Hydration Loss Rate Calculator Formula
Understand the math behind the hydration loss rate calculator. Each variable explained with a worked example.
Formulas Used
Sweat Rate
sweat_rate_ml_hr = total_sweat_loss_ml / (exercise_duration_min / 60)Total Fluid Lost
total_fluid_lost = total_sweat_loss_mlDehydration Level
dehydration_pct = (pre_weight_kg - post_weight_kg) / pre_weight_kg * 100Variables
| Variable | Description | Default |
|---|---|---|
pre_weight_kg | Pre-Exercise Weight(kg) | 75 |
post_weight_kg | Post-Exercise Weight(kg) | 73.5 |
fluid_consumed_ml | Fluid Consumed During(mL) | 500 |
exercise_duration_min | Exercise Duration(min) | 60 |
total_sweat_loss_ml | Derived value= (pre_weight_kg - post_weight_kg) * 1000 + fluid_consumed_ml | calculated |
How It Works
Calculating Sweat Rate
Sweat rate determines how much fluid you need to replace during exercise. Weight loss during exercise is primarily from sweat.
Formula
Total Sweat Loss (mL) = (Pre-Weight - Post-Weight) x 1000 + Fluid Consumed
Sweat Rate (mL/hr) = Total Sweat Loss / Duration (hours)
Dehydration beyond 2% of body weight impairs performance. Athletes typically sweat 500-2500 mL/hour depending on intensity, heat, and individual physiology.
Worked Example
An athlete drops from 75 kg to 73.5 kg during 60 minutes of exercise while drinking 500 mL.
- 01Weight loss = 75 - 73.5 = 1.5 kg = 1,500 mL
- 02Total sweat = 1,500 + 500 = 2,000 mL
- 03Sweat rate = 2,000 / 1 hour = 2,000 mL/hr
- 04Dehydration = 1.5 / 75 x 100 = 2.0%
Frequently Asked Questions
How much should I drink during exercise?
Aim to replace 50-80% of your sweat losses. Complete replacement is often impractical and unnecessary. Drinking to thirst is reasonable for most recreational athletes.
Does sweat rate change?
Yes. Sweat rate increases with temperature, humidity, exercise intensity, and heat acclimation status. It can vary 2-3x between cool and hot conditions.
When does dehydration hurt performance?
Performance typically declines at 2%+ body weight loss. At 3-4%, strength and endurance drop significantly. Above 5% is a medical concern.
Ready to run the numbers?
Open Hydration Loss Rate Calculator