Sweat Rate Calculator
Use this sweat rate calculator to estimate how much fluid you lose during exercise and how much you should aim to drink per hour during training and racing.
Pre-Weight
Post-Weight
Session Duration
Estimated sweat rate: --
Fluid Consumed
Understanding Your Sweat Rate
Sweat rate varies significantly between individuals and is affected by exercise intensity, air temperature, humidity, and acclimatisation. Two athletes of similar fitness completing the same session can have very different fluid losses - which is why generic hydration advice is often unhelpful.
The most reliable way to estimate your sweat rate is to weigh yourself before and after a training session, accounting for any fluid consumed during the session. Each kilogram of bodyweight lost approximately equals one litre of fluid loss.
How to Use the Calculator
Weigh yourself without clothing before your session. Train for a known duration without drinking, or record exactly how much fluid you consume during the session. Weigh yourself immediately after the session. Enter these values into the calculator to estimate your sweat rate in litres per hour.
Aim to conduct this test across a range of conditions - a warm-weather session will produce a very different result from a cool winter run.
What Dehydration Does to Performance
Even modest dehydration - a body mass loss of around 2% - is associated with a measurable reduction in endurance performance. At higher levels, the effects on thermoregulation, cognitive function, and perceived effort become more significant.
The goal is not to replace every drop of fluid lost - that is neither practical nor necessary. Rather, the aim is to keep fluid losses within a manageable range and to arrive at key points in a race (aid stations, feed zones) with a coherent hydration plan rather than simply drinking to thirst.
Hydration strategy is one of the most overlooked elements of race preparation. For tailored guidance, explore coaching at stevebarbour.com.