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Sweat Rate Calculator

Estimate how much fluid and sodium you lose during exercise, then model an example electrolyte profile around a conservative replacement target. No login, no email gate, no upsell.

·By Croix

Run the calculator

Tune the inputs to your body and session. Click “Model Session Losses” for estimated losses and a per-serving example target.

This is an estimate based on published sports science data. Individual sweat rate and sodium concentration vary enormously. A professional sweat test gives the most accurate results.

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What this calculator does

Most generic hydration advice (“drink 8 cups a day”) is built for sedentary baselines. The moment you start exercising — especially in heat — your sodium and fluid losses can dwarf the casual advice by an order of magnitude. A 70 kg runner doing a vigorous 90-minute session in a hot climate can plausibly lose 1.5–2 liters of fluid and 1500–3000 mg of sodium. Replacing those losses with a beverage tuned for office hydration is the wrong tool.

This calculator turns four inputs (body weight, duration, intensity, climate, sweat tendency) into a four-number output: estimated sweat rate in L/hr, total fluid loss in mL, sodium loss per hour in mg, and a per-serving sodium / potassium / magnesium example target you can model in the builder. The estimate is grounded in published sports-science ranges (Baker 2017, ACSM position stand), with conservative midpoints and physiological clamps so the output is constrained to common electrolyte-drink ranges.

The modeled output target assumes ~80% of estimated sodium losses across the session — not 100% in a single bottle. That ratio is a conservative formula-modeling assumption, not individualized medical or nutrition advice.

The science behind the inputs

Body weight + intensity

Sweat rate scales with metabolic heat production. A 90 kg athlete generates roughly 30% more heat than a 70 kg athlete at the same relative intensity, and dissipates it through proportionally more sweat. The base rates here come from Baker (2017): ~0.6 L/hr at moderate intensity for a 70 kg adult, climbing to ~1.5 L/hr at extreme intensity.

Climate

Heat and humidity multiply sweat rate. Cool conditions get a 0.75x multiplier (less evaporative demand), temperate is 1.0x, hot is 1.35x, and hot-and-humid is 1.55x. Humidity matters specifically because evaporation slows when ambient water vapor is high — your skin keeps producing sweat even though it isn't cooling efficiently.

Sweat tendency

Self-reported sweat tendency captures both volume and sodium concentration. Light sweaters lose ~30 mmol/L sodium (~690 mg/L); average is ~40 mmol/L (~920 mg/L); heavy sweaters with visible salt stains are commonly at 50+ mmol/L (~1150 mg/L). Heat acclimation lowers sweat sodium over weeks of consistent training in heat.

How to test your real sweat rate (10 minutes, no gear)

  1. 1.Pre-weigh.Weigh yourself naked (or in dry clothes you'll weigh again later) immediately before exercise. Use the same scale, same surface, same orientation as your post-weigh.
  2. 2.Track fluid in. Note every milliliter of liquid you drink during the session. A bottle that says 750 ml on the label is fine; estimating with a sip is not.
  3. 3.Train.Do the session you actually want to fuel for — don't test on an easy day if your hard sessions are the ones you're trying to plan.
  4. 4.Towel off + post-weigh. Get fully dry — wet hair, wet clothes, sweat on skin all confound the result. Step on the same scale.
  5. 5.Compute.Sweat rate (L/hr) = (pre-weight kg − post-weight kg + fluid drunk in L) ÷ duration in hours. A 0.6 kg loss + 0.5 L drunk over 60 min → 1.1 L/hr sweat rate. Plug it back in here as a sanity check, or use the builder to design around your measured number.

This calculator describes general physiology and is not medical advice. If you have a medical condition (cardiac, renal, blood pressure), are pregnant, are caring for a child, or take prescription medications that affect electrolyte balance, consult a healthcare professional before changing your hydration strategy.

Frequently asked

How accurate is a sweat rate calculator?+
This calculator gives a rough planning estimate, not a validated replacement for a sweat test. Two things drive the bulk of individual variation: total sweat volume (~0.5-2.5 L/hr for most adults during exercise) and sweat sodium concentration (~460-1840 mg per liter). Body weight, exercise intensity, climate, and self-reported sweat tendency can approximate volume, but sodium concentration in particular is genetically influenced and can only be precisely measured with a lab or patch-based sweat test.
How do I measure my sweat rate without a lab?+
Weigh yourself naked (or in dry clothes you'll re-weigh after) immediately before exercise. Track every milliliter of fluid you drink during the session. After exercise, towel off and weigh yourself again. Sweat rate in L/hr ≈ (pre-weight − post-weight in kg + fluid drunk in L) ÷ duration in hours. A 0.5 kg loss over a 60-minute session with 0.5 L drunk equals a 1.0 L/hr sweat rate.
Why do I lose so much sodium when I sweat?+
Sweat is plasma-derived but hypotonic — it carries sodium out of the blood and onto the skin even though sodium is the body's primary extracellular electrolyte. The reabsorption channels in the sweat duct work harder when you're heat-acclimated, so seasoned exercisers typically have lower sweat sodium concentrations than novices doing the same workout. Heavy sweaters with visible salt stains are commonly losing 1000–1800 mg of sodium per liter — meaningful losses across a long session.
Is more sodium during exercise always better?+
No. Per-serving amounts above ~1500 mg can cause GI distress for some people, especially when paired with concentrated carbohydrate. This calculator models a conservative 70-80% replacement profile spread across the session, not 100% in a single bottle. It caps per-serving sodium at 2000 mg as a hard ceiling.
Do I need potassium and magnesium during exercise?+
Potassium losses through sweat are usually smaller than sodium losses, and magnesium losses are smaller still. The calculator includes potassium and magnesium only as conservative example amounts so the modeled drink resembles common electrolyte profiles rather than a medical replacement plan.
Should I use this calculator if I'm not exercising?+
The calculator is tuned for exercise-driven sweat losses. It is not designed to advise on sedentary hot-climate hydration, fasting, keto adaptation, illness, or clinical dehydration.

Sources & references

  1. Baker, L.B. (2017). Sweating rate and sweat sodium concentration in athletes: a review of methodology and intra/interindividual variability. — Sports Medicine
  2. Baker et al. (2009). Comparison of regional patch collection vs. whole body washdown for measuring sweat sodium concentration. — Journal of Applied Physiology
  3. Sawka, M.N. et al. (2007). ACSM Position Stand: Exercise and Fluid Replacement. — Medicine & Science in Sports & Exercise

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