📖 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.

ExampleExample: a 52-minute 10K projects to about 1:55:36 for a half marathon using the standard Riegel exponent.
InputBase CaseHigher Pressure Case
Known Distance (km)1011.5
Known Time (minutes)5262.4
Target Distance (km)21.124.27

⚙️ 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
T₁Known finish time for the reference distance
D₁Reference distance (e.g. 5K, 10K)
T₂Projected finish time for the target distance
D₂Target distance (e.g. half marathon, marathon)
1.06Riegel 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 TimeProj 10KProj HMProj Marathon
20:0041:391:33:103:14:24
25:0052:041:56:274:03:00
30:001:02:292:19:444:51:36
35:001:12:532:43:015:40:12

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.

Common Mistakes

  • Projecting from a stale race result.
  • Using an unrealistic target distance jump.
  • Ignoring heat, elevation, terrain, and training specificity.

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

❓ How accurate is the Riegel formula?

It is a useful estimate when the known race is recent and the target distance is reasonably close.

❓ Can I project marathon time from a 5K?

You can, but the estimate is less reliable because endurance and fueling matter much more.

❓ What exponent does Riegel use?

A common exponent is 1.06, meaning pace slows as race distance increases.

❓ 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.