— Sodium per hour (mg/h)
— Sodium total (mg)
— Potassium per hour (mg/h)
— Magnesium total (mg)
— Total electrolyte needs (mg)
Methodology
This calculator uses sweat volume (duration × sweat rate) combined with sweat sodium concentration to estimate electrolyte losses. Sodium concentration in sweat ranges from approximately 230 mg/L for low-saltiness sweaters to 1,150 mg/L for high-saltiness sweaters, with a moderate midpoint around 690 mg/L — consistent with ranges reported by Shirreffs & Sawka (2011) and Baker & Jeukendrup (2014). Potassium losses are estimated at roughly 15–20% of sodium losses. Magnesium is calculated as a session-level need based on sweat volume, as magnesium concentration in sweat is relatively stable (~8–12 mg/L). Total electrolyte needs sum sodium, potassium, and magnesium across the full session.
FAQ
How do I know my sweat saltiness without a lab test?
You can make a reliable judgment from everyday training observations. Low saltiness: your skin feels relatively clean after a session, no white marks on dark clothing, and your eyes don't sting from dripping sweat. Moderate saltiness: you occasionally see faint salt rings on your kit after long sessions or in the heat, and sweat in your eyes produces mild irritation. High saltiness: your dark shirts regularly show white crystalline residue, your sweat stings your eyes noticeably, and you can taste salt on your lips during hard efforts. Genetics play a large role — high-salt sweaters often have family members with the same trait.
Why does sodium matter more than other electrolytes during exercise?
Sodium is the dominant electrolyte in sweat — you lose far more of it per litre than potassium or magnesium. More importantly, sodium is the key driver of thirst and fluid retention. Without adequate sodium replacement during long efforts, you risk hyponatraemia (low blood sodium), which can cause nausea, confusion, and in severe cases is life-threatening. Potassium and magnesium losses are real but typically smaller in magnitude, and they can often be topped up through post-exercise food. Sodium replacement during the session itself is usually the priority.
When is electrolyte supplementation actually necessary?
For sessions under 60–75 minutes at moderate intensity, plain water is usually sufficient. The case for deliberate electrolyte supplementation becomes clear when: your session exceeds 90 minutes, you're training or racing in hot or humid conditions, your sweat rate is high (above ~1.5 L/h), or you're a salty sweater. Races and multi-session training days also increase the stakes — recovering your electrolyte balance before your next session matters. For casual exercise, a balanced diet covers most electrolyte needs without supplementation.
What's the practical difference between sports drinks and electrolyte tablets?
Sports drinks combine electrolytes with carbohydrates, which is useful when you need both fuel and hydration simultaneously — for example, in races lasting over 75–90 minutes. The downside is that you're locked into a fixed carb-to-electrolyte ratio that may not match your needs. Electrolyte tablets or capsules give you independent control: you can add them to plain water and adjust dosing without affecting your carbohydrate strategy. They're also easier to carry for long events. Neither is universally better — it comes down to session length, your fuelling strategy, and personal taste preferences.
What are the signs of electrolyte imbalance during exercise?
Muscle cramps are the classic sign, though research shows cramps have multiple causes and aren't always electrolyte-driven. More reliable warning signs include: unusual fatigue that goes beyond expected tiredness for the effort, nausea or a bloated feeling (often from drinking too much plain water without sodium), headaches during or after a session, and in more serious cases, confusion, slurred speech, or swelling in the hands and feet. If you feel fine drinking lots of water but your performance is still dropping, low sodium is worth considering. Always treat confusion or extreme symptoms as a medical emergency.
How do I combine this with a sweat rate calculator for more accurate results?
The most accurate approach is to measure your actual sweat rate rather than using a default estimate. Weigh yourself (in minimal clothing) immediately before and after a training session of known duration, without eating or drinking during the session. Each kilogram lost equals approximately 1 litre of sweat. Divide total sweat loss by hours to get your L/h rate. Then enter that measured value into this calculator's sweat rate field. Repeating the test in different conditions (heat, cold, easy effort, hard effort) builds a useful personal profile, since sweat rate can vary significantly with environment and intensity.
Results are estimates based on population averages and published research. Individual sweat composition varies considerably. These figures are intended as a planning guide, not a clinical prescription. Consult a sports dietitian or healthcare professional for personalized electrolyte strategies, especially for competitions, extreme heat, or if you have any health condition.
