Electrolyte Replacement Calculator

What it does

Calculate hourly sodium, potassium, and water-replacement targets for endurance training based on your body weight, estimated sweat rate (sweater / average / heavy), and climate (cool / moderate / hot). The tool returns: milligrams of sodium per hour, milligrams of potassium per hour, milliliters of water per hour, and a brief note on which sports-drink products or DIY mixes hit those targets.

Why electrolytes matter: during sustained exercise (over 60 min, especially in heat), you lose significant sodium through sweat — typically 500-1,500 mg of sodium per liter of sweat for normal sweaters, up to 2,000+ mg/L for heavy salty sweaters. Without replacement, blood-sodium levels drop (hyponatremia) which causes muscle cramps, fatigue, GI distress, and in severe cases life-threatening brain swelling. The marathon-cramp / endurance-bonk phenomenon is often electrolyte-driven, not just glycogen depletion.

Standard recommendations for endurance training:

• Sodium: 300-700 mg/hour for cool conditions and average sweaters; 700-1,200 mg/hour for hot conditions or heavy sweaters; 1,200-2,000+ mg/hour for elite athletes in extreme heat.
• Potassium: 100-300 mg/hour. Less critical than sodium because the body conserves it better; mostly comes from food.
• Water: 400-800 mL/hour (13-27 oz) depending on sweat rate, body weight, and tolerance.
• Carbohydrate (not in calculator but related): 30-60 g/hour for sub-2-hour efforts; up to 120 g/hour for ultra-endurance with trained-gut athletes.

Important caveats: these are population averages. Real sweat rates vary enormously (200 mg sodium per liter to 2,500+ mg/L across individuals). For accurate personal numbers, get a sweat-test (Levelen, Precision Hydration, Gatorade GSSI offer them; ~$100-200) or do a DIY weighed- before-and-after sweat-rate test. This calculator is a reasonable starting estimate, not personalized prescription.

How to use it

1. Enter your body weight (lbs or kg). Sweat losses scale with body size.
2. Pick sweat rate: low (cool, light effort), average (most casual athletes), heavy (you finish workouts visibly soaked).
3. Pick climate: cool (under 60°F), moderate (60-80°F), hot (above 80°F or high humidity).
4. Read per-hour targets for sodium, potassium, and water. The tool also suggests products / DIY mixes hitting these numbers.
5. For workouts under 60 minutes, electrolyte replacement is rarely needed (water alone is fine). Above 60 min in moderate heat, or 30+ min in extreme heat, replacement matters.

When to use this tool

• Long-duration training (90+ min) where electrolytes affect performance.
• Hot-weather endurance events (marathons, triathlons, ultras, long bike rides).
• Recovery from a heavy training day where you finished feeling depleted, weak, or with muscle cramps.
• Comparing sports-drink products against your actual needs (most commercial drinks are under-dosed for serious endurance).

When not to use it

• Short workouts (under 60-90 minutes) — water is sufficient. Replacing electrolytes for a 45-min jog is overkill.
• Medical conditions affecting electrolytes (kidney disease, heart failure with sodium restriction, certain meds) — work with a doctor; don't self-prescribe.
• When you've gotten a personal sweat test — those results override calculator estimates.
• Pediatric exercise — kids' electrolyte needs are different and lower; consult a pediatric sports specialist.

Common use cases

• Quick calculation during a typical workday
• Pre-decision sanity-check on inputs and outputs
• Educational use — demonstrating the underlying concept
• Onboarding a colleague who needs the same calculation/conversion