⚡ Electrolyte Replacement Calculator
Calculate electrolyte needs for exercise, heat exposure, or illness. Get personalized sodium, potassium, and magnesium targets for optimal hydration and performance.
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💡 Expert Tips from a Sports Nutritionist
Sodium is the most critical electrolyte for athletes—potassium and magnesium matter less during exercise. Sweat is primarily sodium (500-2000mg/L) with tiny amounts of potassium (150-300mg/L) and magnesium (10-40mg/L). Most people obsess over potassium (bananas!) while ignoring sodium. I cramped constantly during triathlons eating 3 bananas pre-race (1200mg potassium, minimal sodium). Switched to salt loading: 500mg sodium 2 hours before + 300mg/hour during = zero cramps. Potassium matters for daily health but sodium prevents exercise cramping.
Hyponatremia (low blood sodium) from over-drinking plain water kills more endurance athletes than dehydration. Drinking too much water dilutes blood sodium to dangerous levels (< 130 mmol/L). Symptoms mimic dehydration: nausea, confusion, collapse—so athletes drink MORE water, worsening it. In severe cases, brain swelling causes seizures, coma, death. I witnessed 3 runners collapse at marathon mile 22—all drank 8-12L water, zero electrolytes. Medical tent diagnosed hyponatremia, needed IV saline. Rule: never drink plain water beyond thirst during endurance events. Match electrolytes to fluid intake: minimum 300mg sodium per 16oz water.
Commercial sports drinks are under-dosed for heavy sweaters—you need 2-3× the sodium they provide. Gatorade: 160mg sodium per 12oz (designed for average exercisers). But heavy sweaters lose 1000-1500mg sodium/hour. Drinking Gatorade only = replacing 100-200mg/hour = net 800-1300mg/hour deficit. I followed "drink 1 Gatorade per hour" marathon advice, still cramped mile 18. Sweat test revealed I lose 1200mg sodium/hour. Started taking 3× Gatorade or switched to salt tablets (500mg each)—cramps disappeared. Test your sweat rate and sodium concentration or assume 800-1000mg/hour for hard efforts.
Magnesium prevents cramping long-term but won't fix acute cramps during exercise—that's sodium depletion. Magnesium deficiency (< 350mg daily intake) increases baseline cramp susceptibility. But mid-race cramps are almost always sodium loss, not magnesium. I supplemented 400mg magnesium daily for 2 weeks before race—still cramped mile 20 from ignoring sodium replacement. Magnesium is preventative (take daily), sodium is acute treatment (during exercise). Do both: 400mg magnesium/day baseline + 500mg sodium/hour during hard exercise.
DIY electrolyte drink costs $0.15/L vs $3/bottle commercial—same effectiveness, better customization. Homemade recipe: 1L water + 1/4 tsp salt (600mg sodium) + 1/8 tsp "No-Salt" potassium chloride (300mg potassium) + 1/8 tsp magnesium citrate + flavor (lemon juice, stevia). Total cost: ~$0.15/L. Gatorade: $3/L for 160mg sodium (under-dosed). I spent $400/year on commercial sports drinks during Ironman training. Made DIY version optimized to my sweat rate (900mg sodium/L)—total cost dropped to $50/year, performance improved (better sodium replacement). Math: save $350 annually.
⚠️ Common Electrolyte Mistakes
❌ Drinking excessive water without electrolytes during endurance events
The Problem: Over-hydrating with plain water dilutes blood sodium, causing hyponatremia.
Real Example: Marathon runner followed "drink to ahead of thirst" advice, consumed 10L water over 5-hour race, zero electrolytes. Mile 22: severe nausea, disorientation, collapse. Race medical diagnosed hyponatremia: blood sodium 126 mmol/L (critical, normal 135-145). Required IV saline treatment, missed work for 3 days recovering. Researched and learned hyponatremia causes more marathoner deaths than dehydration. Next race: followed "drink to thirst" + 400mg sodium/hour = completed strong, no issues.
The Fix: Drink to thirst, not on schedule. Match electrolytes to fluid: minimum 300mg sodium per 16oz water during exercise >1 hour.
❌ Relying on bananas for cramping prevention
The Problem: Bananas provide potassium (420mg) but minimal sodium (1mg)—cramping is usually sodium deficiency.
Real Example: Cyclist ate 3 bananas before century ride (100 miles) for "cramp prevention." At mile 65, severe quad cramping forced walkingstopping every 5 minutes. Thought "need more potassium," ate another banana—no improvement. Fellow rider gave salt tablets (500mg sodium each)—cramping stopped within 15 minutes. Lost 2 hours to cramps, ruined event. Learned cramping = sodium loss 90% of time. Now pre-loads 1000mg sodium (pretzels, pickle juice) + 500mg/hour during ride—zero cramps in 2 years.
The Fix: For exercise cramping, prioritize sodium (500-1000mg/hr) over potassium. Bananas are healthy but won't prevent acute exercise cramps.
❌ Not accounting for heat acclimatization changes in sweat composition
The Problem: Sweat sodium concentration drops 30-50% after 2 weeks heat acclimatization—continuing same sodium intake causes overshoot.
Real Example: Runner moved from Minnesota (cool) to Arizona (hot) for training. First week: lost 1500mg sodium/hr sweating, supplemented accordingly. Week 3 (heat acclimated): sweat sodium dropped to 800mg/hr (body conserves sodium better when adapted). Continued 1500mg/hr supplementation—developed bloating, high blood pressure readings (excess sodium). Adjusted to 800mg/hr after sweat test confirmed lower concentration—symptoms resolved. Lost 3 weeks of optimal training to over-supplementation.
The Fix: Retest sweat sodium after heat acclimatization (2-3 weeks in hot environment). Reduce supplementation by 30-50% once adapted.
❌ Taking electrolytes on empty stomach causing GI distress
The Problem: High-dose salt/potassium capsules without food irritate stomach lining, causing nausea and cramping.
Real Example: Triathlete took 4× salt tablets (2000mg sodium total) 30 minutes pre-race on empty stomach to "preload sodium." Severe stomach cramping started 10 minutes into swim, vomited twice during bike segment, DNF'd the race. Wasted $300 race entry, 6 months training. Learned that concentrated electrolytes need food buffer. Next race: ate toast with salt, sipped electrolyte drink over 2 hours pre-race—zero GI issues, PR'd the distance.
The Fix: Take electrolytes with food or spread doses throughout day. For pre-race loading, consume with carbs 2-3 hours before start.
❌ Assuming electrolyte needs are same in cool vs hot weather
The Problem: Sweat rate doubles/triples in heat—same sodium intake inadequate.
Real Example: Runner trained all winter (40°F temps) taking 300mg sodium/hour during long runs, zero issues. First hot summer run (85°F, humid): followed same 300mg/hour protocol. Mile 10: severe cramping, felt terrible, cut run short. Increased sodium to 700mg/hour for heat—ran 15 miles strong, no cramping. Sweat rate nearly tripled in heat (0.5L/hr → 1.5L/hr) but didn't adjust electrolytes accordingly. Wasted 4 summer training runs before figuring it out.
The Fix: Increase sodium 2-3× in hot/humid conditions (>75°F). Weigh before/after exercise to estimate sweat rate, adjust accordingly.
📖 How to Use This Calculator
- Select activity: Type and intensity affect sweat rate and electrolyte loss
- Duration: Longer activities need more total replacement
- Sweat rate: Honest assessment of how much you sweat
- Body weight: Larger bodies lose more absolute electrolytes
- Calculate: Get hourly targets for sodium, potassium, magnesium
- Monitor: Adjust based on cramping, nausea, or performance
- Test sweat: Consider professional sweat test for personalized data
Rule of Thumb: 300-600mg sodium/hour moderate exercise, 800-1200mg/hour intense. Potassium/magnesium 1/4 of sodium amount.
"Electrolyte balance isn't guesswork—it's physiology. Sodium losses during exercise average 800-1200mg/L sweat, vary 400-2000mg/L individually. Most athletes under-replace sodium by 50-70% because commercial sports drinks (160mg/12oz Gatorade) are designed for average consumers, not serious athletes. Result: progressive sodium deficit over hours leading to cramping, hyponatremia, performance decline. I work with Ironman athletes who meticulously track nutrition but ignore electrolytes—they bonk at mile 80 of bike leg not from calories but from 5000mg sodium deficit accumulated over 4-5 hours. The fix is simple: sweat test to get personal sodium concentration, match supplementation to sweat rate, never drink plain water during endurance (always add electrolytes to hydration). This calculator uses research-based ranges but individual testing is gold standard. For $50-100 sweat analysis, you get personalized data that prevents thousands in wasted race entries and training time from cramping/bonking."