Race Time Predictor
Enter a recent race result to predict your finish times for 5K, 10K, half marathon, and marathon.
How Race Time Prediction Works
This calculator uses the Riegel formula, published by Peter Riegel in 1977 and still the most widely used race prediction model:
T₂ = T₁ × (D₂ / D₁)1.06
Where T₁ is your known time, D₁ is the known distance, T₂ is the predicted time, and D₂ is the target distance. The exponent 1.06 reflects the fact that runners slow down as distance increases — longer races are disproportionately harder, not just proportionally harder.
The Riegel formula is most accurate for distances within 2–3× of your known race. Using a 5K to predict a marathon introduces more error than using a half marathon to predict a marathon. The prediction also assumes similar conditions, pacing strategy, and fitness level.
Example Predictions from a 45-Minute 10K
| Distance | Predicted time | Avg pace (min/km) |
|---|---|---|
| 5K | 21:28 | 4:18/km |
| 10K | 45:00 | 4:30/km (known) |
| Half marathon | 1:40:18 | 4:45/km |
| Marathon | 3:29:15 | 4:58/km |
Worked Examples
Example 1: 5K in 22:30 → predict half marathon. T₂ = 22.5 × (21.0975 / 5)1.06 = 22.5 × 4.21951.06 = 22.5 × 4.578 = 103.0 min = 1:43:02.
Example 2: Half marathon in 1:55:00 → predict marathon. T₂ = 115 × (42.195 / 21.0975)1.06 = 115 × 21.06 = 115 × 2.085 = 239.8 min = 3:59:49.
Example 3: Marathon in 4:30:00 → predict 10K. T₂ = 270 × (10 / 42.195)1.06 = 270 × 0.23701.06 = 270 × 0.219 = 59.1 min = 59:06.
Frequently Asked Questions
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The Riegel formula is accurate to within a few percent for most runners when predicting across similar distances (e.g. 10K to half marathon). Accuracy decreases for very short or very long target distances relative to the known race. It also assumes the runner is equally well-trained for both distances and runs both at maximum effort with similar pacing.
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The Riegel formula is based on average performance data and does not account for fuelling, hydration, heat, or the specific physiological demands of ultra-endurance events. Most recreational runners slow more than the formula predicts in a marathon due to inadequate fuelling strategy, pacing errors, and insufficient long-run training. Elite marathon runners often match or beat the prediction.
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Yes. Divide the predicted finish time by the race distance (in km) to get your target pace per kilometre. For example, a predicted marathon time of 3:30:00 over 42.195 km gives a target pace of 210 min ÷ 42.195 = 4:58 per km. Use our running pace calculator to convert between pace and speed formats.
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Context matters — age, sex, and training history all affect what counts as "good". Rough population benchmarks: 5K under 25 min (male) / 30 min (female) is above average for recreational runners. A sub-2-hour half marathon and sub-4-hour marathon are common goal milestones. Elite times are: 5K ~13 min, 10K ~27 min, half marathon ~58 min, marathon ~2:01.
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The most effective training strategies are: consistent weekly mileage (most runners benefit from simply running more), one weekly long run at easy pace (builds aerobic base), one weekly interval or tempo session (builds speed and lactate threshold), and adequate recovery. For marathon improvement specifically, long runs of 30+ km are essential in the final 8–12 weeks.