How to use the race time predictor
- Choose the distance you ran in a recent race, or pick another distance and enter it in kilometers or miles.
- Enter your finish time in hours, minutes and seconds.
- Choose the distance to predict for. Leave the fatigue exponent at 1.06 unless you have a reason to change it.
- Read the predicted time and pace, and the table of predicted times for the 1 mile, 5K, 10K, 10 mile, half marathon and marathon.
Formula
T2 = T1 x (D2 / D1)^k
T1 = your known time, D1 = known distance, D2 = target distance, k = exponent
Pace per mile = time / distance in miles; pace per km = time / distance in kilometers
1 mile = 1,609.344 meters
The Riegel formula is a published rule of thumb with k = 1.06. Setting k = 1 would assume you hold exactly the same pace at any distance.
Worked example
A runner finishes a 5K in 25:00 (1,500 seconds). For a marathon of 42,195 meters, T2 = 1,500 x (42,195 / 5,000)^1.06 = 14,386.65 seconds, which is 3:59:47, a pace of 9:09 per mile.
A 10K in 50:00 predicts a half marathon of 1:50:19 at 8:25 per mile. Setting the exponent to 1 turns a 25:00 5K into a 50:00 10K.
An estimate, not a promise
A predicted time assumes similar conditions and the training to run the longer distance. Predictions for distances far longer than the race you ran, like a marathon from a mile, are the least reliable. Use a recent race run at a hard effort for the best guess, and treat the result as a best case for the longer race.
Choosing the exponent
Riegel's published formula uses an exponent of 1.06, which is the default here. Runners who are very well trained for endurance may slow down less, and others more. If you have results at two distances you can test which exponent fits you. Treat the formula as a well known rule of thumb, not a law.
Pace and training
Convert any time into pace with the pace calculator, estimate your aerobic fitness with the VO2 max calculator, or turn steps into distance with the steps to miles converter. Training plans, heat, hills and race day nutrition all change what you can run.
Assumptions and limits
- The Riegel formula with the exponent you set. The default 1.06 is the exponent in Riegel's published formula, but it is a general figure, not a measure of you.
- You are racing at a similar effort and in similar conditions at both distances.
- Times above 99 hours, minutes above 59 and a zero time are rejected, as is an exponent outside 1 to 1.3.
- Course profile, weather and training for the longer distance are not modeled.
Frequently asked questions
How do I predict my marathon time from a 5K?
Multiply your 5K time by the distance ratio (42,195 / 5,000) raised to the power 1.06. A 25:00 5K gives about 3:59:47. Treat it as a best case.
What is the Riegel formula?
It is a rule of thumb that says race time grows with distance raised to about 1.06, because most runners slow down a little as the distance increases.
Is the predicted time accurate?
It is an estimate. It works best for nearby distances and when you have trained for the longer race. It is less reliable for much longer distances.
What pace is a 3:59:47 marathon?
About 9:09 per mile or 5:41 per kilometer. Enter your own time to see yours.