Hydration for lifters: how much water you really need

Numbers for before, during and after training, a simple weigh-in test that gives you your own sweat rate, and when salt matters more than plain water.

One person tells you three litres a day, no exceptions. The next tells you to drink when you're thirsty and ignore the rest. Neither answer covers the thing you actually want to know: how much you need, on a session that runs an hour and a quarter, in a gym with no air conditioning, in August.

Below are numbers you can use at your next session, plus a simple test that gives you your own sweat rate within a fortnight instead of somebody else's average.

Step 1: set your daily baseline

A sensible starting point for most adults is roughly 30 to 35 ml of fluid per kilogram of body weight per day, before you count training at all. That puts a 70 kg person at about 2.1 to 2.5 litres and a 95 kg person at about 2.9 to 3.3 litres.

That figure is total fluid, not bottled water alone. Coffee, tea, milk, yoghurt, soup and fruit all count. Food supplies somewhere between a fifth and a third of most people's intake, because fruit and vegetables are often 85 to 90 per cent water — a large peach is not just sugar and fibre, it's close to 200 ml of fluid.

Treat the baseline as a starting position, not a target to hit exactly. Its job is to tell you where you stand before you add what training takes out.

Step 2: measure what you actually sweat

This is the only part that takes effort, and the only part that gives you a personal number.

Weigh yourself before training, naked or close to it, after a trip to the toilet.
Note how much you drink during the session (say 750 ml from your bottle).
Towel off afterwards, take off the wet kit, and weigh again.

The sum is: (weight before − weight after) + fluid drunk = sweat loss. Start at 82.0 kg, finish at 81.2 kg, drink half a litre, and you lost 0.8 + 0.5 = 1.3 litres. A kilo of weight lost immediately after training is roughly a litre of fluid. It is not fat.

Do it three times: once in a cool gym, once in the heat, once on the hardest session of the week. Most people land between 0.5 and 2 litres per hour, and the spread between individuals is huge — two people doing the same session can differ threefold.

Step 3: what to drink beforehand

The aim is to arrive normally hydrated, not full. In practice: 400 to 600 ml two to three hours before, then another 200 to 300 ml in the last 20 to 30 minutes.

Sink a litre five minutes before your first set and all you get is a heavy stomach and a toilet break between sets. If you train fasted in the morning, take at least 300 to 500 ml on waking — eight hours of sleep is eight hours without fluid.

Step 4: during the session

This is where most people get it wrong, in both directions. The rule of thumb that holds up: once you lose more than about 2 per cent of body weight in a single session, endurance, concentration and heat tolerance start to fall off measurably. For an 80 kg lifter that's 1.6 kg, so about 1.6 litres.

Up to 60 minutes in a normal gym: 300 to 500 ml, or simply drink to thirst. Nothing to overthink here.
60 to 90 minutes, or hot and humid: 400 to 800 ml per hour, taken as 150 to 250 ml every 15 to 20 minutes.
Beyond 90 minutes, serious heat, or two sessions a day: plan against your measured sweat rate and add sodium.

Past the 90-minute mark, 30 to 60 g of carbohydrate per hour is worth having alongside the fluid. Sports drinks at 6 to 8 per cent carbohydrate (60 to 80 g per litre) cover both jobs at once. Those are real calories, so if you track calories and macros, log them like any other food.

Step 5: afterwards

With 24 hours until your next session, drinking with meals and to thirst is enough — you'll replace the deficit without thinking about it. If the next session or match is inside 8 to 12 hours, be deliberate: about 1.25 to 1.5 litres for every kilo you were down, spread across the following two to four hours.

Why more than you lost? Some of what you drink goes straight out as urine. Sodium and a meal alongside it improve retention compared with plain water, which is why an ordinary salted meal often does the job better than anything from a sachet.

Salt and electrolytes: when they matter

Sweat isn't just water. Sodium concentration ranges from roughly 400 to 1,200 mg per litre, and it's largely an individual trait that training doesn't change much. White marks on your cap and shirt, sweat that stings your eyes and tastes distinctly salty — that usually means you're at the higher end.

For sessions under an hour, on a normal diet that already contains salt, you don't need an electrolyte product. For long hot sessions they earn their place: half a gram to a gram of salt per 500 ml gives roughly 400 to 800 mg of sodium per litre, which is in the range of what you're losing. Potassium and magnesium come from the plate — fruit, dairy, pulses and nuts handle it without supplements.

How to check you're getting it right

No app required. Three signs, read together:

Urine colour: pale straw is fine, apple-juice colour means you're behind. The first pass of the morning is naturally darker, so don't judge by that one. B vitamins turn it bright yellow with no relation to hydration.
Frequency: four to seven trips a day is normal.
Morning weight: if you're about 1 per cent lighter than usual two mornings running and nothing has changed in your eating, you're probably short on fluid.

Common mistakes

Downing a litre before the first set. It can't be absorbed that fast; it sits in your bladder and bothers you for the rest of the hour.
Reading too much into the scale. A kilo down after training, or two kilos up after a salty meal, is water. If it looks like your fat loss has stalled, judge it on the weekly average rather than a single morning.
Plain water only on long sessions in heat. Litres of water taken over several hours with no sodium can push blood sodium too low. It's rare, but it's dangerous, and it mostly catches slower recreational runners on long events.
Four litres "just in case". More water doesn't buy better skin, a faster metabolism or a flushed-out system. It buys more toilet breaks and, taken far enough, unnecessary risk.
A sports drink for a 45-minute gym session. That's 150 to 250 kcal you didn't earn. If you're in a cutting phase, it's often the difference between a deficit and maintenance.
Drinking alcohol the night before a hard session. It increases urine output, so you start the session already down.

When to see a doctor

Get help if effort in the heat brings on dizziness, nausea, a headache that won't lift, confusion, or sweating that suddenly stops. Those point to heat exhaustion, and another bottle of water is not the answer.

See a doctor too if you have constant thirst and frequent urination that your drinking doesn't explain, if urine stays dark despite a normal intake, or if a long race leaves you with headache, nausea and swollen fingers — that combination needs assessment, not more fluid.

If you have kidney or heart disease, or take diuretics, agree your fluid intake with your doctor. Everything here is written for healthy people who train.

Common questions

Does coffee dehydrate you?

Not in normal amounts. Two or three coffees a day in someone who drinks them regularly produce no net fluid loss, and they count towards your daily intake. A large caffeine dose in someone who never drinks it will raise urine output a little, but not dramatically.

Do I need extra water on creatine?

Creatine pulls water into muscle cells, and gaining 1 to 2 kg in the first few weeks is expected. That's water inside the muscle, not bloating, and it doesn't call for special litres. Stick to your baseline and replace what you lose after training.

Is thirst a good enough guide?

For most gym sessions, yes. Thirst lags slightly behind your actual state and is blunter in older people and in cold weather, so for anything over 90 minutes, sessions in heat, or a day when performance matters, drink to a plan rather than waiting for the signal.

Does a high-protein diet need more water?

Slightly, since the nitrogen from protein is cleared by the kidneys along with water, but the difference is a few hundred millilitres, not litres. With healthy kidneys, high protein intake isn't a concern in itself — the problem would only be chronic under-drinking on top of it.

The short version

Baseline: 30 to 35 ml per kilo per day, counting food, coffee and tea.
Weigh in before and after three sessions and work out your own hourly sweat loss.
Before: 400 to 600 ml two to three hours out, plus 200 to 300 ml just before you start.
During: drink to thirst under an hour; 400 to 800 ml per hour in sips when it's longer or hot.
After: 1.25 to 1.5 litres per kilo of deficit if you train again soon, otherwise water with meals.
Salt only when the session is long and hot; food covers the rest.
Check: pale urine through the day, steady morning weight, no headache at the end of a session.