Weigh yourself before and after a session and get your sweat rate in litres per hour, then how much to drink on race day so you stay under a 2 percent loss.
Fluid is always in litres. A bottle is usually 0.5 to 0.75 L.
h:mm, for example 1:00
After using the toilet, before you start.
Usually 0.
Not used in the maths. Sweat rate changes with heat, so test in conditions like race day.
Your sweat rate
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Enter your weight before and after, the fluid you drank and how long you trained.
Weigh yourself nude after using the toilet, train for an hour at race effort, then dry off and weigh yourself again. Note how much you drank and whether you urinated. Your sweat loss is weight before, minus weight after, plus the fluid you drank, minus any urine. One kilogram of weight lost counts as one litre of sweat. Divide by the hours and you have litres per hour.
Sweat rate changes with heat, humidity and effort, so repeat the test indoors, on a cool day and on a warm one. Use the number from the conditions closest to your race.
You do not need to replace all of what you sweat. The ACSM position stand sets the goal as avoiding a loss of more than 2 percent of body weight. A common rule of thumb, and the one used here as our estimate, is to replace roughly 60 to 80 percent of your sweat loss. The calculator caps the target at 1 litre an hour, our own cautious ceiling, because drinking more than you sweat is what causes dangerously low blood sodium.
Pair the number with your carbs and sodium in the triathlon nutrition calculator and turn the carbs into a gel count. Bottles go on the packing list.
It varies a lot. In a study of 506 athletes the average was about 1.2 litres an hour, and individual values ranged from 0.3 to 5.7. Heat, effort, body size and fitness all matter, so your own number from a test is the one that matters.
Weigh yourself nude before and after a session. Sweat loss in litres is weight before, minus weight after, plus the fluid you drank, minus any urine. Divide by the session length in hours to get litres per hour.
The ACSM aim is to lose less than 2 percent of your body weight, not to stay at zero loss. As a rule of thumb that is replacing about 60 to 80 percent of your sweat loss, and as a cautious ceiling never more than about 1 litre an hour.
Yes. Drinking more than you sweat dilutes the sodium in your blood, which can cause hyponatraemia, a dangerous condition. Drink to a plan based on your sweat rate and thirst, and as a cautious rule do not force fluid beyond about 1 litre an hour.
The ACSM position stand uses 2 percent of body weight as the line above which dehydration may compromise performance, especially in the heat. Staying under it is the standard target, though how much a given athlete is affected varies. A smaller loss is fine and does not need to be replaced.
