SplitLab

Sweat Rate Calculator

46.7fl oz/hr

1.3%body weight

160.0 → 158.0 lb · drank 16 fl oz · 1:00 run

lb
Nude or minimal-clothing weight right before the run, in lb. Default 160.0 lb (72.6 kg).
lb
Towel off, then weigh in the same clothing, in lb. Default 158.0 lb (71.7 kg).
fl oz
Fluid drunk during the run, in fl oz. 0 if none.
fl oz
Optional. Urine passed during the run, in fl oz. Blank counts as 0.
h:mm
h:mm, for example 1:00 or 1:30.
Drink every 15 min
8.8 fl oz

75% of losses · 259 ml

Body weight lost
1.3%

2.0 lb · flag above 2%

Sweat lost
46.7 fl oz

in 1:00 · 1.38 L

Sweat rate
1.38

L/hr

Under 2% of body weight lost — the range where most runners hold their pace.

Long run & marathon at this sweat rate

3:30 run · drink
123 fl oz

sweat 163 fl oz · ends −1.7%

4:30 run · drink
158 fl oz

sweat 210 fl oz · ends −2.1%

Drinking 75% of losses, 35 fl oz per hour. Heat, humidity, and pace change sweat rate, so test in race-like conditions.

Sweat test

Lost 2.0 lb, drank 16 fl oz in 1:00: sweat 46.7 fl oz, 46.7 fl oz/hr. Drink about 8.8 fl oz every 15 minutes.

Sweat-test estimate — not medical advice. Related: race fueling · heat pace · marathon pace · half marathon pace · calories burned.

Worked examples

  • 160.0 lb → 158.0 lb · drank 16 fl oz · 1:00 run

    46.7 fl oz/hr · 1.38 L/hr · 1.3% body weight · drink 8.8 fl oz (259 ml) every 15 min

  • Same rate over a 3:30 and 4:30 marathon

    3:30 sweats 4.83 L (163 fl oz), drink 3.62 L · 4:30 sweats 6.21 L (210 fl oz), drink 4.66 L

  • 70.0 kg → 69.0 kg · drank 500 ml · 1:30 run

    1.00 L/hr · 1.4% body weight

  • 70.0 kg → 68.3 kg · nothing drunk · 2:00 run

    2.4% body weight — flagged above 2%

How it works

A sweat test is the simplest honest way to learn how much you sweat. Weigh yourself right before a run and again straight after, towelled dry and in the same minimal clothing. Every kilogram you lose is treated as about one litre of water (1 lb ≈ 15.34 fl oz). Add back what you drank, because that fluid also left as sweat, and subtract any urine you passed, because that did not. Divide by the hours you ran and you have a sweat rate.

The drinking guide replaces 75% of your losses — enough to keep most runners near or under 2% body-weight loss on long efforts without drinking more than you sweat. It is split into sips every 15 minutes. The long-run projection holds your measured rate steady for a 3:30 and a 4:30 run and shows where you would finish on body weight if you followed the 75% guide. Notice that at 1.38 L/hr even 75% replacement ends a 4:30 run just over 2%.

  • sweat loss = (pre − post weight) + fluid drunk − urine
  • sweat rate = sweat loss ÷ hours
  • % body weight = (pre − post) ÷ pre
  • drink per 15 min = sweat rate × 75% ÷ 4
  • 1 kg ≈ 1 L · 1 lb ≈ 15.34 fl oz · 1 fl oz = 29.5735 ml

Heat, humidity, pace, and clothing all change sweat rate, so repeat the test in conditions like your race. Pair it with the race fueling calculator · heat pace · marathon pace · half marathon pace · calories burned · run/walk intervals. Not medical advice — if you have a heart, kidney, or other health condition, ask a doctor or sports dietitian before changing how much you drink.

FAQ

How do I calculate my sweat rate?

Weigh yourself right before a run and again right after, towelled dry and in the same minimal clothing. Sweat loss is the weight you lost, plus everything you drank, minus any urine you passed. Divide by the hours you ran. Default 160.0 lb before, 158.0 lb after, 16 fl oz drunk in 1:00 is 46.7 fl oz (1.38 L) per hour. Results update live; there is no Calculate button.

What is a normal sweat rate for runners?

Most runners land somewhere between about 0.5 and 2.0 litres (17 to 68 fl oz) per hour, and it swings a lot with heat, humidity, pace, body size, and fitness. That is why a personal sweat test beats a generic number. Repeat it in a few different conditions.

How much should I drink during a run?

This page suggests replacing about 75% of what you sweat, split into sips every 15 minutes. At the default 1.38 L per hour that is about 8.8 fl oz (259 ml) every 15 minutes. Replacing every drop is not the goal; drinking far more than you sweat is not safe either. Treat it as a starting point, not a prescription.

Why does losing more than 2% of body weight matter?

Many runners start to feel slower and work harder once they have lost around 2% of body weight to sweat over a session, especially in the heat. The page flags anything above 2% so you can plan to drink a little more next time. It is a performance guide, not a health diagnosis.

How much fluid will I need for a marathon?

The page projects your measured rate over a 3:30 and a 4:30 run. At the default 1.38 L per hour, 3:30 sweats about 4.83 L (163 fl oz) and the 75% guide is about 3.62 L (123 fl oz) to drink. Sweat rate changes with weather and pace, so test in race-like conditions. Not medical advice — talk to a doctor or sports dietitian if you have a health condition.