Running Time Calculator
Predict race finish times at 1K, 5K, 10K, half, marathon, and ultra distances using the Riegel formula from a single known race time.
Running Time Calculator
Predict your race finish times at any distance from 1K to 100 miles using the Riegel formula from a single known race time. Also shows pace per km/mile and speed in km/h and mph.
Your Known Race Time
Predicted Times
| Distance | km | Predicted Time | Pace /km | Pace /mi |
|---|---|---|---|---|
| 1K | 1.00 | 4:32 | 4:32 | 7:18 |
| 1 Mile | 1.61 | 7:31 | 4:40 | 7:31 |
| 5K Input | 5.00 | 25:00 | 5:00 | 8:03 |
| 10K | 10.00 | 52:07 | 5:13 | 8:23 |
| Half Marathon | 21.10 | 1:55:00 | 5:27 | 8:46 |
| Marathon | 42.20 | 3:59:47 | 5:41 | 9:09 |
| 50K Ultra | 50.00 | 4:47:02 | 5:44 | 9:14 |
| 50 Mile | 80.47 | 7:55:19 | 5:54 | 9:30 |
| 100K | 100.00 | 9:58:27 | 5:59 | 9:38 |
| 100 Mile | 160.93 | 16:31:01 | 6:09 | 9:55 |
The Riegel formula (Peter Riegel, 1981) predicts race times via T₂ = T₁ × (D₂/D₁)^1.06. It is most accurate when predicting distances within a factor of 4 of your known race, and assumes you are adequately trained for the target distance. Ultramarathons (50K+) often produce less accurate predictions because pacing dynamics shift significantly.
Frequently Asked Questions
What is the Riegel formula?
Peter Riegel (1981): T2 = T1 x (D2/D1)^1.06. Predicts finish time at a new distance from a known race time. Published in Marathoner magazine and widely validated.
How accurate is the prediction?
Most accurate when predicting within 4x your known distance and you are properly trained for the target. A 5K PR predicts half marathons well; less reliable for ultras.
Does it account for training?
No. The prediction assumes you have specific training for the target distance. Running a 20-minute 5K does not automatically mean you can run the predicted marathon time without marathon-specific training.
Is the Running Time Calculator free to use?
Yes, the Running Time Calculator is 100% free with no registration, no hidden fees, and no usage limits. All processing happens locally in your browser, ensuring complete privacy.
Is my data safe with this tool?
Absolutely. The Running Time Calculator processes everything client-side in your browser. No data is uploaded to or stored on any server. Your content remains private on your device at all times.
Does the Running Time Calculator work on mobile devices?
Yes, the Running Time Calculator is fully responsive and works on smartphones and tablets. You can use it on any device with a modern web browser -- no app download required.
How do I use the Running Time Calculator?
Simply enter your input in the provided field, adjust any settings to your preference, and the tool will process it instantly. You can then copy the result to your clipboard or download it.
Which browsers are supported?
The Running Time Calculator works in all modern browsers including Chrome, Firefox, Safari, Edge, and Opera. For the best experience, use the latest version of your preferred browser.
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About Running Time Calculator
Running Time Calculator is a free, browser-based tool in our Calculator Tools collection. Everything runs locally on your device — no uploads, no sign-up, and your data stays private.