Race time predictor
Run one race, project the rest. Enter a recent result and see your estimated times and paces from the mile to the marathon.
How it works
This predictor uses the Riegel formula, the running world's standard for projecting race times:T₂ = T₁ × (D₂ ÷ D₁)1.06. In plain terms, it takes a result you've already run and scales it up or down to another distance, accounting for the fact that your pace naturally slows as the race gets longer.
The 1.06 exponent is the fatigue factor Peter Riegel derived from thousands of race results. A pure "same pace at every distance" model would use 1.0; the extra 0.06 is what makes the estimate realistic.
How to use your estimate
- Enter a recent, hard race. A time trial from last month predicts better than a PR from two years ago.
- Pick the closest distance. Predicting a half from a 10K is more reliable than predicting a marathon from a 5K.
- Use the pace column to plan. The predicted pace is your target — punch it into thepace calculator to build even splits for race day.
Where predictions break down
Every model has limits. Riegel assumes you're equally well trained for both distances, which is rarely true — a fast 5K runner who hasn't built mileage won't hit the marathon time the formula suggests. Long-course estimates also ignore fueling, heat, and elevation. Use the number as a starting point, then let real training data refine it.
That's exactly what a coach does: watch how you actually absorb the work and adjust. Trackside Training reads every run you sync and turns these estimates into an adaptive plan that rebuilds every two weeks to keep up with your real fitness.
Frequently asked questions
How accurate is a race time predictor?
A prediction is a well-grounded estimate, not a guarantee. It's most accurate when the race you enter is recent, run at a hard effort, and close in distance to the one you're predicting. Estimates across very different distances — a 5K used to predict a marathon — carry more error because endurance, fueling, and pacing matter more the longer you go.
What formula does this use?
It uses Peter Riegel's formula, T2 = T1 × (D2 ÷ D1)^1.06, where the 1.06 exponent models how pace fades as distance grows. It's the most widely used running prediction model because it's simple and holds up well across the 5K-to-half range.
Why does it over-predict the marathon?
The marathon is limited by fueling and muscular endurance in ways shorter races aren't, so a time projected from a 5K or 10K usually comes out faster than what most runners actually run. Treat the marathon estimate as a ceiling that assumes marathon-specific training and disciplined pacing.
Which race should I enter?
Use your most recent all-out effort at any distance from a mile to the half marathon. If you have several, the one closest in distance to your goal race will give the sharpest estimate.
Your numbers, turned into a plan.
A calculator gives you a target. Trackside reads the runs you're already logging from Garmin, Wahoo, and Apple Health and rebuilds your plan every two weeks around them — adaptive, ad-free, and free to start.