📖 Overview
Use this calculator to estimate equivalent performance across race distances.
🧪 Example Scenarios
Use these default and higher-pressure example inputs to explore how sensitive this calculator is before using your real numbers.
| Input | Base Case | Higher Pressure Case |
|---|---|---|
| Known Distance (km) | 10 | 11.5 |
| Known Time (minutes) | 52 | 62.4 |
| Target Distance (km) | 21.1 | 24.27 |
⚙️ How It Works
This uses the Riegel projection model to estimate race time at a target distance from known race performance.
The Formula
T₂ = T₁ × (D₂ ÷ D₁)^1.06
| T₁ | Known finish time for the reference distance |
| D₁ | Reference distance (e.g. 5K, 10K) |
| T₂ | Projected finish time for the target distance |
| D₂ | Target distance (e.g. half marathon, marathon) |
| 1.06 | Riegel exponent — accounts for fatigue at longer distances |
💡The Riegel formula assumes consistent pacing and training level. Projections over large distance jumps (5K → marathon) tend to be optimistic. Use for directional planning.
Quick Reference
| 5K Time | Proj 10K | Proj HM | Proj Marathon |
|---|---|---|---|
| 20:00 | 41:39 | 1:33:10 | 3:14:24 |
| 25:00 | 52:04 | 1:56:27 | 4:03:00 |
| 30:00 | 1:02:29 | 2:19:44 | 4:51:36 |
| 35:00 | 1:12:53 | 2:43:01 | 5:40:12 |
When To Use This
- Use this tool when you need a fast decision during active planning or execution.
- Use this before committing money, time, or tradeoffs that are hard to reverse.
- Use this to compare options using the same assumptions across scenarios.
Edge Cases To Watch
- Results can be misleading if key inputs are missing, stale, or unrealistic.
- Very small or very large values may amplify rounding effects and interpretation risk.
- If assumptions change mid-decision, recalculate before acting.
Practical Tips
💡 Use a recent all-out effort as your input baseline.
💡 Longer projection jumps usually increase uncertainty.
💡 Run a best-case, base-case, and worst-case scenario before deciding.
💡 Use recent real values, not ideal assumptions, for better accuracy.
Frequently Asked Questions
❓ Is this exact race-day prediction?
No, terrain, weather, and pacing strategy can alter outcomes.
❓ Why use exponent 1.06?
It is a common endurance projection coefficient for distance scaling.