How Much Water to Drink When You Train

Weigh yourself before a 60 minute summer run and again afterwards, and you will often see a difference of a kilo or more. That gap is almost entirely water. Lose around 2 percent of your body weight in sweat, about 1.5 kilos for a 75 kilo person, and most studies find endurance and concentration start to slip.

So how much water to drink when you train is a real question, but the answer is not the eight glasses a day rule or a two litre bottle you force down before lunch. It depends on how much you sweat, how long you train and how hot the room is. Here is how I work it out with the people I coach, using a scale, a watch and a bit of honesty about thirst.

What Sweat Loss Actually Looks Like

Sweat rates vary more than most people expect. A light session in a cool gym might cost 300 to 500 millilitres an hour. A hard spin class in a warm studio can push past 1.5 litres an hour for heavy sweaters. Two people doing the same workout side by side can lose very different amounts.

These are ranges, not targets. The only way to know your own number is to measure it.

Measure Your Own Sweat Rate in One Session

This takes one workout and a bathroom scale. Do it during a typical session, not your hardest one.

1. Use the toilet, then weigh yourself in minimal clothing before training.
2. Train for about an hour. Note exactly how much you drink, in millilitres.
3. Towel off and weigh yourself again in the same clothing.
4. Subtract the second weight from the first. Each kilo lost equals about one litre of fluid.
5. Add the amount you drank during the session. That total is your hourly sweat rate.

Example: you start at 72.0 kilos, finish at 71.2, and drank 400 millilitres. You lost 0.8 litres plus the 0.4 you drank, so your sweat rate is about 1.2 litres an hour in those conditions. Repeat it once in summer and once in winter, because the number can nearly double in heat.

How Much Water to Drink Before, During and After

With your sweat rate in hand, the rest is fairly simple. These are practical starting points that most active adults can use.

Before training

Drink around 400 to 600 millilitres in the two to three hours before a session, then stop gulping in the last half hour so you are not training with a sloshing stomach. If you train first thing in the morning, a large glass on waking is usually enough. Your urine is a decent guide: pale yellow means you are in a good place, dark means drink a bit more.

During training

For sessions under an hour, most people do fine sipping to thirst. For longer or hotter sessions, aim to replace somewhere between half and three quarters of your sweat rate. If you lose 1.2 litres an hour, that is roughly 600 to 900 millilitres, taken in small amounts every 15 to 20 minutes rather than all at once.

After training

Replace about 1.25 to 1.5 litres for every kilo you lost. The extra covers the fluid you will keep losing as urine over the next few hours. Spread it over two to three hours with a meal; food helps you hold on to fluid. Our guide to eating before and after a workout covers what that meal can look like.

The Advice to Drink Ahead of Thirst Is Overdone

Gym culture loves the line "if you are thirsty, you are already dehydrated." It sounds wise, and for most people training under 90 minutes in normal conditions it is not true. Thirst kicks in at a small level of fluid loss, well before performance suffers.

Forcing water on a fixed schedule has a real downside. Drinking much more than you sweat, especially during long endurance events, can dilute the sodium in your blood. This condition, called hyponatraemia, is rare but serious, and it has turned up among slower marathon runners who drank at every station for four or five hours. Symptoms include headache, nausea, confusion and swelling in the hands.

A better rule for most training: drink to thirst during shorter sessions, and use your measured sweat rate for long or hot ones. If you weigh more after a session than before, you drank too much.

When Water Alone Is Not Enough

Sweat is not just water. It carries sodium, roughly 0.5 to 1 gram per litre for most people and more for "salty sweaters," the ones whose shirts dry with white streaks. For a typical one hour gym session, ordinary meals replace that easily.

Electrolytes start to matter when sessions run past 90 minutes, when you sweat heavily in heat, or when you train twice in one day. In those cases, a drink with around 300 to 600 milligrams of sodium per 500 millilitres helps you absorb and keep the fluid. Sports drinks also add carbohydrate, which is useful for long endurance work and unnecessary for a 45 minute strength session.

Cheaper option: a pinch of salt and a splash of juice in a bottle of water works for most recreational training. You do not need expensive tablets unless you prefer the convenience.

Signs You Are Getting It Wrong

Under drinking and over drinking both leave clues. Watch for these after a few weeks of training:

• Headaches in the evening after training days often point to a fluid shortfall
• Dark urine the morning after a hard session suggests you did not catch up
• Weight up after training, or constant trips to the toilet mid session, means you are drinking more than you need
• Muscle cramps late in long sessions may relate to sodium loss as much as water
• Feeling bloated or nauseous while running usually means too much fluid too fast

Hydration also links to everything else you eat. If you are adjusting your diet around training, our article on how much protein you actually need in a day pairs well with this one, and the rest of our nutrition guides cover fuelling in more depth.

FAQ

Does coffee count toward my fluid intake?

Yes. Regular coffee drinkers adapt to caffeine, and a normal cup still adds more fluid than it takes away. It is fine as part of your pre training drink, within your usual amount.

Should I drink cold or room temperature water while training?

Cool water, around 10 to 15 degrees, tends to be more pleasant, so people drink more of it, and it may help a little in hot conditions. Either is fine for normal sessions.

Do I need to drink more on rest days?

Not specifically. Drink to thirst and keep your urine pale. The bigger needs come on training days and in the hours after hard or hot sessions.

Weigh In Once, Then Stop Guessing

Pick one normal workout this week, weigh yourself before and after, and note what you drank. That single number tells you more than any generic rule. Drink to thirst for short sessions, replace half to three quarters of your sweat rate for long ones, add salt when you train long or hot, and adjust by how you feel and what the scale says.