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Race Time Predictor

See what finish time your current form can deliver.

How it is calculated

The tool uses the Riegel formula: T2 = T1 × (D2 ÷ D1)1.06. T1 is your known time as a plain number of minutes (50 for 50:00; 80 for 1 hour 20), D1 is that race's distance in kilometres and D2 is the target distance in kilometres. The exponent 1.06 makes the prediction slower than simple proportion, because pace fades over longer distances. The predicted time is printed as h:mm:ss, rounded to the whole second, and the pace as m:ss per km, which is the predicted time divided by the target distance. All three values must be above zero; text such as 0:50 is not read as a time. The estimate assumes you train properly for the target distance and gets less reliable when it is far from the known race.

Worked example

You ran 10 km in 50 minutes and want a half marathon (21.0975 km) forecast. Distance ratio: 21.0975 ÷ 10 = 2.10975. Raised to the power 1.06 this is about 2.2064. Multiply: 50 × 2.2064 = 110.32 minutes, which is 1 hour 50 minutes 19 seconds. The tool shows 1:50:19. Dividing 110.32 minutes by 21.0975 km gives about 5.23 minutes per km, shown as 5:14 /km. The same 10 km time forecasts 3:50:01 for 42.195 km.

Try this example →

Common mistakes

  • Typing the known time as hours and minutes (0:50 or 1.20) instead of a total in minutes. A 1 hour 20 minute result is entered as 80, and 0:50 gives no result.
  • Entering miles. Distances must be in kilometres, so a 10-mile race is 16.09 and a half marathon is 21.0975.
  • Treating the pace line as a time per mile. It is minutes and seconds per kilometre.
  • Predicting a marathon from a 5 km race and expecting accuracy. The farther the target from the known distance, the less reliable the Riegel estimate.
  • Using a race time from a hilly or hot day as the base. The tool assumes the known time reflects your current fitness.

About this calculator

The Riegel formula, developed by exercise physiologist Pete Riegel in 1977, predicts a runner's finish time at one distance from a known result at another: T2 = T1 × (D2/D1)^1.06, where the exponent 1.06 captures the fact that pace naturally slows as distance increases — running twice as far takes more than twice as long, even at consistent fitness, because fatigue and glycogen depletion compound over longer efforts. It's remarkably accurate for predicting between reasonably similar distances (a 10K time predicting a half marathon, for example) but grows less reliable across very different distance ranges, since factors like fueling, pacing strategy and heat management matter increasingly at marathon-plus distances.

Runners and coaches use Riegel-based predictions to set realistic goal times for an upcoming race based on a recent tune-up race or time trial, to gauge fitness progress between training blocks, and to choose appropriate pacing splits going into race day. It's one of several prediction models (alongside the VDOT and Cameron formulas) that coaches cross-check against each other, since no single formula perfectly captures every runner's individual fatigue profile.

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