If you’ve just raced a 5K, 10K or half-marathon all-out, you can estimate your finish time at any other road distance — including the full marathon — without running it first. This predictor uses the Riegel formula, the equivalent-performance model runners and coaches have relied on for decades, and shows the per-kilometre pace your target time implies.
How it works
Pete Riegel’s formula relates a known time at one distance to a predicted time at another:
T2 = T1 × (D2 ÷ D1) ^ 1.06
where T1 is your known time over distance D1, D2 is the target distance, and the
exponent 1.06 captures how pace fades as distance grows. An exponent of exactly 1
would assume identical pace at every distance; 1.06 adds a realistic slowdown. The tool
converts your hours/minutes/seconds to total seconds, applies the formula, then divides
the predicted time by the target distance to give pace per kilometre.
Example
You run a 5 km in 25:00 (1,500 seconds) and want a marathon prediction:
T2 = 1500 × (42.195 ÷ 5) ^ 1.06 = 1500 × 8.439 ^ 1.06 ≈ 15,090 seconds ≈ 4:11:30, which is a pace of about 5:58 per km.
| From | Time | To | Predicted |
|---|---|---|---|
| 5 km | 25:00 | 10 km | ~51:55 |
| 5 km | 25:00 | Half | ~1:55:30 |
| 5 km | 25:00 | Marathon | ~4:11:30 |
Every calculation runs locally in your browser and nothing is uploaded.
How accurate is the Riegel prediction?
The Riegel formula is well-validated for comparing performances across the four standard road distances when:
- The reference race was run at maximum effort — a training run or a paced long run will underpredict your actual race fitness.
- The reference race was run recently — fitness changes over months. A spring half-marathon time does not reliably predict an autumn marathon if training volume changed.
- The two distances are not too far apart — predicting a marathon from a 5K extrapolates across a factor of 8.4 in distance, which amplifies any individual deviation from the formula. A half-marathon to marathon prediction (factor of 2) is more reliable than a 5K to marathon prediction.
Individual runners deviate from the Riegel exponent of 1.06 based on their physiology. Runners who are naturally strong at shorter distances (fast-twitch, speed-dominant) often find real marathon times slower than predicted. Runners with high aerobic base and efficient long-distance mechanics often run closer to or faster than the prediction.
Using predictions alongside VDOT
Jack Daniels’ VDOT system (see the VDOT Running Calculator) is another widely used equivalent-performance model. It is calibrated to a larger dataset and gives very similar results to Riegel for most runners. If both models agree, the prediction is more robust. If they diverge significantly, investigate why — the most common reason is that one of the input races was not run at full effort.
Planning your marathon pace
Once you have a predicted marathon finish time, divide by 42.195 to get the average pace per kilometre (or divide the time in minutes by 26.22 for minutes per mile). For example, a predicted 4:00:00 marathon is 60 × 4 ÷ 42.195 ≈ 5:41/km average pace. Most coaches recommend starting the first 5–10 km at roughly 10–15 seconds per kilometre slower than goal pace to avoid blowup in miles 18–22.
Distances you can compare
| Distance | Km |
|---|---|
| 5 km | 5.000 |
| 10 km | 10.000 |
| Half Marathon | 21.0975 |
| Marathon | 42.195 |
The formula is symmetric — you can run it in either direction. Entering a slower target time than your current fitness suggests will give you the equivalent shorter-distance time to aim for in a tune-up race.