How much should you drink before, during, and after a run? The honest answer depends on how long you're running, how hot it is, and how much you sweat. This guide gives you the full UK protocol, plus an interactive calculator that estimates your hydration needs for any run.
TL;DR
- Under 60 minutes of easy running: nothing to drink during is fine. Sip water before and after.
- Above 60 minutes or in heat above 20°C: 400 to 800ml per hour, with electrolytes if you sweat heavily.
- Post-run: weigh yourself before and after. Drink 1.5L for every 1kg lost.
- Do not force water. Over-hydration is rare but dangerous for slower marathoners. Drink to thirst, not to a schedule.
Why hydration matters (and when it doesn't)
Water moves heat out of your body when you run. As you warm up, your blood carries heat to the skin, and sweat evaporates to cool you down. Lose too much fluid, and your blood thickens, your heart works harder, your core temperature rises, and your pace slows. Research suggests every 1% drop in body weight from sweat costs you roughly 0.5 to 1.5% of performance, with the drop steeper in heat.
That sounds dramatic, but for short, easy runs the practical impact is small. A 30 minute jog in cool weather will not drop your body weight by much, and your body has plenty of reserve. The mistake is assuming the same rules apply to every run. A 45 minute easy run in March is not the same as a two hour long run in July, and trying to drink a fixed amount across all conditions makes things worse, not better.
The other side of the coin is over-drinking. Slower marathoners who religiously drink at every aid station can dilute their blood sodium and develop hyponatraemia, a condition that can be serious or fatal. Hydration is not "drink more, get faster." It is "match your intake to your sweat losses, especially as runs get longer." The goal is balance.
Pre-run hydration
Most runners arrive at a run slightly under-hydrated, especially first thing in the morning. The fix is simple. Drink 400 to 600ml of water in the two hours before your run, plus a small top-up of 100 to 200ml about 15 to 20 minutes before you head out the door. That gives your body time to absorb the fluid and pass the excess so you do not start with a sloshing stomach.
If you wake up and run within 30 minutes, aim for 250 to 400ml on waking, sipped slowly. For longer or harder sessions in heat, add a pinch of salt to one of those drinks or use an electrolyte tablet. Check your urine before heading out. Pale straw colour is the target. Dark yellow means you are behind. Completely clear means you may have over-drunk and could be diluting your sodium.
Avoid loading large volumes in the last 30 minutes. It will not absorb in time and will slosh in your stomach. Spread your pre-run fluid across the two hours leading up to the run instead.
During-run hydration
Runs under 60 minutes: nothing usually needed
For easy runs under an hour, you do not need to drink during the run. Your body has enough reserve, and the stomach absorption rate during exercise is limited anyway. Carrying water adds weight and faff for no real benefit. Hydrate well before, and rehydrate after. That is the whole protocol.
The one exception is heat. Above 25°C, even a 45 minute run can pull significant sweat losses out of you. Carry a small handheld bottle or plan a route past a water fountain if you are running for 45 minutes or more in warm conditions.
Runs 60 to 90 minutes: 200 to 400ml plus optional gel
Once you cross the hour mark, sweat losses start to matter. Aim for 200 to 400ml of fluid across the run, sipped little and often rather than gulped at one point. Water is fine for cooler conditions. In heat above 20°C, switch to an electrolyte drink or add a tablet to your bottle.
If you are doing a quality session, like a tempo or threshold run, a single 30 to 40g carb gel taken around the 45 minute mark can boost performance, even if you are well fuelled. The carb hit signals your brain to push harder. Pair the gel with a few sips of water for absorption.
Runs over 90 minutes: 400 to 800ml/hour plus electrolytes
This is where structured hydration starts to matter. For long runs and marathon training, aim for 400 to 800ml per hour, split into roughly 150 to 200ml every 15 to 20 minutes. The exact volume depends on heat, body weight, and sweat rate, which is where the calculator below comes in.
Electrolytes are essential at this duration. You are losing sodium, potassium, and magnesium through sweat, and replacing only water can lower your blood sodium. Use a sports drink (typical sodium 250 to 500mg per 500ml) or add an electrolyte tablet to plain water. Carbohydrate is also helpful here, ideally 30 to 60g per hour for runs of 90 to 180 minutes, scaling up for longer.
Hydration calculator
Post-run rehydration
The most accurate way to know how much to drink after a run is to weigh yourself before and after. Strip down to running kit, weigh, run, towel off, and weigh again. Every kilogram lost equals roughly one litre of sweat. Replace 1.5 litres for every 1kg of body weight lost, spread across the next two to four hours. The extra 0.5 litres covers the fluid you continue to lose through urine and sweat as you cool down.
If you lost 1.5kg on a hot long run, that is 2.25 litres to replace. Drink it gradually with a meal, ideally one that includes some salt. A bowl of soup, salted potatoes, or a sports drink alongside food all work. Plain water alone after heavy sweating can flush out remaining sodium and leave you peeing it straight back out.
The weigh-yourself protocol is gold standard for long runs and race-pace efforts. For short easy runs, you can skip it. Just drink to thirst, eat a normal meal, and you will be fine by the next day.
Water vs sports drink vs electrolytes
Plain water
Right for: runs under 60 minutes, cooler conditions, light sweaters, hydrating throughout the day. Plain water absorbs quickly, costs nothing, and is the default choice for most training. The mistake is assuming it is always enough. For long runs or heat, water alone can leave you short on sodium.
Sports drink
Right for: runs over 75 minutes, races, hot conditions, sessions where you want both carbs and electrolytes in one. A standard sports drink gives you roughly 30 to 60g of carbs and 250 to 500mg of sodium per 500ml. That is exactly what you need for marathon training long runs and race day. The downside is sugar, so it is not the right call for short easy runs.
Electrolyte tablets
Right for: heavy sweaters, hot weather, long runs where you do not want the carbs, daily hydration in heatwaves. A tablet dropped into water gives you 250 to 500mg of sodium plus potassium and magnesium without the sugar. Useful if you are doing a long fasted run, or if your stomach struggles with sports drinks.
Hyponatraemia: when too much water is dangerous
Hyponatraemia is low blood sodium, and in runners it is almost always caused by drinking too much plain water over a long event. The classic case is a slower marathoner who walks through every aid station, drinks a cup at each, and ends up with several litres of plain water sloshing in their gut by mile 20. Their blood sodium drops, fluid moves into their cells, and in serious cases their brain swells.
Warning signs include nausea, headache, confusion, puffiness in the hands and face, and unusually frequent urination during the race. If you feel worse after drinking more, that is a red flag. Stop drinking water, find some salt, and if symptoms worsen, get medical help.
The prevention is simple. Drink to thirst, not to a schedule. Use electrolyte drinks rather than plain water for runs over 90 minutes. If you are a slower marathoner expecting to be on course for four hours or more, especially in cooler weather, do not drink a cup at every station. This is one of the only running conditions where less is genuinely safer. If you have any history of hyponatraemia or are unsure, speak to a sports dietitian or GP before race day.
Caffeine and alcohol
Caffeine is a mild diuretic in large doses, but in normal training amounts, a coffee or pre-run gel will not dehydrate you. The fluid in your coffee largely offsets the diuretic effect. A pre-run espresso is fine, and caffeine itself can give a small performance boost. Just do not rely on it as your only fluid before a long run.
Alcohol is the bigger problem. A few pints the night before a long run can leave you dehydrated 12 to 24 hours later, and heavy drinking pushes that out to 36 to 48 hours. If you have been drinking the night before a long run, expect to need more fluid the next morning. The signs are obvious: dark urine on waking, dry mouth, heavier breathing for the same pace. If you are training for a goal race, cut alcohol back in the final two weeks and avoid it entirely the night before key sessions.
Common hydration mistakes
- Forcing water when not thirsty. Thirst is your most reliable signal. Drinking on a schedule when you do not feel thirsty risks hyponatraemia. Use the calculator as a guide, not a quota.
- No electrolytes for long runs. Plain water for runs over 90 minutes is the most common mistake. You lose sodium through sweat, and you need to replace it.
- Drinking too cold. Ice cold fluid empties from the stomach slowly and can cramp. Cool but not freezing is the target. Insulated bottles overshoot. Plastic bottles are fine.
- Ignoring your sweat rate. A heavy sweater and a light sweater have completely different needs. Weigh yourself a few times after long runs to learn your rate, then drink accordingly.
- Drinking sports drink for easy runs. You do not need carbs and sodium for a 30 minute easy run. Plain water is fine, and the unnecessary sugar adds up across a training week.
- Dehydration carrying over to the next day. Skipping rehydration after a hot long run leaves you under-hydrated for the next session. Use the weigh-yourself protocol after hot or long runs and front-load fluid the next morning.
How Edge handles hydration
Honestly: Edge does not build hydration guidance into every plan, does not track your hydration, does not calculate your sweat rate, and does not auto-adjust your sessions for hot weather. Hydration is your responsibility as a runner, and that is where this guide comes in. Use the calculator above for any run that worries you, weigh yourself before and after long runs in heat, and learn your own sweat rate over a few sessions.
What Edge does do is give you an adaptive starting plan, Flexi Swap so you can shift a long run to a cooler day if a heatwave hits, direct sync with Strava, Garmin, Apple Watch, and Coros so your hydration learnings stack alongside your training data, and Edge AI for quick training questions, including general guidance on how to approach a hot weekend long run. We have over 17,000 members in the UK using Edge to train consistently, with a free 7 day trial and plans at £19.99 monthly or £119.99 annually.
For personalised nutrition or hydration plans, especially if you have any medical history that affects fluid balance, speak to a sports dietitian or registered nutritionist. Use this guide as the protocol, and Edge for the training around it.
FAQ
How much water should runners drink per day?
Aim for roughly 30 to 40ml per kg of body weight per day as a baseline, which is around 2 to 3 litres for most adults, then add 400 to 800ml per hour of running on top of that. Heat, altitude, and heavy training all push your needs up. Use urine colour as your check. Pale straw means you are on track. Dark means drink more. Completely clear means you may be over-doing it.
What should I drink during a run?
Under 60 minutes in cool weather, nothing is needed during the run. Between 60 and 90 minutes, plain water or a light electrolyte drink in small sips. Over 90 minutes, use a sports drink or electrolyte tablet with carbs, taking 400 to 800ml per hour depending on heat and sweat rate.
Do I need electrolytes for running?
Not for short, easy runs. Plain water is fine for anything under an hour in cool weather. For runs over 90 minutes, in heat above 20°C, or if you are a heavy sweater, electrolytes are essential. Sodium is the key one, with potassium and magnesium playing supporting roles.
Can you drink too much water running?
Yes. Over-drinking plain water during long events can cause hyponatraemia, which is dangerously low blood sodium. It is most common in slower marathoners who drink at every aid station. Drink to thirst, use electrolyte drinks for long runs, and do not force water if you feel nauseous or puffy.
Is sports drink better than water for running?
Only for runs over 75 to 90 minutes, races, or hot conditions where you need both carbs and sodium. For short easy runs, water is the better default. Sports drinks add unnecessary sugar to short sessions and can upset your stomach if used routinely on easy days.
Should you drink during a 5K?
Almost never. A 5K takes most runners 20 to 40 minutes, which is well within the duration where pre-race hydration covers you. Drink 200 to 400ml in the 90 minutes before, sip a small top-up about 15 minutes out, and run the race without a bottle. Rehydrate properly afterwards.
