Plan on 400 to 800 ml (14 to 27 oz) of water per hour of effort: about 500 ml (17 oz) for one hour, 1 liter (34 oz) for two hours, 1.5 to 2 liters (51 to 68 oz) for three to four hours without a water source. Mileage alone tells you nothing about what you need. What matters is the time between two refills, the heat and how much you sweat.
The basic rule: think in time, not distance
Two runners on the same 20 km (12 mi) course don't need the same amount. One finishes in 1:45 on a fast, rolling route. The other takes 3:30 on a course with 900 m of elevation gain. Same distance, nearly twice the time in the sun, nearly twice the water.
Your body doesn't sweat by the mile: it sweats by the minute, depending on intensity and heat. So "how much water for 20 km?" has no answer until you've answered three other questions: how long you'll be running, in what temperature, and where the next water source is.
The method takes three steps:
- Estimate your actual running time, not the distance. On rough terrain, take your usual pace and add 10 to 15 minutes per 300 m of elevation gain.
- Break that time into segments, from one water source to the next. An aid station at km 12 changes everything: you never carry the water for the whole race.
- Apply an hourly rate to your longest segment, then add a 15 to 20% safety margin.
The reference chart: time on your feet → water volume
Here are the standard benchmarks, expressed in liters of water to drink, not liters of storage. Don't mix the two up: when brands talk about a "5 L vest" or a "10 L pack," they mean the pack's storage volume. The "2L" on a hydration bladder, on the other hand, really is its water capacity. Those two numbers have nothing to do with each other. If you don't have a pack yet, this distinction is the first filter in our guide to choosing the right hydration vest for your runs, before size and storage.
| Time without a refill | Cool (5-15 °C / 41-59 °F) | Mild (15-22 °C / 59-72 °F) | Hot (25 °C / 77 °F and up) | Best container |
|---|---|---|---|---|
| 30 to 45 min | 0 to 250 ml | 250 ml | 350 to 500 ml | 1 soft flask, 500 ml |
| 1 hr | 250 to 400 ml | 400 to 500 ml | 500 to 700 ml | 1 to 2 soft flasks |
| 1 hr 30 min | 400 to 600 ml | 600 to 750 ml | 750 ml to 1 L | 2 soft flasks or hydration bladder |
| 2 hr | 500 to 800 ml | 800 ml to 1 L | 1 to 1.4 L | 1.5L or 2L hydration bladder |
| 3 hr | 750 ml to 1.2 L | 1.2 to 1.5 L | 1.5 to 2 L | 2L hydration bladder |
| 4 hr | 1 to 1.6 L | 1.6 to 2 L | 2 to 2.8 L | 2L hydration bladder + 1 soft flask |
| 5 hr and up | Never carry the total: split it into segments of 2 to 3 hours max between water sources. | 2L hydration bladder, refilled | ||
Read this chart by segment, never by whole race: a 50 km (31 mi) with three aid stations runs just fine with 1.5 liters on your back, while a fully self-supported 25 km (16 mi) at 30 °C (86 °F) calls for more.
The 4 variables that change everything
Temperature and humidity
Heat increases your sweat rate, but it's humidity that does the most damage. At 28 °C (82 °F) in dry air, your sweat evaporates and cools you down. At 24 °C (75 °F) in saturated air — in the woods after rain, with a storm building — it stops evaporating: you sweat more and cool down less.
Rule of thumb: above 25 °C (77 °F), increase your volume by 30 to 50% compared with a cool-weather run. If the air is heavy and still, go for the top of the range.
Elevation gain
Climbing works on you in two ways. First, it stretches out the time: a 20 km (12 mi) route with twelve hundred meters of elevation gain takes as long as a flat 35 km (22 mi). Second, on the way up you produce a lot of heat at low speed, without the airflow that cools you on the flats. High heat output, poor heat loss: the worst of both worlds.
Distance between aid stations
This is the most overlooked variable, and yet it's the only one that decides what you carry. Before every race, pull out the course guide and note the time gap — not the distance — between water sources. Your volume is the longest of those segments, plus a margin.
In training, apply the same logic with park drinking fountains and spigots at sports fields: plenty of long loops become easy to manage once you redraw them around two water sources you know.
Your sweat rate, and how to measure it with a scale
The ranges above are population averages. Your personal number can land well outside the average, in either direction. The good news: you can measure it in one run, with a bathroom scale.
- Weigh yourself naked, right before you head out, after using the bathroom.
- Run for 1 hour at a typical intensity, in typical conditions.
- Write down exactly what you drank during that hour, in milliliters.
- Come back, dry off, and weigh yourself naked again.
- Hourly sweat rate = (weight before − weight after, in grams) + volume drunk in ml.
Example: you lost 700 g and drank 400 ml. That means you sweat roughly eleven hundred milliliters (37 oz) per hour in those conditions. Do the test once in winter and once in summer: you'll have your two benchmarks and you'll never have to guess again.
One important point: the goal isn't to replace 100% of that loss. Losing around 1 to 2% of your body weight during a long effort is normal and well tolerated. Trying to replace it all mostly means carrying extra weight and overloading your stomach.
Is 2 liters enough?
In the vast majority of cases, yes — as long as you think in segments.
Short trail runs, 10 to 25 km (6 to 16 mi)
Between 1 and 3 hours of effort depending on the terrain. In cool weather, 500 ml to 1 liter is plenty. In the middle of summer, you'll go up to 1.5 liters, sometimes 2 if you're fully self-supported and you sweat a lot. So two liters covers the whole range for this format, with room to spare.
Mid-distance trail runs, 25 to 45 km (16 to 28 mi)
From 3 to 6 hours of effort. This is the format where the question gets serious. With one or two aid stations, 1.5 to 2 liters of water at the start is enough: you refill along the way. Fully self-supported in serious heat, 2 liters becomes the bare minimum and you need to plan a water stop, even if it means a 500 m detour.
Beyond 45 km (28 mi)
Nobody carries the water for a 60 km (37 mi) race. Usable capacity tops out around 2 to 2.5 liters because beyond that, the weight works against you: 2 liters of water is already 2 kg (4.4 lb) on your shoulders. The logic changes completely: you manage segments of 2 to 3 hours and refill every time, even if your bladder is still half full.
That's exactly how the Sentia hydration vest is designed to work: one size, adjustable, and it comes with its 2L hydration bladder, hose and bite valve, so segments of 2 to 4 hours are covered without a second thought. If you want to switch between plain water and sports drink without rinsing in between, a 2L replacement bladder (BPA-free TPU, compatible with the Sentia vest and most trail running packs) solves the problem. The rest of the lineup is in Our Essentials.
Plain water, sports drink or both
It's not a matter of philosophy, it's mechanics. Plain water hydrates. A sports drink also delivers carbs and sodium, useful beyond about 1:30 of effort, and essential when it's hot and you're sweating a lot.
- Under 1:30: plain water, period. Nothing to add.
- 1:30 to 3 hours: a drink with a little carbohydrate and sodium starts to make sense, especially in serious heat.
- Over 3 hours: you need sodium and carbs, whether from drinks, gels or solid food. Otherwise, you drink a lot and still feel empty.
The most practical setup: sports drink in the front soft flasks, plain water in the hydration bladder on your back. You mix what you want, you can rinse your mouth, splash the back of your neck, and you don't turn your bladder into a sugar farm. Because sugar in a hydration bladder means guaranteed mold if you don't clean it within 48 hours — we cover it in detail in our method for cleaning your hydration bladder.
If you're still torn on the container itself, compare the two systems in our article hydration bladder vs. soft flasks.
Carrying too much water: the hidden cost in weight and bounce
A liter of water weighs a kilo (2.2 lb). That's the only number to remember. Heading out with 2.5 liters when you'll drink 1.2 means running with 1.3 kg (2.9 lb) of dead weight for the entire run.
The weight isn't even the worst part. The real problem is how the pack behaves:
- Sloshing. A half-full bladder lets the water shift with every stride. The pack slams into your back on descents and your footing gets unstable.
- Chafing. More mass and more movement mean more shear on the shoulder straps, so a higher risk of irritation under your arms and on your collarbones.
- The energy cost. You pay for every extra kilo on the climbs, exactly where you're already at your limit.
Two habits are all it takes: burp the air out of the bladder before you head out by squeezing it with the hose pointing up, and strap in properly. If your pack still bounces around, the cause is almost always the fit — we cover it in detail in running vest bouncing, and storage volume (not to be confused with liters of water) is covered in what vest capacity to choose.
The right amount of water isn't the most you can carry. It's the minimum that keeps you safe, plus a small margin.
FAQ
How much water should I carry for a 20 km trail run?
First, convert those 20 km (12 mi) into time. On a rolling course you finish in 2 hours in cool weather, 500 to 800 ml will do. On a mountain route taking 3:30 at 28 °C (82 °F), you'll go up to 1.5 or even 2 liters. Distance alone can't settle it: the time between two water sources decides.
Is 2 liters of water enough for a trail run?
Yes for the vast majority of runs, up to about 4 hours of effort in mild conditions, or 3 hours in serious heat. Beyond that, it's not the capacity you should increase but how often you refill: carrying more than 2 to 2.5 liters becomes a weight penalty.
How much should you drink per hour trail running?
A range of 400 to 800 ml (14 to 27 oz) per hour covers most situations: closer to 400 to 500 ml in cool weather, closer to 600 to 800 ml in serious heat or for a heavier build. Drink small sips regularly rather than a lot at once.
2L or 1.5L hydration bladder: which one should I choose?
A 2L bladder is more versatile: you can always fill it halfway, but not the other way around. The difference in empty weight between the two sizes is negligible. Go for 1.5L only if you never run more than 2 hours and want the most compact pack possible.
How do I know if I'm drinking enough during a run?
Three simple, free indicators: the color of your urine (pale is good; dark means you're falling behind), a dry, sticky mouth that shows up too early, and weighing yourself before and after a long run. Losing more than 2% of your body weight on a run signals a deficit to correct next time.