Formula, in Hydration

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 variables. Published May 14, 2026. By AI Fit Hub.

Education, not medical advice. The result comes only from the published formula and your inputs. How we check our formulas.

Formula

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 2007 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

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

Pre-weight: 75.5 kg.
2

Track fluid intake during exercise (bottle volume).

Drank 1.5 L water + sports drink during 2-hour long run.
3

Weigh-in immediately after exercise, same state.

Post-weight: 74.2 kg.
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

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 2015): 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 2007).

Run the numbers next

Recovery Sweat Rate Calculator
Calculate your personal sweat rate from pre/post-exercise weigh-ins and estimate fluid and sodium losses using ACSM guidelines.
Nutrition Water Intake Calculator
Calculate daily water intake based on weight, activity level, and climate.
Cardio Marathon Pace + Elevation Calculator
Convert a target marathon time on a hilly course into the flat-equivalent pace you actually need.

Questions people ask next

How do you calculate your sweat rate for a marathon?

Weigh yourself nude or in minimal dry clothing immediately pre-run, then again right after the run, track fluid you drink, then apply sweat_rate_L_per_hr = (pre_weight_kg - post_weight_kg + fluid_intake_L) / hours_exercised. For example, 75.5 kg pre, 74.2 kg post, 1.5 L drunk over 2 hours gives (75.5 - 74.2 + 1.5) / 2 = 1.4 L/h. The pre/post weigh-in is the most accurate way to get your personal rate.

How much fluid and sodium should I replace during a marathon?

ACSM 2007 and Casa 2007 recommend replacing 70-90% of your sweat losses, so multiply your measured sweat rate by 0.70-0.90 for fluid. For sodium, multiply sweat rate by your sweat sodium concentration (typical 460-1840 mg/L; default 800-1000 mg/L if unknown). A 1.4 L/h sweat rate works out to roughly 1.0-1.25 L/h fluid plus around 1,100 mg sodium/hour. As a safe default for moderate-effort marathons, aim for 500-750 ml/h water plus 500-700 mg sodium/h.

How do I know if I am a salty sweater?

Look for visible white salt residue on clothes or cap, or salt-crystal buildup around the hairline after exercise; salty sweaters average about 1200-1800 mg/L sodium versus 460-700 mg/L for low-sweat-sodium individuals. Lab testing via a Macroduct or PNN patch confirms it. If you are a known salty sweater, plan toward the upper end of the sodium range.

How do I avoid hyponatremia while hydrating?

Do not drink ad libitum with zero sodium during events over 2 hours, the combination that has caused confirmed deaths in marathons and Ironman (Casa 2007; Hew-Butler 2015). Keep sodium above the 500 mg/hour floor for long events with high water intake, since falling below it risks blood sodium dropping under 135 mmol/L. Replacing 70-90% of losses (not 100%+) guards against both dehydration and over-drinking.

Sources

Casa et al. (2007). American College of Sports Medicine position stand: exertional heat illness during training and competition. , Medicine & Science in Sports & Exercise — ACSM 2007 hydration position
Maughan et al. (2007). Errors in the estimation of hydration status from changes in body mass. , Journal of Sports Sciences — sweat-rate measurement methodology
Hew-Butler et al. (2015). Statement of the Third International Exercise-Associated Hyponatremia Consensus Development Conference, Carlsbad, California, 2015. , Clinical Journal of Sport Medicine — hyponatremia consensus
Sawka et al. (2007). American College of Sports Medicine position stand: exercise and fluid replacement. , Medicine & Science in Sports & Exercise — ACSM fluid replacement guidelines