Race Time Predictor Formula

Understand the math behind the race time predictor. Each variable explained with a worked example.

Use the Race Time Predictor

Formulas Used

Predicted Hours

predicted_hours = floor(predicted_time_min / 60)

Predicted Minutes

predicted_minutes = floor(predicted_time_min - floor(predicted_time_min / 60) * 60)

Predicted Seconds

predicted_seconds = round((predicted_time_min - floor(predicted_time_min)) * 60)

Predicted Pace

predicted_pace = predicted_time_min / target_distance_miles

Variables

VariableDescriptionDefault
known_distance_milesKnown Race Distance(miles)3.1
known_time_minKnown Race Time(min)25
target_distance_milesTarget Race Distance(miles)13.1
predicted_time_minDerived value= known_time_min * pow(target_distance_miles / known_distance_miles, 1.06)calculated

How It Works

How the Riegel Race Prediction Formula Works

The Riegel formula predicts race times across distances based on a known performance.

Formula

T2 = T1 x (D2 / D1) ^ 1.06

Where:

  • T1 = Known race time
  • D1 = Known race distance
  • D2 = Target race distance
  • 1.06 = Fatigue exponent

    • The formula works best for distances between 1500 m and the marathon, assuming similar fitness and training for both distances.

    Worked Example

    You ran a 5K (3.1 miles) in 25 minutes and want to predict your half marathon time.

    known_distance_miles = 3.1known_time_min = 25target_distance_miles = 13.1
    1. 01Distance ratio = 13.1 / 3.1 = 4.2258
    2. 02Fatigue factor = 4.2258 ^ 1.06 = 4.6013
    3. 03Predicted time = 25 * 4.6013 = 115.03 minutes
    4. 04That is 1 hour, 55 minutes, and 2 seconds
    5. 05Predicted pace = 115.03 / 13.1 = 8.78 min/mile

    Frequently Asked Questions

    How accurate is the Riegel formula?

    It is typically accurate within 2-5% for trained runners racing distances from the mile to the marathon, assuming proper training for the target distance.

    What is the fatigue exponent?

    The 1.06 exponent models the fact that you cannot maintain the same pace over longer distances. Higher exponents predict more slowdown.

    Can I use this for ultramarathons?

    The formula becomes less reliable beyond the marathon because fatigue factors like nutrition, sleep, and terrain play a much larger role.

    Learn More

    Guide

    How to Calculate Running Pace - A Complete Guide for Runners

    Learn how to calculate running pace in minutes per mile or kilometer. Covers pace formulas, race pacing strategies, treadmill conversions, and training zones.

    Ready to run the numbers?

    Open Race Time Predictor