Hydration Calculator
Calculate exactly how much water you need before, during, and after your run based on your weight, weather, and intensity.
Hydration Dialed In. Now Make Every Run Count for Your City.
A solid hydration plan means you can run longer and recover faster. Motera makes those extra miles worthwhile: capture blocks on your city map, reveal streets hidden in the Fog of War, and compete with neighbors on the local leaderboard.
MoteraFrequently Asked Questions
How much water should I drink while running?
A general guideline is 150 to 250ml (5 to 8 oz) every 15 to 20 minutes during a run. However, your actual needs depend on your weight, the temperature, humidity, and your intensity. Heavier runners, hot weather, and harder efforts all increase your fluid needs. Use this calculator to get a personalized recommendation.
Should I drink water or sports drinks while running?
For runs under 60 minutes, water is usually sufficient. For runs longer than 60 minutes, a sports drink with electrolytes and carbohydrates helps replace sodium lost through sweat and provides energy. If you are a heavy sweater or running in hot conditions, consider adding electrolytes even for shorter runs.
How do I know if I am dehydrated during a run?
Early signs of dehydration include increased thirst, dry mouth, darker urine, headache, and a noticeable drop in performance. More severe signs include dizziness, confusion, rapid heartbeat, and stopping sweating. Check your urine color before and after runs as a simple hydration gauge.
Can you drink too much water while running?
Yes. Overhydration, called hyponatremia, occurs when you drink so much water that your blood sodium levels drop dangerously low. This is especially risky during long, slow events. Symptoms include nausea, headache, confusion, and in severe cases, seizures. Drink to thirst and include sodium in your hydration plan for runs over 90 minutes.
How much should I drink before a run?
Drink about 5 to 7ml per kilogram of body weight 2 to 4 hours before running. For a 70kg runner, that is roughly 350 to 500ml. Then sip another 150 to 200ml in the 15 minutes before you start. Avoid chugging large amounts right before running as this can cause stomach discomfort.
How much fluid should I drink after running?
Aim to replace 150 percent of the fluid you lost during your run. A simple method is to weigh yourself before and after running. For every kilogram lost, drink 1.5 liters over the next 2 to 4 hours. Include sodium in your post-run fluids to help your body retain the water you drink.
Does humidity affect how much I need to drink?
Yes, high humidity significantly increases your hydration needs. When humidity is high, sweat does not evaporate as efficiently, so your body produces more sweat to try to cool down. This can increase your sweat rate by 20 percent or more. In humid conditions, start drinking earlier and drink more frequently.
What are electrolytes and why do runners need them?
Electrolytes are minerals like sodium, potassium, magnesium, and calcium that help regulate fluid balance, muscle contractions, and nerve signaling. You lose electrolytes through sweat, especially sodium. Replacing them during longer runs prevents cramping, maintains performance, and helps your body absorb and retain water more effectively.
How does temperature affect hydration needs?
Higher temperatures increase your sweat rate significantly. Running in 90F (32C) heat can increase your fluid needs by 50 percent compared to running in 60F (15C). Cold weather also has effects since dry cold air and layered clothing can cause dehydration that is easy to miss because you do not feel as thirsty.
Is this hydration calculator free?
Yes, this running hydration calculator is completely free with no sign up required. Calculate your personalized hydration plan for any run distance, weather condition, or intensity level. Bookmark this page and use it before every run.
More Running Tools
The Short Answer
Most runners lose somewhere between 0.5 and 1.5 liters of sweat per hour, and the general guideline is to replace 150 to 250ml (5 to 8oz) every 15 to 20 minutes during a run. Your actual number depends on your body weight, the temperature, the humidity, and how hard you are running, which is exactly what the calculator above works out for you. For anything over 60 minutes, add electrolytes to plain water, and for anything under 60 minutes in mild weather, water on its own is usually enough.
How This Calculator Works
The calculator starts from a baseline sweat rate of 0.8 liters per hour for a 70kg runner at a moderate effort in mild conditions, then adjusts that baseline with four multipliers: your body weight, the temperature, the humidity, and your running intensity.
Sweat Rate (L/hr) = 0.8 x (Your Weight in kg / 70) x Temperature Factor x Humidity Factor x Intensity Factor
From that hourly sweat rate, the tool derives your total fluid need for the full run duration, a drinking schedule in 15 minute intervals capped at 300ml per interval (drinking more than that at once rarely gets absorbed any faster), and a sodium target. Sodium starts at 500mg per liter-equivalent and scales with the same weight, temperature, and humidity factors, then gets a further 20 percent bump for hard efforts and races, since harder efforts increase sweat sodium concentration.
Temperature factor
| Temperature | Label | Multiplier |
|---|---|---|
| 90F and above | Very Hot | 1.5x |
| 80 to 89F | Hot | 1.35x |
| 70 to 79F | Warm | 1.15x |
| 60 to 69F | Mild | 1.0x |
| 50 to 59F | Cool | 0.85x |
| Below 50F | Cold | 0.7x |
Humidity factor
Low (dry air)
0.85x
Medium (comfortable)
1.0x
High (muggy)
1.2x
Intensity factor
Easy
0.7x
Moderate
1.0x
Hard
1.3x
Race Effort
1.5x
The post-run fluid recommendation applies a 150 percent replacement factor to whatever you lost during the run. That is not an arbitrary number, it matches the American College of Sports Medicine guidance that post-exercise rehydration should replace roughly 150 percent of body mass lost during exercise, since some of what you drink is lost again through continued sweating and urine output before your body fully re-equilibrates.
Worked Examples
Here is exactly what the calculator produces for four different runner profiles, so you can see how much the inputs actually move the output.
70kg runner, 60 minute run, 70F, medium humidity, moderate effort
A typical easy 10K on a mild day
80kg runner, 150 minute run, 88F, high humidity, hard effort
A hard long run in hot, humid marathon training conditions
55kg runner, 40 minute run, 55F, low humidity, easy effort
A short, easy recovery jog on a cool, dry day
65kg runner, 105 minute run, 75F, medium humidity, race effort
A half marathon race in warm conditions
Notice how much bigger the swing is from weather and effort than from body weight alone. The 80kg runner in hot, humid, hard conditions needs more than six times the hourly fluid of the 55kg runner in cool, easy conditions. This is why a generic "drink 500ml an hour" rule fails most runners most of the time.
What the Research Says
American College of Sports Medicine
"Sweat rates can range from about one liter per hour to as much as three liters per hour" and post-exercise rehydration "should be 150 percent of body mass loss during exercise."
View the ACSM Position Stand on Exercise and Fluid ReplacementKorey Stringer Institute, University of Connecticut
"Once an individual loses 2% of their body mass from fluid losses, impairments in performance are noticeable, and these impairments become more extreme with greater levels of dehydration."
Read the Korey Stringer Institute hydration guidancePrecision Hydration
"Most endurance athletes lose between 0.5 and 1.5 litres (16 to 48oz) of sweat per hour, depending on body size, intensity, heat, humidity, and acclimation status."
See the full sweat rate testing guideTrainingPeaks
"Estimating your sweat rate can be a useful exercise when you're trying to figure out how much and what you need to drink during training and events."
Read the TrainingPeaks sweat rate walkthroughWatch: How a Real Sweat Test Works
Global Triathlon Network walks through a full sweat test, from weigh-in to the math, so you can see exactly how the same logic behind this calculator is measured on a real athlete.
Where Does Your Sweat Rate Fall?
Once the calculator gives you a liters-per-hour number, here is roughly how sports scientists classify it, based on the ranges published by Precision Hydration.
| Sweat Rate | Category | What it means for you |
|---|---|---|
| Below 0.5 L/hr | Low | Water alone is usually enough even for longer runs, though you should still drink to thirst. |
| 0.5 to 1.5 L/hr | Low to moderate | This covers most recreational runners. Follow the calculator's drinking schedule and add electrolytes past 60 minutes. |
| 1.5 to 2.5 L/hr | High | You are a heavy sweater. Prioritize sodium replacement and consider a hydration vest or belt for anything past 45 minutes. |
| Above 2.5 L/hr | Very high | Rare but real, usually seen in hot, humid conditions at hard efforts. Plan aid station stops and carry electrolyte capsules for anything long. |
Run Your Own Sweat Test
The calculator gives you a good estimate, but a real sweat test measures your actual number. It takes one run and a bathroom scale.
- Empty your bladder and weigh in.
Weigh yourself nude or in dry underwear right before you head out. Record the number to one decimal place if your scale allows it.
- Weigh any fluid you plan to bring.
Weigh your bottle or flask before you leave. One ml of water weighs almost exactly one gram, so this converts straight to volume.
- Run for 60 to 90 minutes at real effort.
Use the pace and conditions you actually train or race in. A short, easy jog will not give you a representative number.
- Weigh yourself again immediately after.
Towel off sweat and weigh in before you shower, eat, or drink anything else.
- Weigh whatever fluid is left over.
Subtract the leftover weight from your starting fluid weight to find exactly how much you drank.
- Do the math.
Add your body weight lost to the fluid you drank, then divide by the hours you ran. That is your sweat rate in liters per hour. Repeat the test on a hot day and a cool day, since your rate can roughly double between the two.
Common Hydration Mistakes
- Chugging a huge amount right before the start. This overfills your stomach, causes sloshing and cramping, and most of it will not be absorbed in time. Spread pre-run fluid over the two hours before you start instead.
- Using the same plan for every run regardless of weather. A drinking schedule built for a 60F morning will underhydrate you badly on a 90F afternoon. Re-run the calculator whenever conditions change.
- Ignoring sodium on long, hot runs. Drinking large volumes of plain water without replacing sodium can dilute your blood sodium levels and increase your risk of hyponatremia on runs over 90 minutes.
- Waiting until you feel thirsty to start drinking. Thirst is a lagging indicator. By the time you notice it, you are already mildly dehydrated. Start on a schedule from the first 15 minutes of a longer run.
- Skipping the post-run rehydration. Recovery from a hard or hot run is slower if you do not replace fluid afterward. Aim for the 150 percent guideline in the hours after you finish, not just water during the run.
- Copying a training partner's hydration needs. Sweat rate varies enormously between individuals even at the same weight and pace. Your training partner's plan is not your plan.
- Overhydrating on purpose "to be safe." Drinking far beyond your sweat losses does not prevent dehydration any better and raises your risk of hyponatremia, which can be more dangerous than mild dehydration.
Dehydration and Overhydration Signs
Signs of dehydration
- Increased thirst and a dry mouth that does not go away.
- Darker urine color than your normal pale straw baseline.
- Unexplained fatigue or a pace that feels much harder than usual.
- Headache that develops mid-run or shortly after.
- Dizziness or lightheadedness, especially standing up quickly.
Signs of overhydration (hyponatremia)
- Nausea or vomiting despite feeling like you have drunk enough.
- Puffiness or swelling in the hands, feet, or face.
- Confusion or unusual disorientation late in a long event.
- A bloated, sloshing stomach from drinking faster than you can absorb.
- In severe cases, seizures, which require immediate medical attention.
Hydration in Different Conditions
Running in the rain
Rain cools you and can mask how much you are actually sweating, but your fluid losses do not disappear just because you feel cooler. If you are curious about the rest of your rain-day gear plan, see our guide to running in the rain.
Cold weather running
Cold, dry air increases fluid loss through breathing, and you sweat under layers even when you do not feel thirsty. Set the calculator's temperature slider to your real conditions rather than assuming cold weather means low fluid needs.
Treadmill and indoor running
Indoor air is often warmer and less ventilated than outdoor conditions, and there is no wind to help evaporate sweat. Many runners sweat noticeably more on a treadmill at the same effort than they would outside, so lean toward the higher end of the humidity setting.
Hydration Glossary
Sweat rate
The volume of fluid your body loses through sweat per hour of exercise, expressed in liters per hour.
Electrolytes
Minerals like sodium, potassium, magnesium, and calcium that regulate fluid balance and muscle function, and that you lose through sweat.
Hyponatremia
A dangerous drop in blood sodium levels caused by drinking far more fluid than your body loses, most common in long, slow events.
Heat acclimatization
The process by which your body adapts to heat over one to two weeks of repeated exposure, typically lowering your sweat sodium concentration and raising your sweat rate for better cooling.
More Questions Runners Ask
Does caffeine dehydrate you during a run?
Not meaningfully once you are a regular caffeine consumer. Older advice treated caffeine as a diuretic that worsened dehydration, but at typical pre-run doses it does not significantly affect fluid balance for habitual users. It is fine to count coffee toward your fluid intake for most runners.
How should I fuel alongside hydration on a long run?
Fluid and carbohydrate needs are separate but connected, since many sports drinks deliver both. Once you have your fluid plan from this calculator, pair it with a carbohydrate plan using our race fueling calculator.
Should my hydration plan change close to race day?
Yes. The two to three days before a race, many runners intentionally increase fluid and sodium slightly to start fully hydrated, then stick strictly to their tested in-race plan on race morning itself. Race day is not the time to try a new drink or gel for the first time.
Hydration is only half the fueling picture
See our full guide on what to eat before, during, and after running and our race recovery planner for what to do once you finish.
