Predicting how long it will take you to complete a long race is unfortunately a very non-exact science. Multiple variables will always be at play when you hit route, and the less experienced you are and the longer the run, the more difficult it can be to accurately forecast.

Some of the most common variables that affect a runner's performance can be hard to gauge factors like sleep quality the night before, nutrition, performance anxiety, and even the weather. Hilly or high-altitude terrain (think Colorado) will obviously slow a runner down far more than a strip of even, flat ground in Louisiana. And of course, the human body simply gets fatigued over time: your pace at the beginning of a full marathon is not going to be the same as by the final hour.

However, many competitive races do require you to give your predicted pace and finish time when you register. This is done to insert runners of similar ability into staggered “start corral” groups designed to reduce overcrowding along the route. So what are some good methods for us to predict our average race time?

Using a Race Time Predictor like the one below is extremely helpful. As previously outlined, there are of course many factors that can throw a wrench into our ability to predict our performance in a long-distance race, but a Race Time Predictor can definitely assist us in our fitness goals. Using an equation developed in the late 1970s by engineer and marathon runner Peter Riegel, a Race Time Predictor can quite accurately calculate your race times based on your previous performance.

Here’s the breakdown of Riegel's formula that we use for our Race Time Predictor (don’t worry you don’t have to do all the math!): T₁x(D₂/D₁)1.06 = T₂

T₁ was the time achieved on your last recent race D₁
T₂ is the predicted time for your next race D₂
D1 is the distance over which the initial time is achieved.
D2 is the distance for which the time is to be predicted.

Riegel’s formula also makes two assumptions: 1) That the runner has trained appropriately for their intended race beforehand. For example, just doing a 5K run in a short amount of time is not going to be a good baseline prediction of how you’ll perform if your next race is a half marathon. 2) That you don’t lean too heavily one way or another towards Speed or Endurance, but fall nicely within either ends of the scale.

The beauty of Riegel’s formula is that it takes into account distance and endurance for the runner. There’s no way you’re going to be running at the same pace during the second hour as you did during the first. However, this calculator is just a predictor. It’s a useful tool, but if you’re racing competitively be sure to log all your stats over time and see how they ultimately average out.

Enter the time and distance from your previous, most recent run into the calculator below to see your predicted times.