About the Race Time Predictor
If you know how fast you ran one distance, you can estimate how fast you could run another. This calculator uses the Riegel formula, which multiplies your time by the ratio of the distances raised to the power 1.06. Enter a recent race time over a mile, 5K, 10K, half marathon or marathon, and it predicts your times for the other distances, with the pace per kilometre and per mile. Type a recent race distance and your finishing time and the predicted race time appears at once, with a few related figures underneath, and you can copy the result with one tap. The sums run in your browser, so nothing you enter is uploaded.
Updated October 2026
How to use the Race Time Predictor
- 1Enter a recent race distance and your finishing time.
- 2Pick the units or options from the menus if the predicted race time tool offers them.
- 3Read the predicted race time in the large result box.
- 4Look at the extra rows for related figures.
- 5Press Copy result to keep the answer.
When you need the predicted race time
Runners use predictions to choose a goal for an upcoming race and to set a sensible pace for the first mile. Coaches use them to compare training and to track progress between races.
The predictions assume similar training, weather and course. A marathon needs endurance work that a 5K does not, so a runner who trains only for short races will do worse than predicted. Use a recent, all-out race result for the best estimate.
How the predicted race time is worked out
New time = known time × (new distance ÷ known distance) ^ 1.06. The exponent is a bit above 1 because people slow down over longer races. A 25:00 5K predicts a 10K of 52:07, a half marathon of 1:55:00 and a marathon of 3:59:47. The paces for the marathon are about 5:41 per km. A 50-minute 10K predicts a 5K of about 23:59.
Every answer on this page comes from that one rule, so you can check any result by hand with a calculator. The figures shown underneath, such as other units, are conversions of the same answer.
- Five race distances
- Pace per km and per mile
- Hours, minutes and seconds input
Mistakes to avoid with predicted race time
Predictions get less reliable the further the target is from the known distance. Predicting a marathon from a mile time is a stretch; a 10K or half marathon result gives a better basis. Heat, hills and illness can change the outcome too.
On phone and computer
On a phone, type into the boxes one after another and read the predicted race time underneath; Copy result puts it on the clipboard for a message or a note. On a computer the inputs sit on the left and the answer on the right, so you can change one figure and watch the predicted race time move. The race time predictor works the same on both. Plan your training with Pace Calculator and Speed Calculator, check your effort with Target Heart Rate Calculator, and estimate what a run burns with Calories Burned Calculator.
Features
- Use a recent race
- Closer distances predict better
- Training matters most
- Clear messages when the predicted race time inputs make no sense
- Copy the result with one tap
- Private: nothing is uploaded
Frequently asked questions
How do I calculate predicted race time?
New time = known time × (new distance ÷ known distance) ^ 1.06. The exponent is a bit above 1 because people slow down over longer races. A 25:00 5K predicts a 10K of 52:07, a half marathon of 1:55:00 and a marathon of 3:59:47. The paces for the marathon are about 5:41 per km. A 50-minute 10K predicts a 5K of about 23:59.
What is the formula for predicted race time?
New time = known time × (new distance ÷ known distance) ^ 1.06. The exponent is a bit above 1 because people slow down over longer races. The race time predictor applies it exactly and rounds only the displayed answer to six significant figures.
Who created the Riegel formula?
Peter Riegel, an engineer and runner, published the idea in the 1970s and 1980s in articles on athletic records.
Why is the exponent 1.06?
It fits the observation that endurance falls slightly with distance; doubling the distance more than doubles the time.
Can I use it for a mile?
Yes, as the known distance. The predictions for very different distances are less trustworthy.
Is this race time predictor free and private?
Yes. It is free, needs no account and runs in your browser, so the numbers you type are not uploaded or stored.