There is no single correct hourly water or electrolyte dose for every runner. Start the run normally hydrated, drink to thirst rather than forcing a preset volume, and adjust for duration, heat, humidity, pace, sweat rate and access to fluids. For runs lasting more than about an hour, a carbohydrate-electrolyte drink can be useful, but its sodium does not make overdrinking safe. The goal is to limit both excessive dehydration and weight gain from drinking beyond your losses.
Use the ranges below as starting points, then refine them with body-weight and training observations. The 2007 American College of Sports Medicine (ACSM) position stand recommends individualized replacement because sweat rate and sweat-electrolyte concentration vary widely between people and conditions (ACSM, 2007).
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What “enough” means on a long run
A successful hydration plan keeps fluid loss from becoming excessive without replacing more fluid than you lose. The ACSM discussion uses more than 2% body-weight loss from water deficit as a goalpost for avoiding excessive dehydration, while still emphasizing that plans must be individualized (ACSM, 2007). That percentage is a planning reference, not a command to drink until the scale shows exactly the same number.
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- Before running: arrive normally hydrated; do not “bank” water by force.
- During running: drink according to thirst and practical access to fluids, with more attention when the route is hot, humid or unusually long.
- Electrolytes: consider sodium and carbohydrate for prolonged or high-sweat sessions, but treat published ranges as starting guidance rather than a personal prescription.
- After running: replace fluids and food gradually, and investigate unusual symptoms instead of trying to correct them by rapidly drinking.
How much to drink before a long run
An older ACSM position stand suggests about 500 mL (approximately 17 US fl oz) around two hours before exercise (ACSM, 1996). This is a pre-exercise suggestion, not a during-run target and not a requirement for every person. It gives the body time to absorb fluid and allows a bathroom stop if needed.
Use a normal-hydration check
Drink normally with meals and through the day before the run. Pale-yellow urine and the absence of unusual thirst are practical signs that you are not starting obviously dehydrated, although urine color is affected by food, supplements and medications. Avoid beginning a run after deliberately restricting fluids, but also avoid consuming several large bottles immediately before the start.
Adjust for the morning and the route
If the run starts in heat, follows a long flight or work shift, or occurs after illness, your usual routine may not apply. Plan access to a drink, shorten the session, or move it to a cooler time rather than attempting to pre-hydrate aggressively. People with kidney, heart or endocrine disease, or those taking medicines that affect fluid or sodium balance, should obtain individualized medical advice before changing intake.
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There is no evidence-based universal “X milliliters every hour” rule that suits all runners. Sweat losses can differ several-fold between people and can change with temperature, humidity, clothing, pace, terrain and acclimatization. The 2015 consensus therefore supports thirst-guided drinking as a strategy that supplies fluid while limiting excess intake (2015 EAH consensus).
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Runs under about one hour
For many runners, drinking to thirst is sufficient, and carrying fluid may be optional when conditions are mild and water is available at the finish. The 1996 ACSM statement found little evidence of a performance difference between water and a sports drink for exercise under one hour (ACSM, 1996). Heat, a very fast effort or unusually high sweat loss can still justify carrying a drink.
Runs longer than one hour
As duration and intensity increase, carbohydrate and electrolytes may support performance and tolerance. The 1996 ACSM guidance discusses 30–60 g of carbohydrate per hour during intense exercise lasting longer than one hour to help maintain carbohydrate oxidation and delay fatigue (ACSM, 1996). That carbohydrate range is a fueling strategy, not a hydration requirement. You can obtain it from a sports drink, gels and food, or a combination, provided your stomach tolerates it.
Drink small amounts when thirsty rather than waiting until you are distressed or forcing a bottle empty on a schedule. On a supported route, take only what you expect to use; at an aid station, a few mouthfuls may be enough. If you finish repeatedly with weight gain, bloating, sloshing or no thirst despite heavy drinking, your plan is too aggressive.
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Very long, hot or humid runs
Ultramarathon guidance recognizes that prolonged sweating in hot or humid conditions can increase sodium needs, but it does not establish one dose for everyone. The International Society of Sports Nutrition (ISSN) position stand gives 500–700 mg sodium per liter of fluid as a long-duration running strategy and describes approximately 300–600 mg sodium per hour in hot or humid conditions (ISSN, 2019). These are context-specific ranges. They should be adjusted for how much you actually drink, your sweat concentration, food intake, weather and gastrointestinal tolerance.
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Estimate your own sweat rate
A simple field assessment is more useful than adopting someone else’s bottle schedule. ACSM describes comparing body weight before and after exercise as one way to estimate sweat rate (ACSM, 2007).
- Choose a representative session. Use a run of at least an hour in conditions similar to your planned race. Record temperature, humidity, clothing, pace and terrain.
- Weigh yourself immediately before. Weigh with minimal clothing, after using the bathroom, and record the result. Keep the scale and method consistent.
- Record every drink. Measure the volume you carry and note what remains. Include sports drink, water and any other fluid.
- Record urine. If you stop to urinate, estimate or measure the amount; otherwise note that there was none.
- Weigh immediately after. Towel off sweat, remove wet clothing and use the same scale. Do not drink or eat before this measurement if you want a clean comparison.
- Interpret the change. A lower post-run weight indicates net fluid loss; a higher weight suggests you drank more than you lost. Compare several sessions before changing your plan.
A practical calculation is: estimated sweat loss equals body-weight loss plus fluid consumed minus urine passed. Divide that amount by run time for an approximate hourly loss. Treat the result as an estimate: food in the stomach, scale error and sweat left on clothing can all distort it. The purpose is to identify whether your intake is plainly too low or too high, not to chase a mathematically perfect replacement rate.
How to decide whether you need electrolytes
Sodium is the electrolyte most relevant to sweat replacement, but sweat sodium varies substantially. A salty film on clothing, visible salt residue, heavy sweating and long exposure to heat suggest that sodium deserves attention; none of these signs alone measures your exact loss.
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Published sodium ranges and their limits
| Guidance | Published amount | How to use it |
|---|---|---|
| ACSM position stand (1996) | 0.5–0.7 g sodium per liter of water during exercise lasting longer than one hour | Historical position-stand guidance; not a universal current prescription for every runner. |
| ISSN ultramarathon position stand (2019) | 500–700 mg sodium per liter of fluid; approximately 300–600 mg per hour in hot or humid conditions | Context for prolonged running, especially heat; adjust for personal sweat, drinking volume and food. |
| Sports-drink comparison in the 2015 EAH consensus | Typical sports drinks: 10–38 mmol/L sodium; serum sodium: approximately 140 mmol/L | Shows why a sports drink cannot make excessive drinking safe. |
The older ACSM concentration is numerically equivalent to 500–700 mg per liter, but keep its 1996 date and context visible. A bottle’s sodium concentration is not the same as the amount you should consume per hour: the hourly amount depends on how much fluid you actually drink. Sodium also comes from gels, food, broth and other products.
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When plain water is reasonable
Water is usually adequate for shorter, easier runs, for runners who are not heavy sweaters, and when normal meals provide sodium. It is also useful when you are thirsty but do not need additional carbohydrate. Do not use thirst as a reason to avoid all drinking in severe heat; it is a safeguard against excess, not a guarantee against every form of dehydration.
When a carbohydrate-electrolyte drink may help
For runs beyond an hour, especially at race intensity, a drink can combine fluid, carbohydrate and sodium in one portable option. Choose a concentration you have practiced, because high sweetness or a large volume can cause nausea or diarrhea. A product’s label tells you its sodium per serving and carbohydrate per serving; use those numbers to calculate the total you will consume, not simply the number of bottles.
Water versus an electrolyte sports drink
| Situation | Practical starting choice | Reason and caution |
|---|---|---|
| Easy run under one hour, mild weather | Drink to thirst; water may be enough | Older ACSM guidance found little performance difference between water and sports drink for exercise under one hour (ACSM, 1996). |
| Hard run over one hour | Water plus separate carbohydrate, or a carbohydrate-electrolyte drink | 30–60 g carbohydrate per hour is an older ACSM strategy for intense exercise longer than one hour; practice gut tolerance. |
| Long run in heat or humidity | Thirst-guided fluid with planned sodium from drink and food | ISSN gives 500–700 mg/L and about 300–600 mg/hour as contextual ultramarathon guidance, not an exact prescription (ISSN, 2019). |
| History of drinking beyond thirst or finishing heavier | Reduce forced drinking; use thirst and reassess | Sodium-containing drinks do not prevent EAH when total fluid exceeds losses (2015 EAH consensus). |
| Limited fluid access | Plan a carry, refill point or shorter loop | Do not compensate for an empty bottle by consuming a large volume at once. |
A practical planning method
- Classify the run. Note expected duration, intensity, temperature, humidity, elevation and whether you can refill.
- Start normally hydrated. If useful, use the 500 mL about two hours before exercise suggestion from ACSM’s 1996 statement, but do not force it if it causes discomfort.
- Set a thirst-led drinking plan. Carry enough for the route, take small drinks when thirsty, and avoid emptying a bottle simply because a clock says it is time.
- Add fuel for duration. For intense sessions beyond an hour, work toward 30–60 g carbohydrate per hour if tolerated, using drink, gels or food.
- Add sodium selectively. In prolonged or hot conditions, compare the label with the ISSN context ranges and account for sodium in food and other products.
- Test in training. Repeat the same plan in comparable weather, record thirst, gastrointestinal symptoms, body weight and performance, then make one change at a time.
- Reassess after the run. Persistent weight gain, swelling, headache or nausea after heavy drinking is a reason to stop experimenting and seek medical guidance.
Recognize overhydration and dehydration emergencies
EAH can occur during or after exercise, particularly when a runner drinks beyond losses. Early symptoms can resemble ordinary fatigue: headache, nausea, vomiting, bloating, dizziness, confusion or unusual weakness. Severe EAH can progress to altered mental status, seizures, breathing problems or collapse. The 2015 consensus treats suspected EAH as requiring urgent medical assessment (2015 EAH consensus).
Do not try to treat suspected EAH by rapidly drinking more water or sports drink. Stop running, alert race staff or emergency services, and report how much you drank and when symptoms began. A runner who is confused, has a severe headache, repeatedly vomits or has a seizure needs immediate emergency care.
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Severe dehydration and heat illness also require urgent help, especially with fainting, inability to sweat, hot skin, confusion or collapse. Hydration planning cannot replace pacing, shade, cooling and medical evaluation.
Common mistakes that make a plan less safe
- Copying another runner’s bottle schedule: their sweat rate, body size, pace and weather may be different.
- Treating sodium as insurance: sodium in a sports drink is far below blood sodium concentration, so excess volume can still dilute blood sodium.
- Confusing concentration with hourly intake: 600 mg/L means 600 mg only if you drink one liter; the total changes with volume.
- Ignoring food: gels, bars and meals contribute carbohydrate and sodium.
- Trying a new mix on race day: gastrointestinal tolerance must be practiced during training.
- Using thirst as a rigid ceiling: in extreme heat, make fluids available and monitor yourself; thirst is a protection against overdrinking, not a complete heat-safety system.
- Chasing a zero weight change: the aim is to limit excessive loss without gaining weight, not to force exact replacement.
How to turn the numbers into a personal plan
Suppose a two-hour training run in warm weather produces a consistent estimated sweat loss of 1.2 liters after accounting for drinks and urine. A sensible next session is not automatically 1.2 liters per hour. Instead, use thirst to decide how much of the available fluid to drink, keep the route supplied, and check whether post-run weight remains lower rather than higher. If the same session repeatedly ends with substantial weight loss, increase access and modestly increase drinking; if it ends heavier with bloating, reduce forced intake.
For sodium, a drink containing 600 mg per liter fits within the ISSN’s 500–700 mg/L ultramarathon context, but the amount consumed per hour depends on your drinking. A runner taking 400 mL per hour would receive about 240 mg sodium from that drink, before counting food. Another runner drinking 800 mL per hour would receive about 480 mg. Neither figure is automatically right; both must be judged against conditions, sweat, tolerance and symptoms.
This iterative approach is safer than selecting a universal formula. Keep a brief log for several long runs: weather, duration, drinks, carbohydrate, sodium, bathroom stops, pre- and post-run weight, thirst, stomach comfort and performance. Patterns across similar sessions are more informative than one unusually hot or cool day.
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