Fuelling · Free tool
Running Sodium & Electrolyte Calculator
Enter how long you expect to be out, roughly how much you sweat, how salty that sweat is, and what each capsule or drink serving carries. We estimate your sodium loss per hour and in total, how much of it to replace, and how often to take a serving.
Why sodium matters on long runs
Sweat is not just water. It carries sodium, and the amount varies enormously between people: measured sweat sodium runs from around 200 mg per litre to over 2,000 mg, with a population average near 950 mg. Combine that with sweat rates of 0.5–2 litres an hour and a three-hour effort can cost anywhere from a few hundred milligrams to more than five grams.
Losing sodium while drinking plain water dilutes the blood, which is what causes exercise-associated hyponatraemia — the dangerous condition behind most serious medical cases at marathons. Replacing a sensible share of what you lose, and drinking to thirst rather than to a schedule, is the guard against it. Cramping and a post-run craving for salty food are the everyday signs you are running short.
Know your own numbers
Sweat rate is easy to measure: weigh yourself naked before and after a one-hour run, add back any fluid you drank, and the difference in kilograms is your loss in litres. Repeat it in different temperatures — the same runner can lose twice as much on a hot day.
Sweat sodium is harder. A lab or sports-nutrition sweat test gives an exact figure; without one, use the presets and the tell-tale signs. White streaks on a black cap or shirt, sweat that stings your eyes, and a strong craving for salt after long runs all point to the salty end of the range.
How much to replace
You do not need to replace every milligram during the run — the body tolerates a moderate deficit and you will eat afterwards. Replacing 50–75% of the loss is the usual target, which for most runners lands in the 300–700 mg per hour range. Genuinely salty sweaters in the heat can need well over 1,000 mg an hour, which is where capsules earn their place.
Spread the intake evenly rather than front-loading it, and take capsules with water. A 250 mg salt capsule and a 500–1,000 mg electrolyte drink serving are the common building blocks; the calculator turns your target into a count and a timing interval for whichever you carry.
Frequently asked questions
How much sodium should a runner take per hour?
It depends on how much you sweat and how salty it is. A typical runner losing a litre an hour at 950 mg per litre who replaces 75% needs about 700 mg an hour. Light, low-sodium sweaters may need only 200–300 mg; heavy, salty sweaters in heat can need 1,500 mg or more.
Do I need salt tablets for a marathon?
Not necessarily. Many runners get enough from sports drinks and gels, especially in cool weather. Capsules make sense when your estimated loss is high — a hot race, a heavy sweat rate, or a salty-sweater profile — because they deliver sodium without extra sugar or fluid.
How do I know if I am a salty sweater?
White salt streaks on dark kit, sweat that stings your eyes, gritty skin after a run, and strong salt cravings afterwards are the usual signs. A sweat test at a sports lab or nutrition clinic confirms it with a number you can plug straight into this calculator.