Sweat rate test

What it does

A sweat rate test estimates how many mL of fluid you lose per hour at a given intensity in given conditions. It is the foundation for the hydration side of the Fueling planner and for race day fluid prescriptions.

URL: /calculators/ then open the Sweat rate test entry. Direct path: /static/calculators/sweat_rate_calculator.html.

Inputs the form asks for

• Weight before test (kg, ideally nude or in dry kit).
• Duration of workout (HH:MM or minutes).
• Quantity of fluid intake during the workout (in mL or oz).
• Weight after test (kg, same conditions as before).
• Sport: swim, bike, run. Drives small adjustments.

Outputs

• Sweat rate in mL per hour for the test conditions.
• (If sodium is logged separately) sweat sodium estimate.

How to run the test

The protocol that produces a useful number is consistent and boring. Skipping steps gives a useless answer.

1. Pre-test weighing. Strip to dry shorts (or get a baseline in your typical kit), wipe off any sweat, weigh accurately (kitchen scale to 0.01 kg if possible).
2. Workout of known duration and intensity. Pick something that mimics race intensity: 60 to 90 minutes of steady tempo on the bike, or a steady tempo run.
3. Log fluid intake to the mL. Take in normally, do not over- or under-drink during the test.
4. No urination during the test if possible (it adds noise).
5. Post-test weighing. Strip again, wipe off sweat, weigh to the same precision.
6. Plug numbers into the calculator.

Sweat rate equals (pre weight minus post weight) plus fluid intake, scaled to per hour by duration. The calculator does the math.

When to retest

• Once per season to set a baseline.
• Again on a hot day (sweat rate doubles in heat).
• After heat acclimation (sweat rate goes up, sweat sodium drops).
• If race conditions are very different from training (e.g. tropical destination from a temperate base).

See also: Triathlon fueling calculator, Race readiness checklist.