26.2 miles is long enough to get it wrong both ways.
A short run forgives a hydration mistake. A marathon compounds it for three, four, five hours — and it can fail in two opposite directions at once. Most advice only warns you about one. Here's how to find your own number and drink to it, honestly.
Get your marathon number →The two ways a marathon breaks.
A salty sweater loses more sodium than gels and sports drinks replace. Over four hours that gap becomes the late-race fade, the calf that seizes at mile 21, the "I trained for this — what happened?"
Why you cramp →Drinking at every aid station — the classic slower-finisher pattern — can dilute your blood sodium into hyponatremia, which at the severe end is more dangerous than dehydration.
Can you drink too much? →The reason generic advice fails here is that the same instruction — "drink X per hour, take Y salt" — pushes one runner toward the first failure and another toward the second. Sweat sodium varies about tenfold between people. There is no one number; there's your number.
Why the distance is the whole problem.
On a 30-minute run, being 20% off on fluid or sodium is invisible. Across a marathon it isn't: over three to four-plus hours a heavy, salty sweater can lose several grams of sodium — a scale of loss a couple of gels can't touch, and one that a single fixed-dose tab was never built for. The error doesn't stay small; it accumulates every mile. That's exactly why a number you measured beats a number you guessed.
The plan that survives mile 22.
- Measure your sweat rate once. One before-and-after weigh-in on a long run tells you how much fluid you actually lose per hour. How to do it →
- Size sodium to your losses, not to a stick. The calculator turns your rate and how salty you sweat into an actual sodium target for the hours you'll be out — then shows the cheap mix that carries it.
- Drink to thirst, not to a schedule. Take sodium with your fluid, but let thirst decide how much you drink. Forcing fluid at every station is how the second failure happens.
- Practice it in training. Nothing new on race day. Rehearse the exact fluid and sodium on your long runs so race morning is a repeat, not an experiment.