How Sweat-Rate + Electrolyte Marathon Plan works

Scope

Computes per-hour sweat rate, recommended in-race fluid intake, and sodium replacement target from a single weighed-in training session. Designed for marathon-length efforts and informed by the climate the race is run in.

Formula

sweat_loss_kg = pre_weight − post_weight + fluid_consumed_kg − urine_kg (we set urine to 0 for <90-min sessions; user can post-correct).

1 kg of sweat ≈ 1 L. sweat_rate_L_per_hour = sweat_loss_L / duration_hours.

Sodium loss per hour = sweat_rate_L × climate_sodium_concentration:

• Cool (<15°C): 600 mg/L
• Temp (15–25°C): 920 mg/L (GSSI median)
• Hot (>25°C): 1300 mg/L

Recommended in-race fluid = min(800 mL/h, 80% of measured sweat rate). The 800 mL/h cap reduces exercise-associated hyponatremia risk per IMMDA 2015.

Data sources

1. Wagner DR, Heyward VH. Fluid balance and dehydration. In: Antonio J et al, eds. Essentials of Sports Nutrition and Supplements. 2008. — Sweat math and weight-loss percentage banding.
2. Maughan RJ, Shirreffs SM. Dehydration and rehydration in competitive sport. Scand J Med Sci Sports. 2010;20(s3):40-47. — Sodium concentration ranges across climates.
3. Hew-Butler T, Rosner MH, Fowkes-Godek S, et al. Statement of the third international exercise-associated hyponatremia consensus development conference. Clin J Sport Med. 2015;25(4):303-320. — PMID 26095810. The 800 mL/h intake cap and EAH risk profile.

Assumptions

• Pre/post weights are measured nude or in dry clothes — wet kit can mask 200–500 g of sweat retention.
• 1 kg of weight loss equals 1 L of sweat (true within ±5% for typical durations).
• Climate sodium-concentration midpoint applies — high-sodium sweaters (genetics) lose 50–100% more.

Approximation range

Sweat rate ±10%; sodium per hour ±30% (individual variation is the dominant error). Using a sweat patch test on race-pace effort is the gold standard for sodium concentration.

Limitations

• Doesn't model gel/chew sodium content separately — those count toward total sodium intake.
• Doesn't address potassium, magnesium, or carb-fluid co-ingestion strategy.
• Single-session prediction; race day will sweat more in early kms (high effort, fresh) and less in later kms (slowed pace).

Reproducibility

70 kg pre, 68.5 kg post, 800 mL fluid, 240 min, temperate. sweat_loss = 70 − 68.5 + 0.8 = 2.3 kg. sweat_rate = 2.3/4 = 0.575 L/h. Sodium = 0.575 × 920 = 529 mg/h. In-race intake = min(800, 0.575 × 1000 × 0.8) = 460 mL/h.

Change log

• 2026-05-08: methodology page first published.

Related tools

• Sweat Rate Calculator — Generic sweat rate without climate sodium banding.
• Water Intake Calculator — Daily hydration baseline outside of race day.
• Race-Week Taper Planner — Final-week prep around race day.

Worked example

Computed by the same engine bundle served at /engines/sweat-rate-electrolyte-marathon.js. Re-runnable: the values below are the literal output of compute(engineInput).

FAQ

What does the Sweat-Rate + Electrolyte Marathon Plan calculate?

Methodology for the Sweat-Rate + Electrolyte Marathon Plan: Wagner & Heyward sweat math, climate-banded sodium loss, and EAH-safe intake caps.

What inputs does the Sweat-Rate + Electrolyte Marathon Plan require?

It takes the following inputs: pre weight kg, post weight kg, fluid consumed ml, duration minutes, climate.

What does the Sweat-Rate + Electrolyte Marathon Plan return?

It returns: fluidIntakePerHourMl, sodiumPerHourMg, sweatRateLPerHour, totalSweatLossL, bodyMassLossPct, riskFlag, preRacePlanMl, postRacePlanMl, notes.

Is the Sweat-Rate + Electrolyte Marathon Plan free to use?

Yes. It runs entirely client-side in your browser with no signup, and is also importable as an ES module engine for AI agents.

What category does the Sweat-Rate + Electrolyte Marathon Plan belong to?

Cardio. See the methodology above for formulas, assumptions, and limitations.