How to predict your marathon time (and actually be right)

Every calculator will happily turn your 5K into a marathon prediction. The number it spits out is often nonsense, and not because the maths is wrong. It is because predicting a marathon is half arithmetic and half honesty about your training.

Here is how I do it, in order of how much I trust each method.

Best: use a recent half marathon

If you have raced a half in the last month or two, you have the single best predictor there is. A half is long enough that your endurance is already in the picture, so the jump to the full is mostly about pacing and fueling, not raw fitness.

A rough rule a lot of coaches use: take your half time, double it, and add somewhere between 10 and 20 minutes. The slower and less marathon-specific your training, the closer to 20. Plug your half into the predictor and you will see it lands in the same neighborhood, because that is roughly what Riegel's formula does too.

I ran a 1:32 half last spring, the calculator said 3:14, I trained properly, and I crossed the line at 3:16. Two minutes off. That is about as good as prediction gets.

Decent: use a 10K, with a pinch of salt

No recent half? A 10K works, but the gap you are extrapolating across is bigger, so the margin for error grows. The prediction assumes you have the endurance to hold a pace for four times longer than you just raced. If your weekly long run is still under 90 minutes, shave your expectations.

Risky: use a 5K

A 5K can predict a marathon, in the same way a sprint can predict a 5K. Technically yes, practically be careful. Use it only as a ceiling, the time you might run if everything about your endurance were as good as your speed. For most of us it is not, yet.

The endurance reality check

Whatever number you get, ask yourself three questions before you believe it.

• Have I run at least a few long runs of 18 miles or more?
• Can I hold the predicted pace for an hour without it feeling hard?
• Have I practiced eating and drinking on the run?

If you answered no to any of these, your real marathon will likely be slower than the prediction, and that is fine. The number is a target. The long runs are how you earn it.

Put it to work

Get your prediction, then build a pace band for it with even or slightly negative splits. The runners who blow up are almost always the ones who banked time early. Trust your pace, start a touch slower than feels right, and let the back half come to you.