Sweat Test Calculator — Your Personal Sweat Rate in oz/hr
Calculate your exact sweat rate and fluid replacement needs using the professional weigh-in/weigh-out sweat test method, with results in oz/hr and sodium estimates.
How to do the sweat test:
- Weigh yourself naked before your run
- Run at race-level effort for the duration below
- Track all fluids consumed (oz)
- Weigh yourself naked immediately after (no shower)
Water + sports drink during run
You finished slightly dehydrated. Increase fluid intake by ~10 oz/hr on your next run.
Your Hourly Fluid Target
Drink this amount every hour in conditions similar to this test run.
1278mg sodium/hr needed
NorthLine Isotonic Drinks — 300mg sodium per 500ml serving
What Is a Sweat Test?
A sweat test is the most accurate way to determine your personal fluid replacement needs. Unlike generic recommendations, your sweat rate is highly individual — it varies by body size, heat acclimatisation, fitness level, and genetics.
Sweat rates among athletes range from 0.5–3.0 litres per hour. Generic "drink 8 oz every 20 minutes" advice may be completely wrong for you.
How to Do the Sweat Test
1. Weigh yourself naked immediately before your run (in lbs, to the nearest 0.1 lb) 2. Run at race-level effort for 60 minutes in representative conditions 3. Track every fluid consumed during the run (oz of water, sports drink, gels) 4. Weigh yourself naked immediately after the run (without showering) 5. Enter the numbers below
Interpreting Your Results
* Fluid loss ≤ 1%: Excellent — your current hydration strategy is working * Fluid loss 1–2%: Mild deficit — minor adjustment needed * Fluid loss 2–3%: Moderate — increasing fluid intake by 20% should close the gap * Fluid loss > 3%: Significant — major strategy overhaul needed; you may be chronically under-hydrated
The Sodium Equation
Every litre of sweat contains 400–1,800mg of sodium. Your sweat rate (measured here) combined with a sodium estimation allows you to calculate your hourly sodium replacement need — critical for races over 90 minutes.
Q: Should I do the sweat test in all conditions? A: Yes — your sweat rate changes with temperature and humidity. A sweat test done in cool October conditions may underestimate your needs for a hot spring marathon by 50–80%. Repeat the test in conditions similar to your target race.
Q: Is 1 lb of weight loss equal to 16 oz of fluid? A: Yes — 1 pound of body mass lost during exercise is approximately 16 oz (473ml) of fluid lost as sweat (assuming no urination). This is the basis of the weigh-in/weigh-out method.
Frequently Asked Questions
Q: What is a normal sweat rate for runners?
A: Most trained runners sweat at 16–40 oz/hr (0.5–1.2L/hr) in temperate conditions. In heat, rates of 40–64+ oz/hr (1.2–2.0L/hr) are common. Elite athletes in hot conditions can exceed 80 oz/hr (2.4L/hr). Your individual rate may differ significantly from these averages.
Q: Should I drink to schedule or to thirst during a marathon?
A: For races under 2 hours: drink to thirst is well-validated and safe. For races over 2 hours in heat: a combination of thirst-based drinking and ensuring you take fluid at every aid station is safer than pure thirst-based intake — as thirst response can lag behind actual needs in hot conditions.
Q: How do I know if I'm a salty sweater?
A: Signs of high sweat sodium: white residue or crust on skin/kit after exercise, frequent cramping, sweat that burns your eyes, sweat that tastes distinctly salty. If you have 2 or more of these signs, you likely lose 1,000–1,500+ mg sodium per litre of sweat and should be especially diligent about sodium replacement.
Verified Data Sources & Authority References
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