision Fuel & Hydration — on the podcast.