How to Calculate Your Sweat Rate for Race Day

Lose 0.9kg during an hour of running, drink 600ml, and produce no urine, and your estimated sweat loss is 1.5 liters. The scale only showed part of it.

To calculate sweat rate, account for the weight change, what you drank, any urine produced, and the time you exercised. Then keep the result attached to those conditions. It tells you about that session, not exactly what to drink for every future race.

When I started training, I didn't think much about hydration. Then I noticed I was crushed for the rest of the day after hard workouts. Thirsty all afternoon, dragging through the evening, never quite feeling recovered. So I started doing informal sweat tests. Indoor workouts, fan blowing on me the entire time, 22oz of water during the session. I'd still step off the scale 2+ pounds lighter afterward, and that was indoors with a fan on me, after eating breakfast and weighing in with food in my stomach.

Those observations got my attention. A more consistent test makes the number easier to interpret.

Set up a test you can repeat

Baker's review of sweat-testing methods describes before-and-after nude body mass as a practical way to estimate whole-body sweat loss, with corrections for drinking and urine. Wet clothing and other mass changes can distort the result.

For a straightforward home test, use a suitable training session around an hour, the same reliable scale, a measured bottle, and a towel. That's a convenient setup, not a proven ideal duration. Wear your normal training kit during exercise, but remove it for both weigh-ins.

1. Empty your bladder, then weigh yourself dry and without clothing. Record kilograms if possible.
2. Do the planned session and record the actual duration. Note the weather, effort, activity and clothing.
3. Measure what you drink. Separate water swallowed from water spilled or poured over yourself.
4. If you need to urinate, don't hold it for the experiment. Measure the volume if practical, or repeat the test another day if you can't account for it.
5. Afterward, remove wet clothing, towel off, and weigh again before eating or drinking anything else.

Keep this simple test free of food intake, rather than trying to weigh snacks halfway through it. If food or a bowel movement changes body mass, the equation below needs additional accounting; don't label that whole change as sweat.

And don't turn a test into a dehydration challenge. Drink normally for the session and record it.

Use the full equation

The NATA fluid-replacement position statement gives this field calculation:

Sweat loss in liters ≈ pre-exercise weight in kg − post-exercise weight in kg + fluid consumed in liters − urine produced in liters.

Sweat rate in liters per hour = estimated sweat loss ÷ exercise duration in hours.

The approximation treats a kilogram of fluid mass as about a liter. Convert milliliters to liters by dividing by 1,000, and minutes to hours by dividing by 60.

Here's a worked example:

Item	Measurement
Dry pre-exercise weight	68.5kg
Dry post-exercise weight	67.6kg
Fluid consumed	600ml = 0.6L
Urine produced	0L
Exercise duration	60 minutes = 1 hour

The calculation is (68.5 − 67.6 + 0.6 − 0) ÷ 1 = 1.5L/hr.

What if the session lasted longer? If those same weight, drink and urine measurements came from a 90-minute session, divide by 1.5 hours instead: 1.5 ÷ 1.5 = 1.0L/hr. Duration changes the answer.

And if the original one-hour test included 200ml of urine, subtract 0.2L: (0.9 + 0.6 − 0.2) ÷ 1 = 1.3L/hr. Otherwise, you'd count that bathroom loss as sweat.

This simple calculator has limits: it has no urine field and only enables calculation when post-exercise weight is lower. Use the manual equation above if you urinated or your weight stayed the same or increased. Don't change a weight just to enable the button.

Stable weight doesn't mean you didn't sweat. For example, unchanged weight with 0.6L consumed, no urine and a one-hour session gives an estimated 0.6L/hr. A weight increase can also coexist with sweating if intake exceeds other losses. That makes the intake record important; it isn't a reason to aim for weight gain while exercising.

Ready to connect the test to race day? Build a carb, fluid, and sodium timeline in about 60 seconds: Use the free EnduranceOS planner.

One measurement isn't your permanent sweat rate

Repeat the test in conditions relevant to your event. Keep the method consistent so a change in the result means more than “I wore wet shorts on the scale this time.”

Baker's review discusses differences associated with effort, weather, body size, clothing and heat acclimation. It also explains why observed differences between men and women cannot be reduced to a universal percentage. A smaller woman doesn't need to subtract 30% from somebody else's test; she needs her own measurements.

My practical preference would be a short log: date, activity, effort, weather, dry weights, drink, urine, duration and result. An indoor ride with a fan and an outdoor run deserve separate entries. You don't need to make either one stand in for every workout you do.

And don't project a cool-day measurement into heat by adding a fixed amount for each few degrees. Repeat the measurement when reasonably possible, and treat an estimate for unfamiliar conditions as uncertain.

Long sessions introduce another limitation. Baker notes that respiratory water and metabolic mass losses become more important in longer tests, particularly beyond about 2–3 hours. A finish-line weight after a marathon is therefore a less tidy sweat measurement than this short field example. Wet kit, food and bathroom stops add more uncertainty.

The result informs drinking; it doesn't prescribe it

Finding 1.5L/hr on the page doesn't mean you must drink 1.5L/hr, or automatically take 70–80% of it.

NATA recommends individual plans that consider losses, conditions and tolerance while avoiding both excessive dehydration and body-weight gain from overdrinking. If your sweat rate is unknown, drinking to thirst is a safer fallback than forcing a fixed volume. The statement also describes performance effects of dehydration that vary with circumstances; losing 2–3% of body weight isn't universally harmless.

So take the measurement into further training. Record how much you actually drink, whether you can tolerate it alongside food, and how conditions compare. A clinician or sports dietitian can help interpret repeated problems or unusually difficult fluid requirements.

Then check access. A sensible practiced plan still needs places to collect the fluid. The marathon hydration guide turns pace and station spacing into cup and bottle decisions. The Ironman hydration guide handles bike storage and run-course pickups.

The cup count is the next job. The scale can't tell you whether the table at mile 8 will be crowded.

Sweat volume doesn't reveal sodium concentration

This test measures volume. It doesn't tell you how many milligrams of sodium are in each liter.

Baker's review describes substantial sodium-concentration variation and testing limitations. A white mark on your hat isn't a laboratory result. You can't calculate an exact replacement dose from that mark or from the sweat-volume number alone.

If concentration has been appropriately estimated, the arithmetic is straightforward: hypothetical sweat loss of 1.0L/hr at 800mg/L equals 800mg/hr lost. That is a loss calculation, not an instruction to swallow 800mg each hour.

Count sodium already present in the drinks and food you consume before considering supplements. NATA supports individual assessment rather than a blanket tablet schedule. And sodium doesn't make overdrinking safe; the hyponatremia guide covers that risk.

Confusion, a severe or worsening headache, repeated vomiting, or difficulty breathing needs medical help. NATA warns that dehydration and overhydration symptoms can overlap. More water or salt isn't a reliable way to diagnose which problem you have.

What the planner does with your number

The EnduranceOS planner uses estimates based on inputs such as body weight, activity, effort and weather. These are planning estimates; the planner has not been clinically validated.

You can enter a known sweat rate, but the current planner applies environmental adjustments using assumed test conditions of 70°F and 50% relative humidity. It doesn't collect your actual test weather. If your measurement came from different conditions, keep that limitation in mind rather than treating the output as an exact translation of your test.

I'd keep the original test notes beside the plan and rehearse it. A number you can repeat and interpret is more useful than another decimal place, especially when you're deciding how much fluid to carry on a long run.

The EnduranceOS planner builds an hourly carb, fluid, and sodium plan in about 60 seconds. It's free. Use it as a starting point alongside your measurements, thirst, conditions and training experience.

This article is educational, not individual medical advice. Conditions or medications affecting fluid balance, previous hydration emergencies, or recurring symptoms need advice from a qualified clinician.