Calculator · Hydration
Sweat Rate Calculator
Measure how much fluid you actually lose per hour, rather than guessing, and turn that figure into a drinking plan you can rehearse before race day.
Weigh in, run, weigh out
Sweat rate
—
Fluid plan for a long run or race
Plan
—
The method
Sweat rate cannot be estimated from your size, your pace or your postcode with any useful accuracy. Individual rates vary by a factor of four or more between runners under identical conditions. The only reliable approach is to weigh yourself.
sweat rate (ml/hour) = sweat lost ÷ hours run
The arithmetic rests on a convenient fact: one litre of sweat weighs almost exactly one kilogram. So every kilogram you have lost across the run is a litre that left your body, and anything you drank along the way has to be added back because it masked part of the loss.
Getting a clean measurement
- Weigh naked or in dry kit, immediately before and immediately after. Sweat-soaked clothing can hold several hundred grams.
- Empty your bladder before the pre-run weigh-in, and do not urinate during the run if you can help it. If you do, the measurement is compromised.
- Towel off thoroughly before the second weigh-in.
- Record every millilitre you drink. Estimating bottle volumes is where most of the error creeps in.
- Note the temperature, humidity and your pace. A sweat rate without conditions attached is not transferable to race day.
What the number means
Typical sweat rates for distance runners fall between 500 and 2,000 millilitres an hour, with heat, humidity, intensity and body mass all pushing the figure up. Elite marathoners in hot conditions have been measured above three litres an hour. The point of the exercise is not to compare yourself to a range but to know your own figure, because the drinking plan that follows from 700 ml/hour is completely different from the one that follows from 1,800.
Performance decrements become consistently measurable when body mass falls by roughly two percent through fluid loss. For a 70 kg runner that is 1.4 kg — reachable inside ninety minutes on a hot day at a sweat rate most runners would consider unremarkable.
Why full replacement is the wrong target
The instinctive plan is to drink exactly what you lose. It is not the right plan for two reasons. First, part of the mass you lose during long exercise is not water at all: oxidising stored glycogen and fat releases metabolic water and burns off substrate mass, which means scale weight overstates true fluid deficit somewhat. Second, and more seriously, aggressive drinking carries a real risk.
Exercise-associated hyponatraemia is what happens when plasma sodium is diluted below safe concentration by excessive plain fluid intake. It causes nausea, confusion and, in severe cases, seizures and cerebral oedema. It is genuinely dangerous, and unlike mild dehydration it has killed marathon runners. The population most at risk is not the elite field but slower finishers who take a cup at every station across four or five hours.
Replacing sixty to eighty percent of measured losses keeps you comfortably clear of the two percent decrement without approaching the dilution end of the problem.
Turning the rate into a plan
The second tool converts a measured sweat rate into the numbers you actually carry to a start line: total volume, hourly volume, and the amount to take at each drinking opportunity. It targets sixty to eighty percent replacement for the reasons set out above — full replacement is neither necessary nor safe.
The per-opportunity figure is the one that changes behaviour. Knowing you need 900 ml an hour is abstract; knowing you need 300 ml at each of three aid stations is a decision you can execute while tired. If the number comes out above about 250 ml per stop, plan to carry rather than rely on cups, because drinking that volume on the move from a paper cup is not realistic.
Sodium, and the salty-sweater test
Sweat sodium concentration varies between roughly 20 and 80 millimoles per litre across individuals, and unlike sweat rate it is largely genetic. The practical field test costs nothing: run in dark technical fabric on a hot day and look for white crusting afterwards. Visible salt residue means you are at the high end and should be replacing sodium deliberately on efforts beyond about ninety minutes.
Heat acclimatisation improves both variables. Ten to fourteen consecutive days of training in heat increases sweat rate — you cool more aggressively — while lowering sweat sodium, conserving electrolytes. This is why a rate measured in April will underestimate what your July self is losing, and why the test is worth repeating rather than filing away.
Building the race plan
Take your measured hourly rate, take sixty to eighty percent of it, and divide by the number of aid stations or bottle carries available per hour. That gives you a per-station volume to rehearse in training. Practise the plan on long runs at race pace; the gut trains, and a stomach that has never handled 600 ml an hour at effort will not learn on race day.
Estimate your finishing time first with the race time predictor so you know how many hours you are planning for.
Common questions
How much should I actually drink during a run?
Enough to keep body mass loss under about two percent, which for most runners in most conditions means replacing sixty to eighty percent of sweat losses rather than all of them. Some mass loss during exercise is normal and comes partly from stored fuel and metabolic water, not only from sweat.
Can I drink too much?
Yes, and it is more dangerous than mild dehydration. Exercise-associated hyponatraemia occurs when fluid intake dilutes blood sodium, and it is most common in slower marathon finishers who drink at every station out of caution. Finishing a long race heavier than you started is the clearest warning sign.
Does sweat rate change through the season?
Substantially. Heat acclimatisation over ten to fourteen days raises sweat rate while lowering the sodium concentration of that sweat — you cool yourself more efficiently and conserve electrolytes better. Retest in early summer and again in midsummer, or your spring numbers will mislead you.
Do I need electrolyte drinks or is water enough?
For efforts under about ninety minutes in moderate conditions, water is generally sufficient. Beyond that, or in heat, or if you are a visibly salty sweater who finishes with white residue on your kit, sodium replacement starts to matter for both performance and safety.