Race Time Predictor
Predict a race time from a known result.
Your finishing time for that distance, in seconds.
Results update as you type.
How it works
A race time predictor estimates how fast you could run a race at one distance based on a result you have already achieved at another. This tool uses the Riegel formula: enter a distance you have raced and your time for it, then the target distance you are aiming at, and it projects a realistic finishing time. Times are handled in seconds so the maths stays clean. This matters because runners constantly want to translate one performance into a goal for another race. A recent 5K result can suggest a sensible 10K or half-marathon target, helping you set pacing strategy and avoid starting too fast. Coaches use predictions to write training paces, and runners use them to choose a goal that stretches without being unrealistic on race day. The formula works because pace does not stay constant as distance increases; it slows in a predictable way that the exponent 1.06 captures well across common racing distances. The prediction assumes suitable endurance training for the longer event, so it is most reliable when the two distances are reasonably close and your fitness supports the target. Treat the output as an informed goal rather than a guarantee, since conditions, terrain, and pacing all shape the actual result.
T2 = T1 × (D2 ÷ D1)^1.06
The Riegel model predicts a race time by scaling a known result to a new distance and raising the distance ratio to the power 1.06. That exponent, slightly above one, captures the reality that pace slows as distance grows, because you cannot hold a short race's speed over a longer one. Multiplying your known time by this factor projects a realistic finishing time for the target distance.
Source:Riegel model (race prediction)
We review all our tools from time to time to ensure accuracy.
Worked example
How to use this calculator
- 1
Enter your values
Enter known distance, known time and target distance.
- 2
Adjust the options
Fine-tune any options to match your situation.
- 3
Read your result
Predicted time updates instantly as you type — no button to press.
Frequently asked questions
What is the Riegel formula?
The Riegel formula, published by Peter Riegel, predicts race times across distances by multiplying a known time by the ratio of distances raised to the power 1.06. The exponent reflects that runners slow down slightly as races get longer. It is widely used because it is simple and matches real results well for distances from about 1,500 metres to the marathon, making it a trusted tool for goal setting.
Why must I enter my time in seconds?
The calculator works in seconds to keep the mathematics unambiguous and the output consistent. Convert your finishing time first: multiply hours by 3,600, minutes by 60, and add the remaining seconds. A time of 25 minutes becomes 1,500 seconds, and 1 hour 45 minutes becomes 6,300 seconds. The predicted time is also returned in seconds, which you can convert back into minutes and seconds for a familiar race split.
How accurate are these predictions?
Predictions are most accurate when the known and target distances are close and you have trained appropriately for the longer event. Jumping from a 5K straight to a marathon prediction assumes endurance you may not yet have, so the estimate can be optimistic. Use a recent, well-paced result for the input, treat the output as a goal to train toward, and expect real conditions like heat, hills, and pacing to shift the actual time.
Can it predict both faster and slower distances?
Yes. You can project upward from a short race to a longer one or downward from a long race to a shorter one, since the formula works in both directions. Predicting a shorter, faster race from a longer result usually gives a slightly conservative target because your speed at short distances may exceed what the endurance-based formula assumes. Either way, use a recent result that reflects your current fitness.
Does terrain or weather affect the prediction?
The formula assumes broadly similar conditions between your known race and your target. Hills, heat, humidity, wind, and altitude all slow real performances, so a prediction based on a flat, cool race may be too fast for a hilly or hot one. Treat the output as a baseline for ideal conditions and adjust your expectations upward in time if the target race presents tougher circumstances than your reference.