Race Time Predictor

The Race Time Predictor estimates your finish times across every common running distance — 5K, 10K, 10 mile, half marathon, and marathon — from a single recent race result. It uses Riegel's endurance formula, the most widely used method for projecting race performance, and shows each predicted time and pace on an animated race-lane chart so you can compare them at a glance.

How the Race Time Predictor Works

Enter one race you have already run — say a 10K in 50:00 — and the tool projects how fast you could run the other distances if you raced them under similar conditions and with appropriate training. It works by assuming that your performance scales predictably with distance, following a well-studied power-law relationship between time and distance.

Riegel's Formula

In 1981, engineer and marathoner Pete Riegel published a simple, durable model for predicting endurance performance. It relates a known time and distance to a predicted time at a new distance:

Here \( T_1 \) and \( D_1 \) are your known finish time and distance, \( T_2 \) and \( D_2 \) are the predicted time and target distance, and the exponent 1.06 is the fatigue factor. Because the exponent is greater than 1, the formula correctly predicts that your average pace gets slightly slower as the distance grows — which is exactly what happens to real runners.

What Is the Fatigue Factor?

The fatigue factor (the exponent) controls how much you are expected to slow down over longer distances. The standard value of 1.06 works well for most trained runners, but you can fine-tune it:

• Lower (around 1.04): for runners with strong endurance who hold their pace well over long distances.
• Standard (1.06): Riegel's original value, a good default for most people.
• Higher (around 1.08): for speed-oriented runners or those with limited long-distance training, who fade more over longer races.

Example Predicted Times from a 50:00 10K

Notice how the pace per kilometre steadily rises from 4:49 at 5K to 5:28 at the marathon — the predicted slowdown built into the fatigue factor.

How Accurate Are the Predictions?

Riegel's formula is remarkably reliable for distances close to your known race, especially between 5K and the half marathon. Accuracy drops as the gap between distances grows, because longer races depend heavily on endurance, fuelling, and pacing that a short race cannot reveal. A marathon predicted from a 5K is best treated as an ceiling you could reach only with dedicated marathon training. For the most trustworthy marathon estimate, use a recent half marathon as your input.

Tips for the Best Prediction

How to Use This Calculator

1. Enter your known race: Choose the distance of a recent race (or enter a custom distance) and type in your finish time in hours, minutes, and seconds.
2. Adjust the options: Optionally fine-tune the fatigue factor and choose whether to see pace in min/km or min/mile.
3. Click Predict: The tool applies Riegel's formula to every target distance instantly.
4. Review your predictions: Compare your predicted finish times and paces on the animated race-lane chart, in the detailed table, and in the step-by-step breakdown.

Frequently Asked Questions

How does the Race Time Predictor work?

It uses Riegel's endurance formula: predicted time equals your known time multiplied by the ratio of the two distances raised to a fatigue exponent (T₂ = T₁ × (D₂ / D₁)^1.06). From a single recent race result it estimates your finish time at every other common distance.

How accurate is the race time prediction?

Predictions are most accurate when the target distance is close to your known race and when you have trained appropriately for that distance. The formula assumes similar conditions and adequate endurance, so a marathon predicted from a 5K will be optimistic unless you have built marathon-specific endurance.

What is the Riegel fatigue factor?

The fatigue factor is the exponent in Riegel's formula, typically 1.06. It reflects how much runners slow down as distance increases. A higher value (for example 1.08) predicts a larger slowdown over longer distances, while a lower value (such as 1.04) predicts that you hold pace better.

Can I predict a marathon time from a 5K?

Yes, but the larger the jump in distance, the less reliable the estimate. A marathon predicted from a 5K assumes you have the endurance to sustain effort for the full distance. For the best marathon estimate, use a recent half marathon or a long tempo run as your known race.

What is the best race to use as my input?

Use a recent, hard, even-paced effort on a flat course, ideally from the last few weeks. A 5K, 10K, or half marathon usually works well. The closer your known distance is to the distance you want to predict, the more accurate the result.

Does the predictor account for hills, heat, or training?

No. It produces a flat-course, equal-conditions estimate based purely on your input race. Hills, heat, humidity, altitude, fatigue, and your training level will all affect your actual finish time, so treat the predictions as a target rather than a guarantee.

Additional Resources

• Peter Riegel - Wikipedia
• Running - Wikipedia