Race Time Predictor
Turn one recent race result into a realistic target for any other distance. Uses Riegel's formula — T2 = T1 × (D2 ÷ D1)1.06 — and gives you the pace per kilometre and per mile you need to hold.
Your recent race
Target race
Predictions assume equivalent training for the target distance and comparable course and weather conditions.
How to use this calculator
1. Start from a race you actually ran
Pick the distance and enter the finish time. The prediction is only as good as its input, so use a recent all-out effort on a fair course rather than a personal best from three seasons ago.
2. Choose the distance you are aiming at
Any of the standard road distances, or a custom one in kilometres or miles for a trail race or an odd local event. The result shows the finish time plus the pace you would have to average per kilometre and per mile.
3. Tune the fatigue exponent to yourself
If you have two honest race results, adjust the exponent with the slider until the calculator reproduces the second one from the first. That personal number will predict your other distances far better than the generic 1.06.
4. Read the whole table, not just one number
The prediction table gives every standard distance at once, which is the fastest way to spot a race that suits your current shape. Download it as CSV and keep it beside your training plan as a pace reference.
About race time prediction
What is Riegel's formula?
Riegel's formula predicts a finish time at a new distance from a known result: T2 = T1 × (D2 ÷ D1)1.06. The exponent above 1 is what makes the prediction slower per kilometre as the race gets longer, which is how real runners behave.
What does the fatigue exponent change?
The exponent describes how much you fade as distance grows. Values near 1.02–1.04 suit high-mileage endurance runners whose pace barely drops; 1.08–1.10 suits speed-oriented runners or anyone stepping far beyond their weekly long run. Tune it until the formula reproduces two races you have actually run.
Why was my real marathon slower than the prediction?
The formula assumes the same level of specific training for both distances, similar terrain, and similar weather. A marathon predicted from a 5K also assumes you have built the endurance base to hold that pace for hours, plus fuelling and hydration that hold up. Heat, hills, wind, and going out too fast all cost minutes the formula never sees.