Race Pace Predictor Calculator
Predict 5K to marathon times from one recent race using Riegel's formula.
Enter your values
Choose one of: 5K, 10K, Half marathon, Marathon, 1 mile.
Enter the finish time in hh:mm:ss.
Results update instantly as you type — no submit needed. Your values are remembered on this device, and the shareable link reopens the calculator with exactly these numbers.
- Input pace
- 5:00 per km
- 5K
- 23:59 (4:48 /km, 7:43 /mi)
- 10K
- 50:00 (5:00 /km, 8:03 /mi)
- Half marathon
- 1:50:19 (5:14 /km, 8:25 /mi)
- Marathon
- 3:50:01 (5:27 /km, 8:46 /mi)
Working
- 1marathon = 50:00 x (42.195 / 10)^1.06 = 3:50:01
Quick answer
Riegel's formula predicts race times across distances using T2 = T1 x (D2/D1)^1.06, the exponent that matches how runners slow as distance grows.
T2 = T1 x (D2 / D1)^1.06
At a glance
| What it does | Predict 5K to marathon times from one recent race using Riegel's formula. |
|---|---|
| Category | Fitness |
| Inputs needed | Race you ran, Finish time |
| Main output | Input pace |
| Formula | T2 = T1 x (D2 / D1)^1.06 |
| Cost | Free — no sign-up, no download |
How to use the Race Pace Predictor Calculator
- 1Enter a race you ran hard in the last few weeks, not a training run.
- 2Read the predicted times and the target pace per kilometre and mile.
- 3Treat the marathon prediction as a ceiling — it assumes you have done the long-run training.
Inputs explained
- Race you ran
- Choose one of: 5K, 10K, Half marathon, Marathon, 1 mile.
- Finish time(hh:mm:ss)
- Enter the finish time in hh:mm:ss.
Worked example
Using the values the calculator loads with:
Inputs
- Race you ran10K
- Finish time50:00 hh:mm:ss
Results
- Input pace5:00 per km
- 5K23:59 (4:48 /km, 7:43 /mi)
- 10K50:00 (5:00 /km, 8:03 /mi)
- Half marathon1:50:19 (5:14 /km, 8:25 /mi)
- Marathon3:50:01 (5:27 /km, 8:46 /mi)
- marathon = 50:00 x (42.195 / 10)^1.06 = 3:50:01
Frequently asked questions
How accurate is Riegel's formula?
Very good from 5K to the half. Marathon predictions are optimistic unless your weekly mileage and long runs support the distance.
Should the exponent ever change?
Well-trained, high-mileage runners fit closer to 1.05, while runners with low mileage often fit 1.08 or higher for the marathon.
What do I need to enter into the Race Pace Predictor Calculator?
Just 2 values: race you ran, finish time. Every field starts with a realistic example, so you can change one number at a time and watch the result update instantly.
How does the Race Pace Predictor Calculator work out the answer?
Riegel's formula predicts race times across distances using T2 = T1 x (D2/D1)^1.06, the exponent that matches how runners slow as distance grows. It applies the formula T2 = T1 x (D2 / D1)^1.06 and shows the working so you can check each step by hand.
Is the Race Pace Predictor Calculator free, and do I need an account?
It is completely free with no sign-up, no download and no usage limit. Everything is calculated in your browser, so the numbers you type never leave your device.
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.