Complete Guide & Formula for the Running Pace Predictor
Wondering what your marathon time might look like based on a recent 10K, or how fast you could run a 5K based on your half marathon pace? This running pace predictor uses the Riegel formula, a well-established method for projecting race performance across distances. It's a handy way to predict your marathon time from a recent 10K result, or estimate a 5K time from your half-marathon pace.
💡 The Riegel Formula
Predicted time (T2) = known time (T1) × (target distance (D2) ÷ known distance (D1))1.06. The 1.06 exponent accounts for the natural pace slowdown as distance increases.
Why Race Predictors Are Useful
- Set realistic goals: Base a marathon goal time on actual recent performance rather than guesswork.
- Pace training runs: Use predicted paces to plan tempo runs and long runs.
- Track fitness trends: Compare predictions over time as your training progresses.
- 100% Free & Private: No signup, all math runs in your browser.
Accuracy Notes
| Factor | Effect on Prediction |
|---|---|
| Similar distances (e.g. 5K → 10K) | Most accurate predictions |
| Very different distances (e.g. 5K → Marathon) | Less accurate — endurance training matters more |
| Race conditions (heat, hills, wind) | Not accounted for in the formula |
Step-by-Step Instructions
- Enter a race distance you've recently run and your finish time in decimal minutes.
- Enter the target distance you'd like a prediction for.
- Click ⚡ Predict Race Time to see your projected finish time and pace.
- Use 📋 Copy Summary to save your prediction or 🔗 Share to send this calculator to a training partner.