How Much Should You Drink During a Half Marathon?

The advice you keep hearing is too vague

Search "how much to drink during a half marathon" and you'll get some version of the same answer: 150–250 ml every 15–20 minutes. That range is so wide it's almost useless. At the low end, you'd drink about 600 ml over a two-hour race. At the high end, you'd drink nearly two litres. Those are wildly different hydration strategies, and the wrong one could leave you cramping at mile 10 or, worse, dealing with hyponatremia — a potentially dangerous condition caused by drinking too much.

The problem isn't that the advice is wrong. It's that it's generic. Your hydration needs during a half marathon depend on your body, your pace, and the conditions on race day. A 55 kg runner finishing in 1:45 on a cool morning has completely different fluid needs than an 85 kg runner finishing in 2:30 in the heat.

So here's how to figure out your actual number.

Why half marathon hydration is its own problem

Most hydration guides are written for the full marathon. That makes sense — it's the distance where fueling mistakes are most punishing. But half marathoners face a different challenge.

The race is short enough that many runners skip hydration planning entirely. They figure they can get through 13.1 miles on what they drank before the start. Sometimes that works. Often it doesn't — especially when conditions are warmer than expected, when the course is hillier than anticipated, or when pre-race nerves disrupted their normal morning routine.

On the other end, anxious first-timers sometimes overcompensate. They grab a cup at every aid station, chug their handheld bottle, and finish the race feeling bloated and heavy. Over-drinking dilutes the sodium in your blood. In mild cases, you feel sluggish and nauseous. In severe cases, it's a medical emergency.

The half marathon sits in a tricky middle ground: long enough that hydration matters, short enough that there's less margin to correct mistakes.

Start with your sweat rate

Your sweat rate is the single most important number for your hydration plan. It tells you how much fluid you lose per hour of running, which is the baseline for calculating how much you need to replace.

Here's how to measure it at home:

1. Weigh yourself naked before a run.
2. Run for 60 minutes at your expected race pace.
3. Don't drink anything during the run.
4. Towel off and weigh yourself naked again immediately after.
5. The difference in kilograms is roughly your sweat rate in litres per hour.

If you ran for less than 60 minutes, adjust the math proportionally. If it was 45 minutes and you lost 0.6 kg, your hourly sweat rate is about 0.8 litres.

Most runners fall between 0.5 and 2.0 litres per hour. That's an enormous range — and it's why generic advice doesn't work. A runner losing 0.6 litres per hour needs a completely different plan than one losing 1.5 litres per hour.

Your sweat rate is influenced by body size, fitness level, genetics, exercise intensity, temperature, and humidity. It's not a fixed number. A sweat test you did in January won't be accurate for a May race if the temperature has jumped 15 degrees.

How much of that sweat should you replace?

Here's the part most guides skip: you don't need to replace 100% of the fluid you lose. In fact, trying to replace all of it during a race usually causes more problems than it solves.

Sports science research suggests replacing 50–80% of sweat losses during exercise. A mild fluid deficit of 2–3% of body weight is normal and tolerable for most athletes. Performance starts to decline meaningfully around 3–4% dehydration, but that level is hard to reach during a half marathon unless conditions are extreme.

So if your sweat rate is 1.0 litres per hour, you're looking at replacing roughly 500–800 ml per hour. For a two-hour half marathon, that's 1.0 to 1.6 litres total across the race.

Here's a practical framework:

| Sweat rate | Target replacement (per hour) | For a 2-hour half marathon | |---|---|---| | 0.5 L/hr (light sweater) | 250–400 ml | 500–800 ml total | | 1.0 L/hr (moderate sweater) | 500–800 ml | 1.0–1.6 L total | | 1.5 L/hr (heavy sweater) | 750–1,200 ml | 1.5–2.4 L total | | 2.0 L/hr (very heavy sweater) | 1,000–1,500 ml | 2.0–3.0 L total |

These numbers shift based on how long you'll be racing. A 1:30 finisher needs less total fluid than a 2:30 finisher even at the same sweat rate, simply because they're out there for an hour less.

Temperature and humidity change everything

If you did your sweat test on a 10-degree morning and race day turns out to be 25 degrees and humid, your plan is already outdated.

Heat increases your sweat rate. Humidity makes it worse because sweat doesn't evaporate as efficiently, so your body produces more of it to try to cool down. A 15-degree temperature jump can increase your sweat rate by 30–50%.

This is where most runners get caught. They trained through winter, calculated a sweat rate in cool conditions, and then race in late spring heat without adjusting their plan.

If race day is significantly warmer than your training conditions, bump your fluid target up by roughly 20–30%. If it's cooler, you can pull back slightly. The key is knowing your baseline so you have something to adjust from.

What should you drink — water or electrolytes?

For a half marathon, most runners benefit from an electrolyte drink rather than plain water.

When you sweat, you lose sodium, potassium, magnesium, and chloride — not just water. Sodium is the big one. The average runner loses between 400 and 1,200 mg of sodium per litre of sweat, and some heavy salt sweaters lose even more.

Replacing fluid without replacing sodium dilutes what's left in your blood. That's how hyponatremia happens. An electrolyte drink solves two problems at once: it replaces fluid and replaces the minerals you're losing.

Look for a drink or tablet that provides 300–700 mg of sodium per 500 ml. If you're a visibly salty sweater — white residue on your clothes or skin after a run — aim for the higher end of that range.

If you prefer plain water, pair it with salt tabs or electrolyte capsules. The goal is the same: replace fluid and sodium together.

Build your race-day drinking plan

Now put it together. Here's how to build a practical plan you can execute on course:

The night before: Sip fluids normally with dinner. Don't try to pre-load by drinking extra litres — your body will just excrete the excess overnight.

Race morning (2–3 hours before start): Drink 400–500 ml of water or electrolyte drink with breakfast. This tops off your fluid stores without overloading your stomach.

30 minutes before the gun: Take a few sips — 100–200 ml. Don't chug. If it's hot, lean toward 200 ml.

During the race: Drink at planned intervals based on your sweat rate calculation. Most half marathon courses have aid stations every 2–3 km. You don't need to stop at every one. Map out which stations you'll use and roughly how much you'll drink at each.

For a runner with a 1.0 L/hr sweat rate targeting 600 ml per hour, that's about 150 ml every 15 minutes — roughly one small cup at every other aid station, or a few gulps from a handheld bottle every 15 minutes.

After the race: Drink to thirst. The urgency of precise hydration ends when you stop running. Have a drink with sodium in it — a recovery drink, broth, or even a salty snack with water — and your body will sort itself out.

How to practise this before race day

Your hydration plan is only as good as your ability to execute it while running. If you've never drunk on the move, practise during your long runs.

Train with whatever you plan to use on race day — a handheld bottle, a hydration vest, or simulating aid station pickups by placing bottles along your route.

Pay attention to how your stomach handles fluid at race pace. Some runners can drink 200 ml in one go without issue. Others feel nauseous with anything more than small sips. Find your tolerance in training, not on race morning.

If you're running a race that provides a specific electrolyte drink on course, train with that brand. Switching drinks on race day is a common cause of stomach issues that runners blame on "the gels" when it was actually the unfamiliar drink.

Your watch collects the data — but what do you do with it?

If you run with a Garmin or Apple Watch, your wrist is already tracking heart rate, workout duration, estimated calorie burn, and in some cases environmental conditions. That data contains signals about your hydration needs — how hard you're working, how long you've been at it, how hot it is.

The gap is turning that data into a plan. Your watch shows you numbers. It doesn't tell you to drink 150 ml at the next aid station or to bump your sodium intake because today is hotter than your last long run.

Sweatr bridges that gap. It pulls your wearable data and Strava activity history, layers in your sweat rate profile, and builds a personalised hydration plan you can follow during training and on race day. Instead of doing the maths yourself, you get clear, timed recommendations — what to drink, how much, and when.

If you're training for a half marathon and want a hydration plan that's built for your body and your race, Sweatr does the work for you. Try it free on iOS.

The short version

1. Measure your sweat rate. It's the foundation of everything.
2. Replace 50–80% of losses — not all of it.
3. Adjust for heat. A warm race day means more fluid.
4. Use electrolytes, not just water.
5. Practise during training so race day is routine.
6. Let Sweatr calculate it from your actual data if you want the personalised version without the spreadsheet.

Your first half marathon is hard enough without guessing how much to drink. Get the hydration right and you can focus on what actually matters — running your race.