Race Time Predictor - Predict Finish Times Using Riegel's Formula

Distance	Predicted time	Pace
5K	25:00	8:03 /mi
10K	52:07	8:23 /mi
Half Marathon	1:55:00	8:46 /mi
MarathonSelected target	3:59:47	9:09 /mi

How to use this tool

Choose your known race. Select the distance of a race you have completed.
Enter your result. Enter the hours, minutes, and seconds from that race.
Choose a target distance. Select the race distance you want to estimate.
Review and adjust the estimate. Read the predicted time and pace, compare common distances, and adjust the Riegel exponent in Advanced if you want to explore another assumption.

How race time prediction works

This calculator uses Riegel's formula, which predicts your finish time at a new distance based on a known race result. The formula is:

T₂ = T₁ × (D₂ / D₁)^1.06

Where T₁ is your known time, D₁ is the known distance, D₂ is the target distance, and T₂ is the predicted time.

The fatigue factor (1.06)

The exponent 1.06 accounts for the fact that longer races take proportionally more time due to fatigue. A value of 1.0 would imply constant pace regardless of distance: the 1.06 exponent gives a more realistic prediction. The calculator starts at 1.06 and offers an advanced adjustment from 1.03 to 1.12. Raising the exponent predicts more slowdown for longer targets; it is a modeling assumption, not a personal physiological measurement.

Accuracy

Riegel's formula works best for races between 3.5 km and 100 km. Predictions become less accurate for very short sprints or ultra-long distances. The formula also assumes similar training and conditions for both races.

Common use cases

Predict your marathon time from a half marathon PR
Set a realistic goal pace for your first 10K based on 5K results
Compare performance across different race distances

Other race prediction models

Other approaches, including Cameron’s model, use different equations and assumptions. This calculator does not implement Cameron’s model: its live results, calculation steps, and equivalence table all use Riegel’s formula. Treat comparisons from other models as separate estimates rather than as options available here.

Training volume consideration

Riegel’s formula assumes equivalent training for both the reference race and the predicted race. If you have significantly increased weekly mileage, added structured tempo runs, or improved your VO2 max since your reference result, your actual performance may outperform the prediction. Conversely, predicting a marathon from a 5K trained peak without building long-run base will likely underperform.

Race equivalence table

Baseline predictions for a runner with a 25:00 5K using the default Riegel exponent of 1.06. The calculator’s common-distance table updates from your inputs and selected exponent.

Distance	Predicted time	Target pace (per mile)
5K (3.1 mi)	25:00	8:03/mi
10K (6.2 mi)	52:07	8:23/mi
Half marathon (13.1 mi)	1:55:00	8:46/mi
Marathon (26.2 mi)	3:59:47	9:09/mi

Frequently asked questions

How does the race time predictor calculate an equivalent time?

It applies Riegel’s formula: target time equals your known time multiplied by the target-to-known distance ratio raised to an exponent. The default exponent is 1.06, and the Advanced control lets you adjust it from 1.03 to 1.12.

Why might this prediction differ from another calculator?

Tools can use different fatigue exponents or prediction models. This calculator uses Riegel’s formula only, so compare results using the same exponent and treat other models as separate estimates.

Which race result should I enter?

Use a recent race result that reflects your current fitness and was run under conditions similar to the target event. The model does not account for differences in training, terrain, weather, or race preparation.

Are equivalent times guaranteed race results?

No. Riegel’s formula is a statistical estimate. Large distance jumps, very short races, and ultra distances are less reliable, and the prediction is not a training plan or medical assessment.