Use the predictor in five steps
You need a previous race distance and your time for that race. The tool can then estimate a time at another distance using a consistent mathematical formula.
Open the Race Time Predictor
Use the blue button above, or select the tool from the Calculators library.
Choose your previous race distance
Select 1 Mile, 5K, 10K, Half Marathon, or Marathon. Use a recent race completed at a representative effort, ideally with a reliable finishing time.
Enter the previous finish time
Fill in Hours, Minutes, and Seconds. For a 25-minute 5K, enter 0 hours, 25 minutes, and 0 seconds. The total time must be greater than zero.
Select your target race distance
Pick the race you would like to estimate. You can predict a longer or shorter distance. The tool uses fixed standard distances for its race options.
Calculate and review the estimate
Select Predict My Race Time. Read the predicted finish time, average pace per kilometre and mile, and estimated average speed. Treat the result as a starting point for planning, not a guaranteed finish.
Example: Predict a 10K from a 25-minute 5K
Imagine you ran a recent 5K in 25 minutes. You want to estimate a possible time for a 10K race. Enter these values:
The results should be approximately:
Notice that the predicted time is slightly slower than simply doubling 25 minutes. The model assumes that average speed tends to decrease as distance increases.
What does each prediction mean?
Predicted finish time
The estimated time to cover the selected target distance, based on the previous result and the Riegel formula. It is not a measurement of what you will achieve on race day.
Estimated pace per kilometre
The predicted finish time divided by the target kilometres. For the 10K example, approximately 52:07 divided by 10 produces about 5:13 min/km.
Estimated pace per mile
The same average pace expressed for each mile rather than for each kilometre. Both pace values describe the same estimated average speed.
Average speed and target distance
Speed is displayed in kilometres per hour, while target distance shows the length of the selected race in kilometres.
How the Riegel formula works
The predictor uses a common power-law race-performance model. It scales a known finishing time by the ratio between the target and previous distances, raised to an exponent of 1.06.
T1 is your previous race time;D1 is the previous distance;D2 is the target distance; andT2 is the predicted time. The formula works with consistent distance units; CalcStudio uses kilometres.
After predicting the finish time, the calculator divides that time by the target distance to estimate average pace, then converts the result to min/mile and km/h. Displayed time and pace are rounded to whole seconds.
The exponent is a general model assumption, not an individual measurement of your endurance. Different runners may have different relationships between performance and distance.
Example: A half marathon from a 50-minute 10K
Select a previous distance of 10K, enter0:50:00, and choose Half Marathonas the target. The formula estimates a finish of approximately1:50:19, equivalent to roughly5:14 min/km.
This example involves a considerable distance increase. Whether you can sustain the estimated pace depends strongly on your endurance preparation, fitness, fueling, and course conditions.
Tips for more useful predictions
- Use a recent, representative performance.An all-out recent race is more informative than an easy run or a personal best from many years ago.
- Compare reasonably similar distances.A 10K-to-half-marathon estimate may be more useful than a 1-mile-to-marathon estimate, although both remain uncertain.
- Account for race-day conditions.Weather, hills, terrain, congestion, and pacing can change your finishing time.
- Match the training to the target.Longer races place greater demands on endurance, fueling, and preparation than shorter races.
- Use the number as a planning reference.Adjust expectations based on recent training rather than forcing a pace because a calculator predicted it.
Frequently Asked Questions
Can I predict a marathon from a 5K result?
Yes, the formula can calculate an estimate, but the distance difference is large and marathon performance relies heavily on endurance-specific training. The estimate may be unreliable.
Can I estimate a shorter race from a longer one?
Yes. You can use a recent 10K to estimate a 5K, for example. The formula works in both directions, although real-world performance may differ.
What happens if I select the same distance twice?
The ratio of target distance to previous distance becomes one. The prediction therefore matches your entered previous time.
Why is the predicted 10K slower than twice my 5K time?
The Riegel formula uses an exponent greater than one. It models the typical reduction in sustainable average speed as distance increases.
Does the predictor use my age, terrain, or training history?
No. The calculation uses only the selected previous distance, previous finishing time, target distance, and a fixed exponent. It does not personalize for those factors.
Can the result guarantee a personal best?
No. The result is a mathematical estimate, not a guarantee or a personalized race plan. Your actual finish depends on preparation and race conditions.