Sweat Rate Calculator

About this tool

L/hr sweat + hydration plan

Compute personal sweat rate from pre/post-exercise weight + fluid intake. Outputs L/hr sweat rate, dehydration %, and recommended intake for race-day + training.

RT-HLT-014 · Health & Fitness

Exercise session details

Pre-exercise weight (kg)

Post-exercise weight (kg)

Duration (minutes)

Fluid intake during (ml)

Urine output (ml, optional)Usually 0 during exercise

Sweat Rate

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Total sweat loss

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Body weight loss

—

Recommended intake

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Enter pre + post weight + duration to compute sweat rate

How to use the Sweat Rate Calculator

Weigh yourself accurately BEFORE exercise

Empty bladder, minimal clothing (or naked), digital scale, kg to one decimal. Same scale + same conditions every time for consistency. Record the number. Don\'t skip this: visual weight change is unreliable; the math is only as accurate as your measurements.

Track fluid intake during exercise

Note every ml you drink during the session — water, sports drink, gels (each gel contains 25-30 ml fluid in its mix). Bottles with measurement markings make this easier. Don\'t guess: 200ml vs 500ml makes a meaningful difference to the calculation.

Weigh yourself IMMEDIATELY after exercise

Towel off sweat thoroughly, strip wet clothes, weigh again. The smaller the gap between exercise end and post-weigh, the more accurate. Run the calculator on the inputs. Tropical conditions: weigh again after a brief shower-towel to ensure all surface moisture is removed.

Repeat across conditions + calibrate

Sweat rate varies with temperature, humidity, intensity, and acclimatisation. Test in different conditions: cool training day, hot training day, race-day conditions. Build a personal table: "easy run cool 18°C = 0.8 L/hr; tempo hot 30°C = 1.8 L/hr". Use these numbers to plan exact race-day fluid carriage + aid station strategy.

Sweat rate — the most underused number in endurance training

Sweat rate is your body\'s personal hydration baseline — how many millilitres of sweat you lose per hour during exercise. It varies enormously between individuals (from ~400 ml/hr to 3000+ ml/hr) and between conditions (a single athlete can sweat 2-3× more in tropical heat than in cool weather). Despite being the single most-actionable hydration metric, most amateur endurance athletes never measure it. They follow generic advice ("drink 500ml per hour") that\'s typically wrong by 50-150% for their actual physiology. Measuring sweat rate twice — once in your typical training conditions, once in race-condition heat — gives you the exact data needed to plan race-day fluid + electrolyte strategy.

The math is dead simple

Sweat rate (L/hr) = (Pre-weight − Post-weight + Fluid intake − Urine output) / hours. The principle: every kilogram of body weight lost during exercise corresponds to one litre of sweat (the body is mostly water; non-sweat losses like breathing are negligible at this scale). Add any fluid you drank during exercise (that fluid is now sweat or stomach contents), subtract any urine output (typically zero during exercise). Divide by duration in hours. Repeat in different conditions to build your personal sweat-rate profile. Modern endurance training apps (TrainingPeaks, COROS Training Hub, Garmin Connect) accept manual sweat-rate entries for personalised hydration nudges.

Individual sweat rates vary 400-3000 ml/hr. Generic "drink 500ml per hour" guidance is typically wrong by 50-150% for any individual. Measure your own.

The hyponatremia + over-hydration risk

Most amateurs worry about dehydration; the elite-athlete medical literature increasingly worries about over-hydration. Hyponatremia (low blood sodium) results from drinking plain water at high rates without replacing electrolytes — the body dilutes its sodium reserves, leading to headache, nausea, confusion, and in severe cases swelling of the brain (which can be fatal). Famous cases: Hyponatremia-related deaths at Boston Marathon (1985, 2002, 2017), London Marathon (2007), and IronMan races. The mechanism: well-meaning athletes who follow "drink lots of water" advice without electrolyte replacement at high sweat rates. The safe approach: drink to thirst + use electrolyte drinks for sessions over 60 minutes. Sweat contains 200-1500 mg sodium per litre (varies by individual — salty sweaters lose more); target 300-700 mg sodium replacement per hour for long sessions. Branded electrolyte products (Gatorade Endurance, Tailwind, Skratch Labs, LMNT) all provide this; salt tabs (SaltStick) work too.

The ASEAN tropical sweat reality

Tropical conditions across ASEAN markets push sweat rates significantly higher than Western training guides predict. Singapore + KL + Bangkok + Jakarta + Manila at 28-32°C + 80%+ humidity routinely produce sweat rates of 1.5-3 L/hr for endurance athletes — vs the 0.8-1.2 L/hr norm in Western 18-20°C training. Practical implications for ASEAN endurance athletes: (1) Carry significantly more fluid than Western training guides suggest — 600-1000 ml/hr is normal for tropical long runs; (2) Electrolyte intake is non-negotiable for sessions over 60 min — plain water at tropical sweat rates risks hyponatremia; (3) Pre-hydrate the day before with extra 500 ml fluid + 1g sodium; (4) Plan aid-station spacing on long routes — going more than 30-40 min between hydration stops in tropical heat is dangerous; (5) Test sweat rate in race conditions — your "cool weather" sweat rate is irrelevant to a tropical race. Sports clinics in the region (Singapore SGH Sports + Exercise Medicine, Bangkok BNH Hospital, Manila Cardinal Santos) increasingly emphasize this; Western hydration guidance underestimates tropical needs by 50-100%.

10 Things to Know About Sweat Rate

Sweat rate = (pre-weight − post + intake − urine) / hours. Simple math; deeply individual results.

Individual sweat rates vary 400-3000 ml/hr. Generic "500 ml/hr" advice is wrong by 50-150% for most people.

Sweat contains 200-1500 mg sodium per litre. Salty sweaters lose double the non-salty sweaters at same sweat rate.

1% body weight loss → performance starts dropping. 2-3% → meaningful drop. 4%+ → heat illness risk.

Hyponatremia (low blood sodium) from drinking too much plain water has killed marathoners — Boston, London, IronMan cases.

Maximum GI absorption: 1000-1200 ml/hr. Beyond this, fluid sits in stomach and you risk nausea + cramping.

Tropical sweat rates (30°C+, 80%+ humidity): 1.5-3 L/hr for endurance athletes — double cool-weather norm.

Heat acclimatisation (10-14 days of tropical exposure) increases sweat rate but improves cooling efficiency — net positive.

Target sodium replacement: 300-700 mg/hr for sessions over 60 min. Electrolyte drinks or salt tabs essential.

Test sweat rate in race conditions, not training conditions. Cool-weather data is useless for tropical race planning.

Frequently Asked Questions

"Drink to thirst" works for short, low-intensity sessions in cool conditions. It fails in two situations: (1) Long, hot endurance events where thirst signal lags actual dehydration — by the time you feel thirsty, you\'re already 2%+ dehydrated. (2) Cold-weather endurance where thirst is suppressed but you\'re still sweating + losing fluid. Measuring sweat rate gives you a precise replacement target, removing guesswork from race-day planning.
Target ~70-90% of your measured sweat rate, capped at 1000-1200 ml/hr (max GI absorption). Examples: sweat rate 0.8 L/hr → drink 560-720 ml/hr; sweat rate 1.5 L/hr → drink 1000-1200 ml/hr; sweat rate 2.5 L/hr → drink 1000-1200 ml/hr (capped) + accept 1-2% deficit. Don\'t try to match 100% of sweat rate: stomach can\'t absorb fast enough, fluid sits + sloshes, you cramp + nauseate. Accept slight deficit at very high sweat rates.
Sessions under 60 min: plain water is fine. Sessions over 60 min, especially in heat: electrolyte sports drinks are essential. Why: plain water at high sweat rates dilutes your blood sodium, risking hyponatremia. Good options: Gatorade Endurance, Tailwind, Skratch Labs, LMNT, Maurten Drink Mix, Precision Hydration. Mix electrolytes + carbs (60-90g/hr) for sessions over 90 min. Avoid: pure sugar water (insufficient electrolytes); high-fructose options that cause GI distress.
Hyponatremia = low blood sodium, caused by drinking water too fast without sodium replacement. Symptoms: headache, nausea, confusion, fatigue, swollen hands/feet, in severe cases brain swelling + death. Risk factors: long endurance events (4+ hours), high sweat rates, drinking plain water at high rates, weight GAIN during the race (key tell: post-race weight > pre-race weight). Prevention: electrolyte drinks (300-700 mg sodium/hr) for sessions over 60 min; don\'t drink to gain weight; trust thirst more than schedule. If symptoms develop: stop, take salt + minimal fluid, seek medical help.
Counter-intuitive: heat acclimatisation INCREASES sweat rate (sweat starts earlier + more) but improves cooling efficiency + electrolyte conservation. After 10-14 days of tropical training, you sweat MORE but lose LESS sodium per litre (concentration drops). Net effect: better thermal regulation, lower perceived effort at same temperature, lower heart rate. Take-home: track sweat rate before + after a 2-week heat-acclimatisation block; you\'ll see higher litre rates but improved performance. Standard protocol: 90-120 min training daily in heat conditions for 10-14 days.
For serious endurance athletes — yes. "Salty sweaters" lose 800-1500 mg sodium per litre; "non-salty" lose 200-500 mg. Same sweat rate, vastly different electrolyte needs. Field test: lick your forearm after a hot session; if it tastes very salty, you\'re a salty sweater. Lab test: Precision Hydration\'s sweat test ($60-100) measures sodium concentration accurately. For race-day planning: combine sweat rate (this calculator) + sweat sodium concentration → exact mg sodium per hour target. Generic 500 mg/hr advice misses individual variation by 2-3×.
Yes — and it\'s more dangerous than mild dehydration. Over-hydration combined with electrolyte loss → hyponatremia (see above). Red flag: weighing more after a race than before. This means you drank more than you sweated, accumulating fluid. Avoid: forcing fluids on a fixed schedule beyond thirst; pure water at high sweat rates; "front-loading" hydration with excess fluid the morning of a race. Healthier approach: drink to thirst within your measured sweat rate; use electrolytes; weigh before + after to verify you didn\'t over-hydrate.
Simple daily hydration check. Pale yellow / lemonade colour: well-hydrated. Dark amber: dehydrated. Clear/colourless: over-hydrated (excessive intake). Best timing: first morning urine reflects overnight hydration status. Limitations: B vitamins (in supplements) cause bright yellow urine; some medications colour urine independently of hydration. Use as one signal among many, not a sole metric.
No. All calculations run in your browser via JavaScript. Open DevTools → Network and confirm zero outbound requests. Weights, fluid intake, and duration stay on your device. Safe for personal health tracking.
Pair with: VO2 Max Calculator (RT-HLT-011) for fitness benchmarking; Race Time Predictor (RT-HLT-012) for race planning; Running Pace Calculator (RT-HLT-013) for race-day pacing; Heart Rate Calculator (RT-HLT-009) for HR-zone effort gauging. External: Precision Hydration sweat test (paid, gold-standard); GU Energy + Tailwind + Skratch Labs fueling products; Sports Dietitians Australia + International Sport Sciences Association nutrition guides.

Sweat Rate Calculator

How to use the Sweat Rate Calculator