Race Time Predictor
Just ran a strong 5K and wondering what it means for your half? Plug in a recent race and get a realistic target for another distance.
This uses Riegel's formula, which assumes you keep training and pace fades a little as distance grows. It's a solid starting estimate, but weather, hills, and how you've trained for the specific distance all move the real number.
How it works
This uses Riegel's formula: t2 = t1 × (d2 / d1)^1.06. The 1.06 exponent bakes in the fact that you can't hold your 5K pace for a marathon — the longer you go, the more pace naturally fades.
Give it an honest, recent race effort for the best guess. A time from a year ago, or a training run you weren't pushing on, will throw the prediction off.
It assumes you've actually trained for the target distance. Predicting a marathon off a mile time is mathematically fine but unrealistic if you've never run long — treat big jumps with a grain of salt.
Frequently asked questions
Why is my predicted marathon slower than I hoped?
The formula assumes pace fades as distance grows, which is realistic — but it also assumes marathon-specific training. If you've built a strong long-run base and fueled well, you can beat the estimate. If you haven't, the real day can be tougher than predicted.
What's the most reliable input distance?
Something close to your target works best. A 10K predicts a half-marathon far better than a mile does, because the physiological demands are more similar. The bigger the jump between distances, the fuzzier the estimate.
Does this account for hills or heat?
It doesn't. The prediction assumes similar conditions to your input race. A hilly or hot target course will run slower than the number here, so adjust your expectations for the day.