Sport Hydration Need Calculator
Knowing your sweat rate allows you to personalise fluid replacement strategies during training and competition. This calculator uses the American College of Sports Medicine (ACSM) pre-post exercise weighing method to determine your sweat rate and recommended fluid intake per hour. Weigh yourself before and after a training session without urinating, note fluid consumed, and enter the values below.
Sweat rate formula (ACSM)
Fluid deficit (L) = pre-exercise weight - post-exercise weight
Sweat loss (L) = fluid deficit + fluid consumed during exercise
Sweat rate (L/hr) = sweat loss / exercise duration (hours)
Recommended intake (mL/hr) = sweat rate x 1000 x 0.80
The 0.80 factor reflects ACSM guidance to replace 80 percent of sweat losses during exercise for most endurance athletes, avoiding both dehydration and overhydration.
Hydration guidelines
- Deficit under 1 percent of body mass: minimal performance impact.
- Deficit of 1-2 percent: modest performance and cognitive impact; increase fluid intake.
- Deficit over 2 percent: significant performance impairment; revise hydration strategy.
- Sweat rate above 2 L/hr: common in heat; electrolyte replacement is critical.
- Rehydrate post-exercise with 1.5 L of fluid per kg of weight lost, as recommended by ACSM.
Sport hydration: frequently asked questions
How is sweat rate calculated?
Sweat rate (L/hr) = (pre-exercise weight - post-exercise weight + fluid consumed during exercise) / exercise duration in hours. This is the standard ACSM method using body mass change to estimate sweat loss. A 1 kg drop in body mass equals approximately 1 litre of fluid loss.
How much should I drink during exercise?
The ACSM recommends drinking to prevent dehydration exceeding 2 percent of body mass, while avoiding overhydration (hyponatremia). For most athletes, 400-800 mL per hour is appropriate, with higher rates in heat and for athletes with high sweat rates. Drink to thirst for sessions under 60 minutes.
Do I need electrolytes as well as water?
For sessions longer than 60-90 minutes, especially in heat, sodium replacement is important. Sweat sodium concentration averages approximately 920 mg/L (range 230-1,750 mg/L). The ACSM recommends consuming 300-600 mg of sodium per hour of exercise in prolonged events to prevent hyponatremia.
What is dehydration and why does it matter?
Dehydration is a fluid deficit relative to body mass. Dehydration of 1-2 percent of body mass impairs cognitive performance. At 2 percent, aerobic performance begins to decline. At 3-5 percent, heat illness risk increases significantly. The ACSM recommends beginning exercise fully hydrated (urine pale yellow).
How do I know if I am well hydrated before exercise?
The simplest check is urine colour. Pale straw yellow indicates good hydration. Dark yellow or amber indicates moderate dehydration. The ACSM also recommends consuming 5-7 mL/kg of fluid in the 2-4 hours before prolonged exercise to ensure adequate pre-exercise hydration status.
Official sources
- American College of Sports Medicine (ACSM), Sawka MN et al. (2007). Exercise and Fluid Replacement. Medicine and Science in Sports and Exercise, 39(2), 377-390. PubMed 17277604.
- Thomas DT, Erdman KA, Burke LM (2016). American College of Sports Medicine joint position statement: nutrition and athletic performance. Medicine and Science in Sports and Exercise, 48(3), 543-568. PubMed 26891166.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.