Predicting a race time
Riegel's formula predicts your time at one distance from a result at another: T2 = T1 × (D2/D1)^1.06. The exponent above 1 reflects that we slow down slightly over longer distances — you can't hold 5 km pace for a marathon.
What it assumes
The prediction assumes you're trained for the target distance and race in similar conditions. Jumping from a 5 km to a marathon without the endurance base will beat the prediction; a well-trained runner often lands close.
Use it to set goals
Plug in a recent race to get a realistic target for your next one, then work backward to the pace you need to hold. Treat it as a guide and adjust for hills, heat and how your training has gone.
Frequently asked questions
Is the prediction guaranteed?
No. It's a well-known estimate that works best when you're properly trained for the longer distance and conditions are similar.
Why the 1.06 exponent?
It captures the slight, predictable slowdown as distance grows. Some runners use a slightly higher value for marathons.
How does a race time predictor work?
It uses a recent race result to estimate your time at another distance with Riegel's formula, which accounts for the natural slowdown over longer distances.
Can it predict a marathon from a 10K?
Yes, though longer predictions assume you have done the endurance training for the distance; otherwise the real time will be slower.
How accurate is the prediction?
It is most reliable between similar distances and for a well-trained runner. Treat it as a target, not a guarantee.