HYDRATION CALCULATOR

Sweat rate range:Published sweat rates in exercising athletes run from 0.3 to 2.5 L/hr (Baker, 2017; Maughan & Shirreffs, 2007). The variation is enormous — genetics, acclimatisation, fitness, body size, and clothing all contribute. The presets here represent population midpoints for each intensity band.

Why 75%, not 100%: Attempting to replace 100% of sweat loss during exercise frequently causes gastrointestinal distress. Current evidence (Noakes, 2012; ACSM position stand) recommends replacing 70–80% of losses, allowing mild dehydration (1–2% body mass) that does not impair performance in most conditions.

Sodium losses: Sweat sodium concentration varies from 200 to 2,000 mg/L between individuals (Baker et al., 2016). This calculator uses 700 mg/L as a population midpoint. If you are a visibly salty sweater (white residue on kit), your true value likely sits above 1,000 mg/L and you should increase sodium supplementation accordingly.

Body weight scaling: Larger riders have more surface area and typically lose more fluid per hour. The weight adjustment here is conservative — a rough 0.5–1.5% body mass loss per hour in hot conditions is consistent with field data (Sawka et al., 2007).

Limitations: Individual variation in sweat rate and composition is the single largest source of error. This calculator produces a reasonable starting point — not a prescription. The gold standard is a supervised sweat test: weigh yourself nude before and after a timed ride, subtract fluid consumed, and divide by duration. Do this in the conditions you race in. Once you know your number, use the advanced override to personalise the output.

Last updated: July 2026 · Tool version 1.0