Triathlon Hydration Calculator
Get an hourly fluid and sodium target for your triathlon, calibrated to body weight, race duration, and expected temperature. Use it as a starting point — then refine with a real sweat rate test.
How much should I drink during a triathlon?
- • Baseline target: 6–17 ml per kg body weight per hour, depending on temperature.
- • Pair fluid with sodium — plain water alone raises your risk of hyponatremia on long races.
- • In hot conditions, aim for the top of the range and include a sodium source in every bottle.
- • The figures below are starting points — test in training, not on race day.
Your Inputs
Fluid-per-hour table
| Weather | Fluid per hour (per 70kg athlete) | Sodium target |
|---|---|---|
| Cool (<15°C) | 400–500 ml | 300–500 mg/hr |
| Moderate (15–25°C) | 600–750 ml | 500–800 mg/hr |
| Hot (25–30°C) | 800–1000 ml | 800–1200 mg/hr |
| Very hot (>30°C) | 1000–1400 ml | 1000–1500 mg/hr |
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Last updated: 2026-04-16Reviewed by: Bryan Collins, founder
How this calculator works
Fluid per hour = body weight (kg) × a weather-adjusted ml/kg coefficient (6–17). Sodium per hour is scaled to fluid and weather. These numbers are population starting points — a sweat rate test gives a more accurate personal value.
Assumptions
- • Normal body composition and no medications that affect sweat or sodium loss.
- • Linear scaling of sweat rate with temperature within each weather band.
- • Sodium targets assume isotonic or moderately hypotonic mixes (not pure water).
Limitations
- • Does not account for humidity, sun exposure, wind, or altitude.
- • Individual sweat rates vary 2–3× at the same temperature.
Who this is for
- • Triathletes without a measured sweat rate who need a safe starting point.
- • First-timers planning how much fluid to carry on bike and run.
- • Age-groupers racing in unfamiliar climates (travel races).
Who this is not for
- • Athletes with a known sodium loss profile — use the Electrolyte Calculator instead.
- • People with clinical conditions affecting fluid or electrolyte balance.