Hydration During Exercise: How Much to Drink

Why Exercise Hydration Is Critical for Performance

During exercise, the body generates heat as a by-product of muscle contraction. Sweating is the primary cooling mechanism — but it comes at the cost of fluid and electrolyte losses that, if not replaced, progressively impair every aspect of physical performance.

Sweat rates during exercise typically range from 0.5 to 1.5 litres per hour under moderate conditions, and can exceed 2.5 litres per hour during high-intensity training in hot, humid environments. The resulting fluid deficit accumulates quickly: in a 70 kg athlete, losing just 1.4 litres — equivalent to a 2% body weight deficit — is sufficient to reduce endurance capacity by 7–10%, impair strength and power output, and increase the subjective perception of effort.

At 5% body weight loss, physical capacity can decline by up to 30%. Beyond this threshold, the risk of heat illness — including heat exhaustion and heat stroke — increases substantially. Effective hydration during exercise is therefore not a comfort issue, but a performance and safety requirement.

Based on: Sawka MN, Burke LM, Eichner ER, et al. "American College of Sports Medicine Position Stand: Exercise and Fluid Replacement." Medicine & Science in Sports & Exercise. 2007;39(2):377–390. PubMed

ACSM Guidelines: Before, During, and After Exercise

The American College of Sports Medicine's Position Stand on exercise and fluid replacement provides the most widely adopted evidence-based hydration protocol for athletes and active individuals. The recommendations are structured around three phases:

Before Exercise

Begin exercise in a euhydrated (adequately hydrated) state. The ACSM recommends consuming 400–600 ml (approximately 2 standard glasses) of water in the 2–3 hours before exercise. This allows time for the fluid to be absorbed and excess to be excreted before training begins. Check urine colour before your session — pale yellow confirms adequate pre-exercise hydration.

During Exercise

The goal during exercise is to prevent dehydration from exceeding 2% of body weight. The ACSM recommends consuming 150–250 ml (approximately one cup) every 15–20 minutes during continuous exercise. This translates to approximately 450–1,000 ml per hour depending on intensity and conditions. Because individual sweat rates vary substantially, the ideal intake should be guided by your personal sweat rate calculation (see Section 3).

After Exercise

Full rehydration after exercise requires replacing approximately 1.5 times the fluid volume lost — not a 1:1 replacement — because ongoing urinary losses occur during recovery. The practical method is to weigh yourself before and after exercise: each kilogram of body weight lost corresponds to approximately 1 litre of fluid deficit. Multiply that volume by 1.5 to get your rehydration target.

How to Calculate Your Sweat Rate

Individual sweat rates vary significantly based on genetics, fitness level, body size, clothing, and environmental conditions. Calculating your personal sweat rate takes one measurement session and gives you a precise hydration target for future training in similar conditions.

Method:

1. Weigh yourself (nude or minimal clothing) immediately before exercise.
2. Train for a fixed duration (e.g., 60 minutes) while tracking all fluid consumed in millilitres.
3. Weigh yourself again immediately after, before any post-exercise fluid intake.
4. Apply the formula: Sweat loss = (pre-weight − post-weight) kg × 1,000 ml + fluid consumed (ml)

Example: pre-weight 75.0 kg, post-weight 74.2 kg, fluid consumed 500 ml. Sweat loss = (0.8 × 1,000) + 500 = 1,300 ml in 60 minutes. This athlete should target approximately 325 ml every 15 minutes during training in similar conditions.

Based on: Shirreffs SM. "Hydration in sport and exercise: water, sports drinks and other drinks." Nutrition Bulletin. 2009;34(4):374–379.

Water vs. Sports Drinks: When Do Electrolytes Matter?

For the majority of exercise sessions, plain water is the optimal choice. However, for prolonged or high-intensity exercise lasting more than 60 minutes, the case for electrolyte replacement becomes physiologically meaningful.

Sweat contains not just water but significant quantities of sodium (the primary electrolyte in sweat), along with smaller amounts of potassium, magnesium, and chloride. During prolonged exercise, cumulative sodium losses increase the risk of hyponatremia — a dilutional drop in blood sodium — if large volumes of plain water are consumed to replace sweat losses without electrolyte replacement.

Commercial sports drinks typically contain 6–8% carbohydrates and 400–1,100 mg of sodium per litre — a concentration designed to promote rapid fluid absorption while providing an energy substrate. Drinks with carbohydrate concentrations above 10% slow gastric emptying and are not recommended during exercise.

For a deeper look at the role of sodium, potassium, and other electrolytes in hydration, see our guide on electrolytes and hydration.

Signs You Are Not Hydrating Enough During Exercise

The following warning signs indicate that fluid intake during a session has been insufficient. Recognising these early allows corrective action before performance and safety are seriously compromised:

• Thirst during exercise — indicates mild dehydration has already begun; drink immediately and increase frequency for the remainder of the session.
• Headache or dizziness — common at 2–3% body weight deficit; reduce intensity and prioritise fluid intake.
• Muscle cramps — can indicate both dehydration and electrolyte imbalance, particularly sodium and potassium deficiency.
• Reduced urine output after exercise — dark amber or orange urine post-training confirms significant dehydration requiring structured rehydration.
• Disproportionate fatigue — performance dropping more than expected given training intensity is a common but often unrecognised sign of inadequate hydration during exercise.

For a complete breakdown of dehydration symptoms by severity, see our guide on signs of dehydration.