Running Race Time Predictor (Riegel Formula)
Predict a race time from a recent result.
Enter your values
Enter the known distance in km.
Enter the finishing time in total minutes.
Enter the target distance in km.
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.
- Predicted time
- 1h 39m 18s
- Predicted pace
- 4.71 min/km
- Total minutes
- 99.3
Quick answer
The Riegel formula scales a known race time to a new distance using a fatigue exponent of about 1.06.
T2 = T1 x (D2/D1)^1.06
At a glance
| What it does | Predict a race time from a recent result. |
|---|---|
| Category | Fitness |
| Inputs needed | Known distance, Known time, Target distance |
| Main output | Predicted time |
| Formula | T2 = T1 x (D2/D1)^1.06 |
| Cost | Free — no sign-up, no download |
How to use the Running Race Time Predictor (Riegel Formula)
- 1Enter a recent race distance and finishing time in minutes.
- 2Enter the target distance you want to predict.
- 3The Riegel formula works best between similar distances (e.g. 10K to half marathon), not extreme jumps.
Inputs explained
- Known distance(km)
- Enter the known distance in km.
- Known time(min)
- Enter the finishing time in total minutes.
- Target distance(km)
- Enter the target distance in km.
Worked example
Using the values the calculator loads with:
Inputs
- Known distance10 km
- Known time45 min
- Target distance21.1 km
Results
- Predicted time1h 39m 18s
- Predicted pace4.71 min/km
- Total minutes99.3
Frequently asked questions
Why not linear scaling?
Fatigue means pace naturally slows over longer distances, which the 1.06 exponent captures.
How accurate is this?
It's a solid estimate for trained runners but can overstate endurance for undertrained long-distance efforts.
What do I need to enter into the Running Race Time Predictor (Riegel Formula)?
Just 3 values: known distance, known time, target distance. 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 Running Race Time Predictor (Riegel Formula) work out the answer?
The Riegel formula scales a known race time to a new distance using a fatigue exponent of about 1.06. 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 Running Race Time Predictor (Riegel Formula) 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.