Skip to main content
aifithub
Hydration Formula

Marathon Sweat Rate + Electrolyte Formula

Marathon sweat rate ranges 0.5-2.5 L/h depending on body mass, ambient temperature, and humidity. Sodium concentration in sweat varies 460-1840 mg/L. ACSM 2007 + Casa 2007 recommend replacing 70-90% of sweat losses to prevent both dehydration and exercise-associated hyponatremia. Pre-race weigh-in + post-race weigh-in produces the most accurate personal sweat rate. Default to 500-750 ml/h water + 500-700 mg sodium/h for moderate-effort marathons.

6 VARIABLESPublished May 14, 2026Live Content
By Orbyd Editorial · AI Fit Hub Team
Best Next MoveCardio

Sweat-Rate + Electrolyte Marathon Plan

Compute marathon sweat rate, fluid intake, and sodium replacement plan from a single training session.

CalculatorOpen ->

On This Page

Formula 6 variables Worked example Variations

Education · Not medical advice. Output is deterministic math from your inputs.Editorial standardsSponsor disclosureCorrections

Formula

Copy the exact expression or work through it step by step below.

sweat_rate_L_per_hr = (pre_weight_kg − post_weight_kg + fluid_intake_L) / hours_exercised fluid_replacement_target = 0.70 to 0.90 × sweat_rate_L_per_hr sodium_replacement_mg_per_hr = sweat_rate_L_per_hr × sweat_sodium_concentration_mg_per_L typical sweat sodium: 460-1840 mg/L (Maughan 2004 range) default for unknown: 800-1000 mg/L hyponatremia_floor_sodium_mg_per_hr = 500 (Casa 2007: below this for events >2h with high water intake risks hyponatremia)

Variables

sweat_rate_L_per_hr

Sweat rate

Liters of sweat per hour of exercise. Range 0.5 (small, cool, easy) to 2.5+ (large, hot, hard). Measure via pre/post weigh-in for personal accuracy.

pre_weight_kg

Pre-exercise body weight

Kilograms, weighed nude or in minimal dry clothing. Subtract any pre-exercise toilet stop weight if relevant.

post_weight_kg

Post-exercise body weight

Kilograms, weighed immediately after exercise in same clothing state. The drop reflects fluid loss (minus fluid intake during exercise).

fluid_intake_L

Fluid consumed during exercise

Liters of water + sports drink consumed during the session. Tracked via bottle volume.

sweat_sodium_concentration

Sweat sodium concentration

Milligrams of sodium per liter of sweat. Highly individual — 'salty sweaters' (white residue on clothes after exercise) average 1200-1800 mg/L vs 460-700 mg/L for low-sweat-sodium individuals. Lab-tested via Macroduct or PNN patch.

hyponatremia_floor

Sodium minimum for safety

Below 500 mg sodium/hour during events >2 hours combined with high water intake increases hyponatremia risk (sodium drops below 135 mmol/L). Casa 2007 threshold.

Step By Step

  1. 1

    Weigh-in immediately before exercise (nude or minimal clothing).

    Pre-weight: 75.5 kg.

  2. 2

    Track fluid intake during exercise (bottle volume).

    Drank 1.5 L water + sports drink during 2-hour long run.

  3. 3

    Weigh-in immediately after exercise, same state.

    Post-weight: 74.2 kg.

  4. 4

    Compute sweat rate: (pre − post + intake) / hours.

    (75.5 − 74.2 + 1.5) / 2 = 2.8 / 2 = 1.4 L/h sweat rate.

  5. 5

    Plan race-day intake: 70-90% of sweat rate as fluid + 500-1000 mg sodium/hour (more if known salty sweater).

    Race-day plan: 1.0-1.25 L/h fluid (~70-90% of 1.4 L/h) + 700-1000 mg sodium/h. Achievable: 2-3 gels (200-300 mg Na each) + 400-500 ml electrolyte drink (300-400 mg Na/L) per hour.

Worked Example

75 kg runner, 2-hour long run in 22°C, 65% humidity

Pre-run weight

75.5 kg

Post-run weight

74.2 kg

Fluid consumed

1.5 L

Duration

2 hours

Sweat rate = (75.5 − 74.2 + 1.5) / 2 = 1.4 L/h Fluid replacement target = 0.70 × 1.4 to 0.90 × 1.4 = 0.98 to 1.26 L/h Sodium target (default 800 mg/L concentration) = 1.4 × 800 = 1,120 mg/h

Race-day plan for similar conditions: 1.0-1.25 L/h fluid + ~1,100 mg sodium/h. Practical: 1 bottle (500 ml) of electrolyte drink at 400 mg sodium/L per 30 min (1.0 L + 800 mg sodium/h) + 2 sodium-loaded gels (~250 mg sodium each) per hour. Total ~1.3 g sodium/h. For hot-weather races (>26°C): increase to 1.3-1.5 L/h + 1,500 mg sodium/h.

Common Variations

Heat acclimation: 10-14 days of progressive heat exposure reduces sodium concentration ~40% and increases sweat rate. Re-measure after acclimation block for accurate race-day target.
Salty sweater diagnostic: visible white salt residue on clothes/cap, or salt-crystal buildup around hairline. Likely 1200-1800 mg/L. Lab confirms.
Hyponatremia avoidance (Casa 2007 + Hew-Butler 2017): avoid drinking ad libitum + zero sodium during events >2 hours. Multiple confirmed deaths in marathon + Ironman from this combination.
Caffeine + sodium combos: gels with 30-50mg caffeine + 200-300mg sodium are common. Caffeine increases sodium reabsorption; combined dose works synergistically.
Carb co-transport: glucose + sodium are absorbed together via SGLT1 transporter. 6-8% carb solution + 18-46 mmol/L sodium maximizes both water and energy uptake (Maughan 2004).

Try These Tools

Run the numbers next

RecoveryCalculator

Sweat Rate Calculator

Calculate your personal sweat rate from pre/post-exercise weigh-ins and estimate fluid and sodium losses using ACSM guidelines.

Launch toolOpen ->
NutritionCalculator

Water Intake Calculator

Calculate daily water intake based on weight, activity level, and climate.

Launch toolOpen ->
CardioCalculator

Marathon Pace + Elevation Calculator

Convert a target marathon time on a hilly course into the flat-equivalent pace you actually need.

Launch toolOpen ->

Sources & References

General fitness estimates — not medical advice. Consult a healthcare professional for medical decisions.