Welcome to our race predictor calculator. This race time converter can predict or calculate the race time for a future run competition based on your previous running scores.

You can use this race time predictor as a:

  • Marathon time predictor: if you input 26.2 miles or 42.2 km (official marathon length) as the new run distance.
  • Half marathon time predictor: if you input 13.1 miles or 21.1 km (official half-marathon length) as the new run distance.

It's important to note that this race time predictor relies on certain assumptions we'll present in the next section.

Assumptions made by this race predictor calculator

Before using this race predictor calculator, there are some points worth considering about the equation used (Riegel formula):

  1. It assumes that the runner has done appropriate training for the distance he wants to run in the future. An excellent result on a 5 km distance doesn't mean the next day you're ready to run a half marathon as a person who's been training for months would do.
  2. Assumes that the athlete doesn't have significantly above-average genetics for speed or endurance sports. That's because those people will always have better results than others.
  3. The calculations for running times under 3.5 minutes and above 4 hours are less accurate.

How this race predictor calculator works - Riegel's formula

This tool uses the Riegel's formula to calculate race time:

T2=T1(D2/D1)1.06T_2 = T_1(D_2/D_1)^{1.06}


  • T1T_1 — Time achieved recently on distance D1D_1;
  • D1D_1 — Distance achieved during T1T_1 time;
  • T2T_2 — Predicted time for distance D2D_2; and
  • D2D_2 — Distance for which T2T_2 time is foreseen.

Note that the equation presents a slight exponential that causes the relationship to not be 100% linear.

Example: Using the formula to calculate race equivalency

Suppose you've run a half-marathon in two hours, and your next goal is to run a full marathon. If you want to predict the time the full marathon will take to you, you can use Riegel's formula to calculate that race equivalency. Follow these steps:

  1. Identify the key terms.
    • As a half-marathon length is 13.1 miles and took two hours, D1=13.1 miD_1 = 13.1 \ \text{mi} and T1=3 hrsT_1 = 3\ \text{hrs}.
    • As a full marathon length is 26.2 miles (twice a half-marathon), D2=26.2 miD_2 = 26.2 \ \text{mi}
  2. Input the information in the Riegel's formula:
    T2=3 hrs(26.2 mi/13.1 mi)1.06=6.255 hrsT_2 = 3\ \text{hrs}(26.2 \ \text{mi}/13.1 \ \text{mi})^{1.06} = 6.255 \ \text{hrs}
  3. If 6.255 hour feels rare to you, use our time unit converter and you'll notice that 6.255 hrs=615min 17s6.255 \ \text{hrs} = 6\text{h}\ 15\text{min}\ 17\text{s}

That's it. You can also use this race time converter and verify the formula works well!

Luis Hoyos
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