Sweat Rate & Hydration Calculator for Trail Running

Free sweat rate calculator for trail runners. Measure it with a before and after weigh-in, or estimate it from heat and effort, then get ml/hour and sodium.

How It Works

Set the expected temperature and humidity
Enter the distance, elevation gain and how long you will be out
Add your body weight and effort level
Read your sweat rate estimate, drinking range, total fluid and sodium target
Optionally measure your real sweat rate: enter your weight before and after a run, the fluid you drank and how long you ran

Measure Your Sweat Rate

Sweat loss = (weight before - weight after) + fluid drunk - urine passed. Sweat rate is that divided by the hours you ran. Example: 70.0 kg before, 69.0 kg after and 500 ml drunk over 90 minutes is 1.5 L of sweat, or 1.0 L per hour. Published averages run from about 0.5 L per hour to more than 2.5 L per hour (ACSM position stand on exercise and fluid replacement), so measuring your own beats using an average.

The Model

Sweat rate starts from a 350 ml/hour baseline in cool conditions and rises by about 30 ml/hour per °C above 5°C. That is then scaled by:

Effort level — easy 0.80x, steady 1.00x, race effort 1.25x
Body mass — relative to a 70 kg reference, clamped between 0.80x and 1.30x
Humidity — dry 0.95x, moderate 1.00x, humid 1.15x
Vertical density — up to +12% for routes with heavy climbing per kilometre

Recommended intake is 65-85% of the estimated sweat rate, capped at 900 ml/hour because absorption during running tops out near a litre an hour.

Typical Numbers

Conditions	Drink per hour
Cool (under 10°C), easy effort	300-450 ml
Mild (15°C), steady effort	400-550 ml
Warm (22°C), steady effort	550-700 ml
Hot (30°C), race effort	750-900 ml

Sodium

Sweat sodium concentration varies from roughly 300 to 1800 mg per litre between individuals. This tool uses 400-700 mg/L as a baseline, rising to 500-900 mg/L when it is hot or the effort goes long. Salt crust on your face after long runs suggests you are at the high end.

Key Principles

Do not try to replace 100% of sweat losses. Keeping total loss under about 2% of body mass is the target, not zero deficit.
Drinking plain water for many hours is how hyponatraemia happens. Past four hours, sodium matters more than volume.
Under an hour in mild conditions, drinking to thirst before and after is enough.
Weigh yourself before and after long runs to calibrate: each kilogram lost is roughly a litre of sweat not replaced.

FAQ

How do I calculate my sweat rate? Weigh yourself before and after a run and note what you drank. Sweat loss is the weight you lost plus the fluid you drank (minus any urine), divided by the hours you ran. The calculator has a section that does the sum for you.

How much water should I drink per hour when trail running? Most trail runners need 400-800 ml per hour. Cool and easy sits at the bottom, hot and hard at the top.

Does elevation gain change how much I need? Yes. Climbing raises metabolic cost, and steep terrain means slow travel with less airflow, so evaporative cooling drops. The calculator adds up to 12%.

Is my data stored? No. The calculator runs entirely in your browser and sends nothing.