Endurance hydration
Fluid intake per hour calculator.
From your sweat rate, body mass and duration, see how much fluid per hour keeps estimated body mass loss within a chosen limit, and how that compares with the published race range.
Inputs
Fluid plan
Enter your sweat rate.
How much should I drink per hour when running?
The ACSM position stand on exercise and fluid replacement says the goal of drinking during exercise is to prevent excessive dehydration, more than 2% of body weight lost from water deficit, and excessive changes in electrolyte balance. It also says sweat rates vary considerably between individuals, so customized plans are recommended, and that individual sweat rates can be estimated by measuring body weight before and after exercise. The calculation here is the difference: total sweat over the duration, minus the loss you allow (2% of body mass by default), spread over the hours.
The ISSN ultra-marathon stand gives 450 to 750 mL per hour (about 150 to 250 mL every 20 minutes) as a race range and warns that drinking fluid with insufficient electrolytes, such as plain water, can cause overhydration and dilutional hyponatraemia. Drinking more than you sweat is not the aim: this tool never suggests more than your sweat rate. It treats 1 kg of body mass as 1 litre of fluid, a simplification that ignores the mass lost through breathing and fuel burned, so treat the result as an approximation.
Frequently asked questions
How much water should I drink per hour while running?
There is no single answer. ACSM recommends an individualized plan based on your sweat rate and aims to prevent more than 2% body weight loss. For ultra racing the ISSN gives 450 to 750 mL per hour. Use your sweat rate and conditions, and practise in training.
Should I replace all of my sweat?
Not necessarily. The ACSM goal is to avoid losing more than 2% of body weight from water deficit, not to match every millilitre. Drinking more than you sweat risks overhydration and hyponatraemia.
What is a normal sweat rate?
It varies widely with fitness, heat, humidity, clothing and intensity, which is why the sweat rate calculator uses your own weigh-in. Test in conditions like your race.
Does this include sodium?
No. It is a fluid volume only. Plain water does not replace sodium lost in sweat; the electrolyte drink mix calculator covers the concentration.
Sources
Checked
- Sawka et al., ACSM position stand: Exercise and fluid replacement, Med Sci Sports Exerc 2007
- Tiller et al., ISSN position stand: nutritional considerations for single-stage ultra-marathon training and racing, J Int Soc Sports Nutr 2019
Ranges are quoted from the sources above and are general research or position-statement figures. Gel, powder and drink contents, spare gels and stop intervals are your own inputs; defaults are placeholders. One kilogram of body mass is treated as one litre of fluid.
Scope
Education only, not medical or dietetic advice. Needs differ by person, event, heat and training, and the published ranges are general. Try any fuelling or hydration plan in training, not for the first time on race day. If you have a medical condition (including diabetes, kidney or heart disease or high blood pressure), are pregnant or breastfeeding, or are under 18, ask your doctor or a registered dietitian first. Stop and get medical help for confusion, vomiting, severe headache, fainting or swelling of the hands and feet during or after a long effort.