Race Time Predictor

The Race Time Predictor uses one recent finish time to estimate how fast you could run across all four standard distances: 5K, 10K, half marathon, and marathon. Enter a result you ran recently and get equivalent predictions for the other distances in seconds.

How to Use

1. Select your benchmark distance (5K, 10K, Half, or Full).
2. Enter your recent finish time in hours, minutes, and seconds.
3. Choose whether to display pace per km or per mile.
4. Switch to the Results tab to see predicted times and paces for all four distances.

Your benchmark distance prediction will match your input exactly. The predictions for other distances are derived from the Riegel formula.

Pick the race you are training for to headline that distance; the table still shows all four so you can compare.

How the Riegel Formula Works

The calculator uses the Riegel formula, published by Peter Riegel in 1981:

$$T_2=T_1\times {\left(\frac{D_2}{D_1}\right)}^{1.06}$$

Where:

• T1 is your known finish time (in seconds)
• D1 is the known distance (in meters)
• T2 is the predicted finish time for a new distance D2

The exponent 1.06 captures the fact that humans slow down more than proportionally as distance increases. A perfectly linear relationship would use an exponent of 1.0; real-world fatigue is slightly higher.

Assumptions and Limitations

The Riegel formula gives reasonable estimates when the following conditions hold:

• Adequate endurance base. The formula assumes you have trained specifically enough for the longer distance. If you have run 5K races but never trained beyond 15K, the marathon prediction will be optimistic.
• Similar course and conditions. A flat road race 5K predicts a flat road marathon well. A hilly trail race is a different matter.
• Consistent fatigue resistance. The formula uses a single exponent for all runners. Elite marathoners often outperform the Riegel prediction because they have exceptional fatigue resistance; beginners who lack endurance base often fall short of it.

The half marathon and marathon predictions are best treated as targets for a well-prepared runner, not guarantees.

Tips for Getting the Most Accurate Prediction

• Use a recent result (within the last 8 to 12 weeks) run at close to full effort.
• A 10K result tends to predict the half marathon more reliably than a 5K result predicts the marathon, because the distance gap is smaller.
• If you are targeting a marathon and your Riegel prediction feels very fast, build your long run base before treating the number as a goal.
• Course type matters. A flat certified road course is the best benchmark.

Data This Is Based On

The Riegel exponent of 1.06 is the value Peter Riegel fit across a broad range of running, swimming, and cycling records (Riegel, 1981); it is the same exponent used throughout this site's running tools for consistency. Predictions assume you are equally trained for each distance, which is why the marathon estimate runs optimistic if you have only raced shorter events. For how your current fitness translates into a realistic trained-up goal (rather than a same-fitness equivalent), see the race-training guide, which layers Daniels' VDOT model on top of this Riegel baseline.

Sources

Peter Riegel, "Athletic Records and Human Endurance," American Scientist 69, no. 3 (1981): 285-290.