Sweat keeps you cool, but losing too much fluid raises heart rate, makes effort feel harder and increases heat-illness risk. Drinking far more than you lose is not the safe alternative either. This guide turns the American College of Sports Medicine’s position stand “Exercise and Fluid Replacement” (Sawka et al., 2007) and the later hyponatremia consensus into a simple, personal plan, with metric and US units side by side.
Not medical advice. This article is general information. If you have heart, kidney or blood-pressure conditions, or take diuretics, agree your fluid and salt intake with your doctor.
What the evidence says in 2026
ACSM’s fluid position stand dates from 2007, and as of 2026 ACSM has not replaced it with a newer fluid-specific stand. Its core guidance was restated in the joint ACSM / Academy of Nutrition and Dietetics / Dietitians of Canada position “Nutrition and Athletic Performance” (Thomas et al., 2016). The risk of overdrinking is covered by the Third International Exercise-Associated Hyponatremia Consensus Statement (Hew-Butler et al., 2015). In the US, the National Athletic Trainers’ Association published its own fluid statement in 2017 with the same overall direction.
The shared message:
- Start euhydrated (normally hydrated).
- Avoid losing more than 2% of body mass from water deficit during exercise.
- Don’t drink more than you sweat, so you never finish heavier than you started.
Before exercise: 5–7 mL per kg
ACSM suggests drinking 5–7 mL per kg of body weight at least about 4 hours before exercise. If you produce no urine or it is dark, add another 3–5 mL/kg about 2 hours before. A little salt (a salty snack or a drink with sodium) helps you retain it. In US units, 5–7 mL/kg works out to roughly 0.08–0.11 fl oz per pound.
| Body weight | 5 mL/kg | 7 mL/kg |
|---|---|---|
| 60 kg / 132 lb | 300 mL / 10 fl oz | 420 mL / 14 fl oz |
| 70 kg / 154 lb | 350 mL / 12 fl oz | 490 mL / 17 fl oz |
| 80 kg / 176 lb | 400 mL / 14 fl oz | 560 mL / 19 fl oz |
| 90 kg / 198 lb | 450 mL / 15 fl oz | 630 mL / 21 fl oz |
So a 154 lb runner heading out at 6 pm needs roughly one 16 oz bottle around 2 pm, not a gallon chugged in the car park.
Measure your sweat rate: worked example
Sweat rates vary widely, from about 0.5 to more than 2 L per hour according to ACSM. The easiest way to find yours is a scale:
Sweat loss = (pre-exercise weight − post-exercise weight) + fluid drunk − urine passed
One kilogram of weight lost is about one litre of fluid; one pound is roughly 16 fl oz (a pint). Weigh in minimal, dry clothing and towel off before the second weigh-in.
Metric example: 60-minute tempo run
- Before: 70.0 kg; after: 68.8 kg → loss 1.2 kg (≈ 1.2 L)
- Drunk during run: 0.5 L; urine during run: 0.1 L
Sweat loss = 1.2 + 0.5 − 0.1 = 1.6 L in one hour, so 1.6 L/h. Body-mass loss = 1.2 ÷ 70 = 1.7%, just under the 2% line (1.4 kg for 70 kg).
US example: same run in pounds
- Before: 154.0 lb; after: 151.4 lb → loss 2.6 lb (≈ 2.6 × 16 = 41.6 fl oz)
- Drunk: 16 fl oz; urine: 4 fl oz
Sweat loss = 41.6 + 16 − 4 = 53.6 fl oz per hour (about 1.6 L). Loss = 2.6 ÷ 154 = 1.7%; 2% of 154 lb is 3.08 lb.
Next time, drinking about 24–27 fl oz (0.7–0.8 L) per hour instead of 16 would keep this runner closer to 1% loss. You do not need to replace all 1.6 L per hour; few stomachs tolerate that. Repeat the test on a cool morning and a hot afternoon, because a July run in Phoenix is not a January run in Manchester. To match fluid to effort, check energy use with the calories burned calculator and plan session length with the pace calculator.
During exercise: how much and what
ACSM’s rough starting range for distance runners is 0.4–0.8 L per hour (about 14–27 fl oz): faster, heavier runners in heat towards the top, slower, lighter runners in cool weather towards the bottom.
- Under 60 minutes, moderate effort: drinking to thirst with water is fine for most people.
- Longer than 60–90 minutes or hot conditions: plan small, regular sips every 15–20 minutes based on your sweat rate.
- Team sports (soccer, basketball, rugby, cricket): use breaks, timeouts and substitutions; keep your own bottle on the bench.
Sodium and electrolytes
Sodium is the main electrolyte lost in sweat. ACSM 2007 cites an average around 35 mmol/L, with a wide individual range of about 10–70 mmol/L; white salt marks on a cap or shirt suggest you are a salty sweater. For long sessions ACSM considers drinks with roughly 20–30 mmol/L sodium, 2–5 mmol/L potassium and 5–10% carbohydrate useful. For a one-hour gym session, a sports drink is usually unnecessary sugar.
The overdrinking risk: hyponatremia
Exercise-associated hyponatremia (EAH) happens when blood sodium is diluted, usually by drinking more than you lose. Early signs include headache, nausea, bloating and confusion; severe cases have caused seizures and deaths. It is most common in long, slow events (marathons beyond about four hours, ultras, long hikes) and in people who drink at every aid station “to be safe”.
The 2015 consensus recommends drinking to thirst. The practical check: if you weigh more after exercise than before, you overdrank. On long days, use salty snacks or sodium drinks rather than plain water alone.
Heat and recovery
Training in summer heat multiplies sweat loss and heat strain:
- Move sessions to early morning or evening.
- Cut volume during the first hot days; heat acclimatisation takes roughly 1–2 weeks.
- Stop at dizziness, cramps, nausea or confusion, get to shade and call 911 (US) or 999 (UK) if symptoms are severe.
After exercise, ACSM suggests about 1.5 L per kg lost (roughly 24 fl oz per pound), because part of what you drink is passed as urine. For the 1.2 kg example that is 1.2 × 1.5 = 1.8 L; in pounds, 2.6 × 24 = 62 fl oz. Spread it over a few hours, ideally with a salty meal. If you are not training hard again the same day, normal meals and drinks usually restore balance.
Quick checklist
| When | What |
|---|---|
| ~4 h before | 5–7 mL/kg (0.08–0.11 fl oz/lb) |
| ~2 h before, if urine is dark | +3–5 mL/kg |
| During | Per sweat rate; keep loss < 2%; start at 0.4–0.8 L/h (14–27 fl oz/h) |
| Long or hot sessions | Drinks with sodium and carbohydrate |
| After | ~1.5 L per kg lost (~24 fl oz per lb) |
| Red flag | Finishing heavier than you started |
Sources
- ACSM, “Exercise and Fluid Replacement” position stand, Sawka et al., Medicine & Science in Sports & Exercise, 2007.
- Thomas, Erdman, Burke, “Nutrition and Athletic Performance”, joint ACSM/AND/DC position, 2016.
- Hew-Butler et al., Third International Exercise-Associated Hyponatremia Consensus, Clinical Journal of Sport Medicine, 2015.
- McDermott et al., NATA position statement on fluid replacement, Journal of Athletic Training, 2017.