Race Time Predictor

Predict your finish time and pace at a new distance from a recent race result, using Riegel's formula T2 = T1 × (D2 ÷ D1)^1.06.

Quick Facts

Method
Riegel's endurance formula: T2 = T1 × (D2 ÷ D1)^1.06
The 1.06 exponent captures the pace slowdown as distance grows. Most accurate when the two distances are close.

Your Results

Calculated
Predicted finish time
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At the target distance
Predicted pace
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Per kilometer
Predicted pace
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Per mile
Average speed
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km/h

Ready

Enter a recent race result and a target distance, then calculate.

About the race time predictor

This calculator predicts how fast you can run a race at one distance based on a recent race result at another distance. It uses Riegel's formula, published by Peter Riegel in the late 1970s and one of the most widely used endurance-prediction models in running. Enter a distance and finish time you have actually raced, choose a target distance, and the tool returns a predicted finish time, pace per kilometer and per mile, and average speed.

The formula

Riegel's relationship between time and distance is:

  • T2 = T1 × (D2 ÷ D1)1.06
  • T1 = your known finish time, D1 = the distance you ran it over
  • T2 = the predicted finish time, D2 = the target distance

The exponent 1.06 is the key. If it were exactly 1.0, the formula would assume you hold the same pace at any distance — but no runner can. The extra 0.06 encodes the observed slowdown in pace each time distance grows. Riegel derived this exponent from world-record and competitive race data across running, swimming and cycling; for running it sits very close to 1.06.

Worked example

Suppose you race a 5K in 22:00 (1320 seconds) and want your 10K prediction. The ratio 10 ÷ 5 = 2, and 21.06 ≈ 2.0851. So T2 ≈ 1320 × 2.0851 ≈ 2752 seconds ≈ 45:52. Notice the predicted 10K time is more than double the 5K time — that is exactly the point: doubling the distance costs you a little pace, so it takes slightly more than twice as long.

Common reference points

From a 20:00 5K, Riegel predicts roughly a 41:42 10K, a 1:32:00 half marathon, and a 3:11:49 marathon. In practice, most runners find the shorter jumps (5K→10K, 10K→half) very accurate, while the full marathon prediction tends to be optimistic unless marathon-specific endurance and fueling are trained. A common rule of thumb is that your marathon is often a few percent slower than a pure Riegel projection from a 5K or 10K.

Standard race distances

  • 5K = 5.0 km
  • 10K = 10.0 km
  • Half marathon = 21.0975 km
  • Marathon = 42.195 km

Frequently Asked Questions

What formula does the race predictor use?
It uses Peter Riegel's endurance formula: T2 = T1 × (D2 ÷ D1)^1.06, where T1 is your known finish time over distance D1 and T2 is the predicted time over the new distance D2. The exponent 1.06 accounts for the fact that pace slows as distance grows, because you cannot hold the same speed for twice the distance.
How accurate is Riegel's race prediction?
It is most accurate when the two distances are reasonably close (for example predicting a 10K from a 5K, or a half marathon from a 10K) and when you have trained specifically for the target distance. Predictions from a 5K straight to a marathon tend to be optimistic because the marathon adds fueling and endurance demands the 1.06 exponent does not model. For big jumps, expect the real time to be a few percent slower than predicted.
Why is the exponent 1.06 and not 1.0?
An exponent of 1.0 would mean you hold identical pace at any distance, which no runner can do. Riegel analyzed race data across running, swimming and cycling and found the time-versus-distance relationship followed a power law with an exponent near 1.06 for running. The extra 0.06 encodes the roughly predictable slowdown in pace as distance doubles.

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