Friday, February 5, 2016

Calculating the Accuracy of Running Calculators

This post was originally featured on Salty Running

I was a math major in college. There was something comforting about numbers and formulas; there was always one right answer, nothing to be argued or evaluated subjectively. (In upper level math courses, where there are more Greek letters than numbers, this falls apart, but I digress.)

When it comes to running, I still love crunching numbers—the weekly total, the paces nailed, the amount of miles on my shoes—but in running, at least, I’ve come to have a serious problem with formulas: running calculators. Specifically, those race equivalency calculators that tell you if you run X pace for a 5K, you should be able to run Y for a 10K and Z for a half. They spit out what seems like a nice, tidy answer, but it isn’t always right.

If you are just starting out and have no idea how much you might slow down for a longer distance or speed up for a shorter one, they can be useful tools. They are based on lots of data, averages from thousands of runners. But they don’t represent every runner. Especially this one.

I’m a marathoner: so purely and completely am I a marathoner that I run nearly the same pace for all race distances. If I put my PRs for shorter distances into any calculator, it tells me I should not be capable of a Trials qualifying time.

To illustrate this point, I put some race times from the fall of 2014 (the season I qualified) into a couple of calculators. In November, one month before my goal marathon, I PR-ed in the half, with a 1:19:28 on a hilly course.

McMillan’s Calculator says that translates to a 2:47:14 marathon.
Runner’s World says 2:45:41.
Slate’s, which claims Runner’s World’s is inaccurate, also takes into account weekly mileage and how difficult the half course was. It says 2:51:30. Slate’s also gives an option for two recent races, so I recalculated to include the ten mile race I ran that October (in a then PR of 1:00:19). That slowed me down to a 2:55:50 prediction.

In reality, I ran 2:42:13.

There are some caveats here. A major one is that I set my season up to peak for the marathon and kind of race myself into shape with the shorter efforts. My PRs from other distances come earlier in the season and aren’t my target races. The calculators don’t know this and obviously can’t take it into consideration.

But another major thing calculators can’t take into consideration? Who you are as a runner. While these calculators are based on averages, I suspect there are a lot of runners out there who excel at one end of the spectrum. If you prefer the 5K, the calculators might not give you an accurate marathon prediction. If you prefer the marathon, your predicted 5K time might be way off. (In the 5K, I should be able to run two minutes faster than my 5K PR.) It’s just not reality that everyone’s times scale up the same exact way from the 5K to the marathon. However, it is true that the closer together two race distances are, the more accurate the calculators will be. Entering your 5K time should give you a more accurate prediction of your 10K time than your marathon time. That’s why, in the case above, I used my half marathon time instead of my 5K. But it was still way off.

If you rely too heavily on the calculators you might unnecessarily become discouraged. Sometimes people tell me they’re impressed I went for the standard, given my half PR. It seemed gutsy and unrealistic. Maybe it was, but I didn’t think so. I was running PRs across the board and hitting the pace I needed to qualify during long solo runs. Why would a calculator be more accurate than actual, individualized data? Why would an online calculator know me and my potential better than me? Had I relied on these calculators, I would have talked myself out of it.

Of course, the calculators can also be encouraging, perhaps giving you reason to shoot for a new goal. Maybe I should take on their challenge to get my 5K PR down. But if I end up with a new PR that’s still not as good as the calculators suggest, does that mean I failed? No. It means I’m a slightly speedier marathoner, but still a marathoner.

My advice is to take the predicted times with a grain of salt. Don’t give these calculators more credit than they deserve. If they encourage you, awesome. If they seem way off and tell you that you can’t do something you already have, or even just slightly discourage you, ignore them. They are generalizations, averages. They don’t apply to every runner.

So I haven’t put any of this season’s times into a running calculator, because I honestly don’t want to know what it would tell me. Clearly the formulas think I can’t run a time I already have. But I do know how my paces in training and races compare to the ones I’ve hit in the past, so maybe I’ve learned to form my own calculator. And that one predicts a PR, which seems like a good answer to me.

Dream big, 


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  2. There are 1000 reasons why I love your blog and a big one of them is that the marathon is the distance you are better at. I could have seriously written this post myself! Exception made for my first marathon, all my marathons have always gone much better than predicted. When I finished my half marathon in 1:42 (7:49) four weeks before my goal marathon last year, people and running calculators thought I was nuts for thinking that I could break 3:30 in the full. 3:34 is the best I could wish for, apparently. Four weeks after, I ran a 3:29 (7:59). Double the distance, but only 10 sec/mile slower than my half, and that wasn't the first time something like that happened. Running calculators are seriously useless for me when it comes to the marathon!

    You are SO right: it all depends on who YOU are as a runner. I know many people who have a marathon PR that is much slower than mine, and yet they do much, much better than me when it comes to shorter distances!

    The only running calculator that is somewhat accurate for me is Jack Daniels' VDOT Running Calculator, which I think is good for those people who are better at longer distances.

    1. I love how aligned we are! I'm not the only one :)

  3. Reading your blog I definitely agree some runners have their "hot" zones where the Daniels predictor fails. I cant help but think of Brian Sell and Boston 2006. His HS 2 mile PR's were relatively slow 10:XX but he turned in a sub 2:20 in Boston and was the First American that year. Best of luck next week. I'll be rooting for you. My son currently attends U of R and I'm running Boston in April.

    1. Brian Sell is a good example and I think Bill Rodgers, too. Thanks for your support and I love the U of R connection!! Good luck in Boston :)

  4. I found your blog a few hours before New Years (2016) and spent the rest of my evening reading this blog and other articles that you have published. Since then, I check every few days or so to read new posts. I enjoy reading about your experiences and ideas.

    I have ran two marathons in the past year and am currently training for the Boston Marathon this April. I, too, someday dream of running the Olympic Trials. However, I have taken into consideration all of the following: I am a busy farmer's wife (yikes), 1st grade teacher, mother of a 3 yr old and a 2 yr old and plan to have at least 2 more children within the next few years. So, it may take sometime...... ;-) But, I am determined to do it. This blog has been inspiring for me to read. You are truly amazing and I look up to you for your hard work and dedication that you have put into your running accomplishments and improvements. I love that you do this for the LOVE of running! I will be cheering for you this weekend! I have all faith in you and I look forward to reading about how awesome you did.

    Also, if you already haven't I recommend checking out the website after the marathon. I don't know how accurate the running calculators are for most but it has been right on with my 5k, half and marathon paces. Very strange! So, it is my GO To running calculator. It's that running website that makes me feel better;-) However, I completely agree with what you had to say. We "runners" know what we are capable of doing.

    1. Yesss, love your goal to get to the Trials!! Keep believing :) And I will definitely check out that site post-marathon.

  5. GOOD LUCK TEAL!!!!! I'm rooting for you on Saturday!!