If you're anything like me, one of the first things you do after setting a new running PR is open up Google on your phone. The destination? A running race predictor calculator. The goal? Figure out how fast you are supposed to be able to run at other distances. 

But which running calculator to use? And, how different are their predictions? Prediction accuracy is - in all reality - difficult to measure until you actually run the new distance. However, it is possible to see how precise the running race calculators are. In other words, how close the running race calculators are together in their predictions. 

So below is the data. I found six different race prediction calculators on the internet. Each uses a different formula and methodology for predicting race times from a current race result. (If your the curious type, please see the links at the bottom of the post for more information on the methodology behind each race calculator. For my purposes, I was less interested in the exact methodology and more interested in the actual predictions!)

I queried the calculators for my predicted 10K, Half Marathon, and Marathon times based on my recent 5K performance. Then, I did the same for a buddy's recent 5K performance.

Here are the results: See attached image. 

Overall, I would say the race predictors are precise - predicting similar minute per mile paces.

In particular, my 10K predictions were 6:22 min/mile plus or minus :01 sec/mile at the 95% confidence interval. A very tight prediction interval. The half marathon prediction was 6:41 min/mile plus or minus :04 sec/mile -- also pretty tight. For the marathon, the predicted pace was 7:00 min/mile plus or minus :05 sec/mile. 

Therefore, a safe strategy would be to choose your favorite calculator and set a pace range:           
       * +/- 5 seconds for half marathon distances or longer. 
       * +/- 2 seconds for 5K to half marathon distances

In the end, what is my favorite calculator? The one that predicts the fastest time of course!