Hydration
Sweat Rate Calculator
Calculate your personal sweat rate from pre/post-exercise weigh-ins and estimate fluid and sodium losses (ACSM and NATA guidance).
Result
Sweat versus drinking
What you sweat each hour, what to drink each hour, and the session total.
Hydration status
Minimal dehydration
You lost 1.1% body weight. ACSM advises keeping losses under 2%. Aim for about 1050 mL/hr next time.
What does the Sweat Rate Calculator calculate, and which formula does it use?
The Sweat Rate Calculator computes fluid loss per hour with the ACSM/NATA weigh-in formula: sweat rate (L/hr) = (pre-exercise weight − post-exercise weight + fluid consumed − urine) ÷ hours exercised. It grades dehydration from percent body weight lost (thresholds at 1%, 2%, and 4%), recommends replacing about 80% of the sweat rate, and estimates sodium loss at 920 mg per liter of sweat.
Last updated .
Questions people usually ask
What is sweat rate?
Sweat rate is the volume of fluid lost through sweating per hour of exercise, measured in litres/hour. Average endurance athletes sweat 0.5–2.5 L/hr; heavy sweaters can exceed 3 L/hr in heat.
Which formula is used?
The ACSM (American College of Sports Medicine) standard formula: Sweat Rate (L/hr) = (Pre-weight − Post-weight + Fluid consumed − Urine produced) / Duration in hours. Each 1 kg body weight loss equals approximately 1 litre of sweat.
How do I weigh myself accurately?
Weigh yourself nude immediately before exercise, then nude again immediately after — before eating or drinking anything post-exercise. Use the same scale both times.
Do I need to track urine output?
For most workouts under 90 minutes, urine output is negligible and you can leave it at 0. For ultra-endurance events, tracking urine output improves accuracy.
Why does sodium loss matter?
Sweat contains sodium (average ~920 mg/L). High sweat rates deplete sodium faster, risking hyponatremia in long events. If you lose more than 800 mg/hr, consider electrolyte drinks or salt tabs.
Is this tool free and private?
Yes. AI Fit Hub tools are free, browser-based, and no-signup. Your inputs stay in your browser.
Does this replace a sports dietitian?
No. These outputs are evidence-based estimates. For personalised hydration plans, especially for elite performance or medical conditions, consult a registered sports dietitian.
The formula, worked through
- Formula Sweat Rate Formula
sweat_rate_L_per_hour = (mass_pre − mass_post + fluid_consumed_L − urine_L) / duration_hours.
- Formula Marathon Sweat Rate + Electrolyte Formula
Marathon hydration: sweat rate 0.5-2.5 L/h, sodium concentration 460-1840 mg/L. Casa 2007 + ACSM 2007 standards. Replace 70-90% of losses; avoid hyponatremia.
- Formula Daily Water Intake Formula
Daily water intake formula: water_ml = 30·mass_kg (sedentary) to 35·mass_kg (active). Add 350–500 ml/hour exercise. Food covers ~20%.
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Background on the Sweat Rate Calculator
- Guide How to Stay Hydrated During Exercise
How to stay Hydrated During Exercise: optimize your performance and prevent dehydration during workouts with expert strategies, personalized fluid intake guid.
- Examples Sweat Rate Examples
Discover practical sweat rate examples from athletes to industrial workers. Learn to calculate your fluid loss for personalized hydration, boosting health.
- Checklist Daily Hydration Checklist
Daily hydration checklist for athletes. Set fluid targets, drink on a schedule, monitor urine color, and stay ahead of training-day water and sodium needs.
- Examples Water Intake Examples
Water intake examples worked through the calculator: an 80 kg person on a moderate day, a heavy training day, a hot climate, and a heavier 95 kg body.
- Formula Swim Pace Formula
Swim pace formula: pace_per_100m = total_seconds × 100 / total_distance_m. Open-water surcharge: typically +5–15 sec per 100 m.
Articles
- 10 min read Heat Acclimatization: Sodium and Plasma Volume
Heat acclimatization shifts plasma volume up 10-12% in 10-14 days. The sodium math that supports the adaptation and the workouts that drive it.
- 7 min read Marathon Sweat Rate: 70kg Runner, 2% Body Mass Loss in Warm Conditions
Marathon sweat rate 0.57 L/h, total loss 2.3 L over a 4-hour temperate race. Body mass loss 2.1%, 460 ml/h fluid plan, 529 mg/h sodium target.
- 11 min read Heart-Rate Zones: Methods Compared
Heart-rate zone methods compared: Karvonen, %HRmax, %HRR, lactate threshold — accuracy by training age, error bands, and which one for which goal.
By goal
Use it from code
The same calculation as a JavaScript module you can import. It runs where you import it, with no request, key or rate limit.
import { compute } from "https://aifithub.io/engines/sweat-rate-calculator.js";Input and output contract, TypeScript types and the guide for agents.