How much you sweat is personal, and guessing at it is how people end up either dehydrated or sloshing with more fluid than they need. The good news is that two weigh-ins and a bit of arithmetic tell you almost exactly how fast you lose fluid.
TL;DR — Weigh yourself before and after a workout, note what you drank and how long you went, then enter it in the sweat rate calculator to see your hourly fluid loss.
What sweat rate tells you
Sweat rate is simply how much fluid you lose per hour of exercise. Knowing it turns hydration from guesswork into a plan: you can work out roughly how much to carry and how often to sip, instead of drinking too little and fading or too much and feeling heavy.
How to measure it
Weigh yourself with little or no clothing right before you start, then again straight after, towelling dry both times. Note the fluid you drank during the session and how long it lasted. The weight you lost, plus what you drank, is close to the fluid you sweated out. One kilogram lost is about one litre of sweat. The calculator does the maths and gives you a rate per hour.
Turning the number into a drinking plan
Once you know your rate, aim to drink enough on long or hot sessions to finish no more than about 2% down on body weight. Many people manage 400 to 800 ml an hour, which is often less than a heavy sweat rate, so full replacement is not always realistic or necessary. Pair the number with your effort: check your pace and target heart rate to judge how hard a session will be, and use the daily water intake calculator for the rest of the day. Then let thirst fine-tune it.
