Race Time Predictor
Predict your finish times for any race distance using the Riegel formula. Enter a known result and get predictions for 5K, 10K, half marathon, and marathon.
For informational purposes only. Not a substitute for professional medical advice. Consult a healthcare provider before making changes to your diet or exercise routine.
About Race Time Predictor
The race time predictor uses the Riegel formula to estimate your finish times across a range of distances. Enter a recent race result - any distance from 1K to a marathon - and see what that fitness level translates to at other race distances.
How It Works
The Riegel formula accounts for the natural slowdown that occurs as race distance increases. It takes your known time and distance, then adjusts for the fatigue factor to predict times at other distances. The standard exponent of 1.06 works well for most runners, but you can adjust it in the advanced settings.
Predicted Times and Paces
The results table shows predicted finish times for 1K, 5K, 10K, half marathon, marathon, and 50K ultra, along with the expected pace per kilometre and per mile. A visual pace comparison makes it easy to see how much slower you will need to run as the distance increases.
Setting Race Goals
Use these predictions to set realistic goals for your next race. If you have recently run a 10K, the half marathon prediction gives you a solid target to train toward. For detailed pace and split planning, pair this with the Pace Calculator. To estimate the calories you will burn, check the Calories Burned Calculator.
All calculations run in your browser. No data is stored or sent anywhere.
Frequently Asked Questions
What is the Riegel formula?
The Riegel formula (T2 = T1 x (D2/D1)^1.06) predicts race times by accounting for the fact that your average pace naturally slows as distance increases. The exponent of 1.06 models the typical fatigue effect. It was developed by Pete Riegel and published in 1977.
How accurate are the predictions?
The Riegel formula is reasonably accurate for distances between 1500m and the marathon, especially for well-trained runners. Predictions become less reliable at extreme distances (ultras) or for very short races. Your actual performance also depends on training, course conditions, weather, and race-day strategy.
What exponent should I use?
The standard exponent is 1.06 and works well for most recreational and competitive runners. Elite runners who maintain pace better over distance might use 1.04-1.05. Less trained runners who slow more at longer distances might use 1.07-1.08. Experiment to see what matches your past results.
Can I use a 5K time to predict a marathon?
Yes, but the further apart the distances, the less precise the prediction. A 10K time is a better predictor for a half marathon than a 5K time. For the most accurate marathon prediction, use a half marathon result as your known time.
Why does pace increase at longer distances?
As distance increases, your body shifts more to aerobic energy systems, glycogen stores deplete, and muscular fatigue accumulates. The Riegel exponent captures this predictable slowdown. That is why a 10K pace is always slower than a 5K pace, even on the same day.