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Sports Hydration & Electrolyte Calculator

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Reviewed by: Martín Rodríguez (editorial policy ) · Last reviewed: Jun 4, 2026

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During exercise, you lose water and electrolytes through sweat. Exercise science shows that a fluid deficit greater than 2% of body weight impairs endurance, strength, and concentration. This calculator tells you how many liters that threshold represents for you, how much to drink per hour to stay within your limit, and when you need electrolytes instead of plain water. Reference values come from the American College of Sports Medicine (ACSM) and peer-reviewed research.

Last reviewed: June 3, 2026 Verified by Martín Rodríguez Source: ACSM Position Stand: Exercise and Fluid Replacement (Medicine & Science in Sports & Exercise), Compositional Aspects of Beverages for Hydration Before, During, and After Exercise — PMC/NCBI, 9 Facts About Hydration & Electrolytes — ACSM, Sweat Rate Calculation Formula — CDC / NCEH Heat Safety, TrueSport: Why and How to Calculate Your Athlete's Sweat Rate 100% private

For most athletes, the safe dehydration limit is 2% of body weight — at 70 kg that equals 1.40 L. Recommended hourly intake = sweat rate × 0.80: at moderate intensity (1.0 L/h) drink 800 mL/h. For sessions over 60 minutes, add 500–700 mg sodium per liter to prevent hyponatremia.

When to use this calculator

Runner calculating how much fluid loss is acceptable during a 10 km race.
Road cyclist planning how many water bottles to carry based on weight and ride duration.
Soccer or tennis player training over 60 minutes who needs to know when to switch to an isotonic drink.
Hiker or trekker calculating the minimum fluid reserve needed for a day in the backcountry.

Example: 72 kg cyclist, 2-hour ride, high intensity

Body weight: 72 kg
Duration: 2 hours
Intensity: high (≈1.5 L/h sweat rate)
Dehydration threshold: 2%
Max safe deficit = 72 × 2 / 100 = 1.44 L
Estimated sweat loss = 1.5 L/h × 2 h = 3.0 L
Sweat loss exceeds the limit → active hydration is critical
Recommended intake: 1.5 × 0.80 = 1.2 L/hour = 300 mL every 15 min
Sodium: ≈ 1,440 mg total (isotonic drink with 500–700 mg Na/L)

Result: Safe deficit limit 1.44 L — drink ≈1.2 L/h and include an isotonic drink with sodium

How it works

2 min read

How It's Calculated

This calculator applies the ACSM (American College of Sports Medicine) guidelines in three steps:

1. Maximum safe fluid deficit

Safe deficit (L) = Body weight (kg) × Dehydration threshold (%) / 100

Example: 70 kg × 2% / 100 = 1.40 L

This is the volume of fluid you can lose before physical and cognitive performance starts to decline measurably (ACSM Position Stand, 2007).

2. Estimated sweat loss

Sweat loss (L) = Sweat rate (L/h) × Duration (h)

Reference sweat rates (ACSM):

Intensity	Sweat rate
Low (light exercise, cool weather)	≈ 0.6 L/h
Moderate (jogging, cycling, recreational sport)	≈ 1.0 L/h
High (intense training or hot weather)	≈ 1.5 L/h
Very high (marathon, extreme heat)	≈ 2.0 L/h

3. Recommended hourly intake during exercise

Intake per hour (L/h) = Sweat rate × 0.80

ACSM recommends replacing 75–100% of sweat losses during exercise. Targeting 80% avoids hyponatremia from overdrinking while preventing excessive dehydration.

Quick-reference table: safe deficit and hourly intake by weight and intensity

Body weight	Safe deficit (2%)	Intake/h — Moderate (1.0 L/h)	Intake/h — High (1.5 L/h)
50 kg	1.00 L	800 mL/h	1,200 mL/h
60 kg	1.20 L	800 mL/h	1,200 mL/h
70 kg	1.40 L	800 mL/h	1,200 mL/h
80 kg	1.60 L	800 mL/h	1,200 mL/h
90 kg	1.80 L	800 mL/h	1,200 mL/h
100 kg	2.00 L	800 mL/h	1,200 mL/h

Hourly intake depends on sweat rate (intensity), not body weight. The safe deficit does scale with weight.

Why is 2% the key threshold?

Classic studies by Armstrong and Maughan, and the ACSM Position Stand, show that:

< 2% loss → performance generally maintained.

2–4% → drops in VO₂max, strength, and concentration.

> 5% → risk of heat stroke, seizures, and collapse.

In extreme heat or for older athletes, lower the threshold to 1.5%.

When to use an isotonic drink instead of water

Plain water is fine for activities under 60 minutes at moderate intensity. For longer or very intense sessions, isotonic drinks (500–700 mg/L sodium, 4–8% carbohydrates) absorb faster and replace sodium lost in sweat, preventing exercise-associated hyponatremia.

Homemade isotonic drink recipe (per liter of water)

Table salt: 1 g (≈ 400 mg sodium)

Sugar or dextrose: 40–60 g

Lemon juice: to taste (potassium + palatability)

This yields an osmolality close to 280–300 mOsm/kg — within the isotonic range.

Disclaimer: Results are guidelines only and do not replace evaluation by a sports nutritionist or physician. For symptoms of severe dehydration (confusion, dark urine, dizziness), seek professional medical attention.

Frequently asked questions

How does this calculator determine how much I should drink per hour?

It uses the ACSM formula: multiply your sweat rate (based on the intensity you selected) by 0.80. That 80% target prevents excessive dehydration without the risk of overdrinking. For example, at high intensity (≈1.5 L/h) you should drink 1.2 L/h — about 300 mL every 15 minutes.

What is an isotonic drink and how does it differ from water?

An isotonic drink has a solute concentration (sodium, sugar) similar to blood plasma: 280–300 mOsm/kg. This allows the intestine to absorb it faster than plain water, and it simultaneously replaces electrolytes — especially sodium and potassium — lost in sweat. Water alone is fine for sessions under 60 minutes; for longer or hotter sessions, an isotonic drink is superior.

Why is 2% of body weight the dehydration limit?

ACSM research and PubMed reviews consistently show that a fluid deficit above 2% of starting body weight measurably reduces VO₂max and muscular power. Below that threshold the body compensates well. In extreme heat or for older athletes the recommended limit is 1.5%.

How do I measure my actual sweat rate?

Weigh yourself undressed before and after exercise (without eating or drinking during the session, or noting exactly how much you drank). The weight difference in grams approximates milliliters of sweat lost. Divide by the session duration in hours to get your rate in mL/h. Example: losing 0.9 kg in 45 min gives 0.9 kg ÷ 0.75 h ≈ 1.2 L/h.

How much sodium do I need to replace per liter of sweat?

Sweat sodium varies widely: 200–1,000 mg/L depending on the individual and conditions, averaging around 460 mg/L. ACSM recommends sports drinks for exercise > 60 min contain 500–700 mg/L of sodium to efficiently replace losses and maintain fluid drive without excess.

Can you drink too much water during exercise?

Yes. Overhydrating with plain water can cause exercise-associated hyponatremia (dangerously low blood sodium), a potentially serious condition causing nausea, confusion, and in extreme cases seizures. The risk is highest in long events (marathons, ultras) where athletes drink past thirst. Hydrate according to estimated losses and thirst — don't exceed them.

Do electrolytes help prevent muscle cramps?

Exercise cramps have multiple causes: neuromuscular fatigue, dehydration, and electrolyte imbalance (mainly sodium and potassium). Replacing electrolytes reduces their incidence, though evidence isn't conclusive for all cramp types. For athletes who sweat heavily and leave white salt marks on their kit, sodium replacement has solid clinical backing.

How often should I drink during exercise?

Current guidelines recommend drinking before you feel thirsty, in small amounts every 15–20 minutes. Don't wait for a dry mouth — thirst appears when you've already lost around 1–2% of your body weight in water. For sessions over 60 minutes, alternating between water and isotonic drinks optimizes absorption and mineral replacement.

Does this calculator work for team sports like soccer, basketball, or tennis?

Yes. Even though intensity varies throughout a game, the cumulative deficit can be significant over 60–90 minutes of intense play. Enter your weight, the full game duration, and select 'high' intensity. In hot or humid conditions, lower the threshold to 1.5% and bring an isotonic drink instead of plain water.

Should I drink differently before a race or competition compared to a training session?

ACSM recommends pre-loading with 5–7 mL/kg of fluid 2–4 hours before competition to start fully hydrated. During the event, follow the same 80% sweat replacement guideline. Post-event, drink 1.25–1.5× any remaining deficit to restore plasma volume within 2–4 hours.