Race time predictor
Enter a recent race result and the distance you want to forecast. The calculator applies the Riegel formula and shows the predicted time with the pace per kilometre and per mile.
Runs in your browser: nothing you enter is sent anywhere.
How to use it
- Pick the distance of your recent race, or choose a custom distance in kilometres, and enter your time.
- Pick the distance you want a forecast for.
- Read the predicted time and the pace per kilometre and per mile. Change the exponent only if you want to test another assumption.
How it works
T2 = T1 × (D2 ÷ D1)^exponent
T1 is your known time, D1 and D2 are the two distances and the exponent is 1.06 by default. Distances used: mile 1.609344 km, half marathon 21.0975 km, marathon 42.195 km. Pace is the predicted time divided by the target distance, rounded to the second.
Worked example
A 5K in 25:00 is 1500 seconds. For a 10K, (10 ÷ 5)^1.06 = 2.085, so the forecast is 1500 × 2.085 = 3127 seconds, about 52:07, a pace of 5:13 per km. With the exponent set to 1.00 the same input gives exactly 50:00.
Tips
Race the longer distance only after training for it: the formula assumes equal preparation. For a marathon forecast, a half marathon is a better starting point than a 5K. Compare the result with your training paces in the pace calculator.
Frequently asked questions
What is the Riegel formula?
Pete Riegel's formula predicts a time at a new distance from a known one - T2 = T1 × (D2 ÷ D1)^1.06. The exponent above 1 reflects that your pace slows as the distance grows.
How accurate is a race time prediction?
It is an estimate, not a promise. It works best between nearby distances, such as 5K to 10K, and for runners with enough weekly mileage for the longer race. It overestimates fitness if you have never trained for the longer distance.
Why can I change the exponent?
1.06 is the commonly quoted default. A higher value predicts a bigger slowdown, which suits runners with a speed bias; 1.00 means a constant pace. Values from 1.00 to 1.20 are accepted.
Which result should I use as the starting point?
Use a recent, all-out race or time trial on a similar course. Training runs, hilly races and races in heat or wind give a slower and less reliable forecast.