Calculator Inputs
Use a recent race, then refine the prediction with training distance, long-run durability, weather, elevation, experience, and strategy.
Example Data Table
These sample inputs show how recent race strength and training load can shift projected half marathon results.
| Recent Race | Race Time | Weekly Distance | Predicted Half Marathon | Average Pace |
|---|---|---|---|---|
| 5K Road Race | 00:24:30 | 32 km/week | 01:54:26 | 05:25 / km |
| 10K Tune-Up | 00:49:15 | 42 km/week | 01:48:43 | 05:09 / km |
| 15K Race | 01:16:40 | 55 km/week | 01:47:09 | 05:05 / km |
| 10 Mile Race | 01:26:20 | 64 km/week | 01:51:31 | 05:17 / km |
Formula Used
1. Base distance prediction: The calculator starts with the Riegel endurance formula:
T₂ = T₁ × (D₂ / D₁)e
Here, T₁ is your recent race time, D₁ is your recent race distance, D₂ is 21.0975 km, and e is the selected exponent.
2. Equivalent pace projection: A second estimate assumes the same pace carries directly to half marathon distance:
Equivalent Time = Recent Time × (21.0975 / Recent Distance)
3. Adjustment model: The Riegel result is modified by training, long-run support, temperature, elevation, experience, and manual tuning:
Final Time = Riegel Base × (1 + Total Adjustment ÷ 100)
4. Split planner: Pacing strategy applies weighted pace multipliers across race segments, then normalizes them so total split time still matches the final prediction.
How to Use This Calculator
- Enter a recent race distance and your finish time.
- Select the correct distance unit for that race.
- Add weekly training distance and longest run for endurance context.
- Set weather and elevation based on expected race-day conditions.
- Enter how many half marathons you have already completed.
- Apply a manual adjustment if you are unusually tired or sharp.
- Choose a pacing strategy for the split planner.
- Press the prediction button to view results above the form.
- Use the CSV or PDF buttons to save your output.
Frequently Asked Questions
1. What does this predictor actually estimate?
It estimates a likely half marathon finish time from recent racing plus training and race-day modifiers. It is a planning tool, not a guarantee.
2. Why use the Riegel formula?
Riegel scaling is widely used for endurance projections because it adjusts for fatigue over longer distances better than a simple pace carryover.
3. Which recent race works best?
A strong recent race between 5K and 15K usually works well. Races closer to half marathon distance often produce more stable predictions.
4. How does weekly distance affect the result?
Higher consistent weekly training generally improves endurance readiness, while low training volume tends to increase the predicted time.
5. Why is there a weather adjustment?
Warm conditions usually slow distance racing, while very cold weather can also reduce efficiency. The calculator adds small percentage penalties outside ideal temperatures.
6. What does the manual adjustment do?
It lets you fine-tune the prediction for current form, travel fatigue, illness recovery, taper quality, or confidence after a strong training block.
7. Which pacing strategy should I choose?
Even pace suits stable race execution. Negative split favors patience. Conservative start reduces early risk. Aggressive start is faster early but harder to hold.
8. Should I trust the optimistic and conservative range?
Use that range as a planning band. It helps with goal setting, split selection, and race expectations when fitness or conditions are uncertain.