Race Time Predictor

What the race time predictor does

One good race tells you a lot about what you could run at other distances. This race time predictor takes a recent result, such as a 5K or a half marathon, and estimates your finish time for another distance using Riegel’s endurance formula. You get:

• A predicted finish time for the race you choose.
• The predicted pace per mile and per kilometer, next to the pace of your recent race.
• An equivalent-times table for the mile, 5K, 10K, 15K, 10 miles, half marathon and marathon.
• The working, showing the distance ratio and exponent with your numbers.

How to use the race time predictor

1. Choose the distance of a recent race you ran hard, or select Custom and enter any distance.
2. Enter your finish time in hours, minutes and seconds.
3. Choose the race you want to predict.
4. Leave the fatigue exponent at 1.06, or raise it to 1.07–1.08 for a more cautious long-race prediction.

Use a result from the last six to eight weeks, run on a reasonably flat course in normal conditions. A time trial on a track works too, as long as it was an all-out effort.

The race time predictor formula

In 1977 the engineer Peter Riegel proposed a simple model of how running time grows with distance, and set it out in detail in American Scientist in 1981:

T₂ = T₁ × (D₂ ÷ D₁)^1.06

If running were perfectly efficient, doubling the distance would double the time and the exponent would be 1. The extra 0.06 captures fatigue: every time the distance doubles, the time grows by a factor of 2^1.06 ≈ 2.085, about 4% more than double. Riegel found the relationship held well for efforts between roughly 3.5 minutes and almost 4 hours.

Predicting a marathon from a 10K by hand

Say you ran a 10K in 48:00.

1. Convert the time to seconds: 48 × 60 = 2,880 s.
2. Distance ratio: 42,195 m ÷ 10,000 m = 4.2195.
3. Raise it to 1.06: 4.2195^1.06 ≈ 4.6002.
4. Multiply: 2,880 × 4.6002 ≈ 13,249 s, which is 3:40:49.

The same 10K predicts 1:45:54 for the half marathon. If your longest runs are under 15 miles, expect the marathon to come out slower than 3:40:49.

Equivalent race times from a 5K

These predictions were calculated with the same formula and rounding as the calculator. The last column shows how much a more cautious exponent of 1.08 changes the marathon.

The gap between the two marathon columns grows from about 8 minutes to about 15, which shows how sensitive long predictions are to the exponent.

How runners use race predictions

• Setting a goal time. Turn the prediction into a target range rather than a single number.
• Choosing training paces. Feed the predicted time into the pace calculator to get per-mile splits for race day.
• Checking progress. If a new 10K predicts a faster half marathon than last season, your fitness has improved.
• Comparing results. The equivalent-times table lets you see whether your 5K or your half marathon is the stronger performance.
• Balancing effort. Long races run largely in the easier heart rate zones; the prediction tells you how long you will be out there.

Common mistakes

• Predicting a marathon from a mile. The bigger the jump, the less reliable the prediction. Use the closest distance you have raced.
• Using a training run. The formula assumes a maximal race effort. An easy run predicts times far slower than you can run.
• Ignoring the course. A hilly or hot race result gives slow predictions; a downhill course gives fast ones.
• Skipping the long runs. Riegel’s formula cannot know whether you have done the training a half or full marathon needs.
• Treating the prediction as a pacing plan. Start conservatively and use the first miles to judge how you feel.

Glossary

• Riegel formula. T₂ = T₁ × (D₂ ÷ D₁)^b, the standard model behind most race time predictors.
• Fatigue exponent (b). How quickly your pace slows as distance increases; 1.06 by default.
• Equivalent performance. Times at different distances that represent the same level of fitness.
• Even pacing. Running each mile or kilometer at the same speed.