Calculating the Accuracy of Running Calculators

imageI 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 1:00:20). 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.

Tea crossing the finish line of the 2014 CIM with her OTQ.
Calculators, you just don’t get me.

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.

How do you feel about running calculators? 

I'm a science journalist with a background in neuroscience and a love of running marathons and baking marathon-worthy feasts. I started out as an over four-hour marathoner but whittled my PR down to qualify for the 2016 Olympic Trials. I write about the importance of chasing big dreams and -- as I'm currently pregnant with my first -- getting ready to chase around a little one.

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  1. My PRs are basically the opposite of yours! Mine starts at my 5k as a high and goes down from there, while yours starts at your 5k and goes up! And I still crack up that we have almost the exact same 5k PR. The Trials?! I wish! For a while though my PRs lined up almost perfectly on the McMillan Calculator. I think my 5k time was the result of introducing really hard track work that was eventually squandered through overtraining … but I also digress 😉

  2. Well… the calculators DO seem to come with the caveat that the prediction assumes one has trained for the race. I’m pretty sure the stipulation applies just as much predicting shorter distances as it does longer. At least that’s what this marathoner keeps telling herself. 😉

  3. One error I think people often make with these calculators is entering a GOAL pace and then trying to hit the prescribed workout paces rather than doing workouts at an appropriate pace and working to earn that goal pace fitness.

    So I’ve recently been using a calculator to plug in my training paces from tempo runs to see what sort of shape I’m in. Well, not exactly — I have to screw around with guessing at race paces to find my corresponding tempo pace. Does that even make sense? I wish there was a calculator that could do that easily — I don’t always have a recent race to plug in!

    1. Also, thanks for the link to that Slate calculator. It’s interesting. It predicted my Columbus 2015 time almost exactly, but when I go back to my 2011 marathon PR it predicts a slower time, even though I’m definitely more of a 5k girl.

  4. I like using them to help me set my goals for races. However, I am not a 5K specialist so I have trouble really racing those hard, so they’re never accurate for me at that end of the spectrum. I’m much more comfortable settling in at a pace and holding it for longer distances. Which as you pointed out is something the calculators don’t take into consideration. Another thing is that they only take a snapshot of who you are as a runner to predict your time. It doesn’t take into consideration whether or not you actually trained for that PR in a similar way you’re training for your next goal race. PS in my training recap for this week I actually discussed using calculators to help set realistic goals!

  5. There’s a 15 minute difference between the slowest and fastest prediction with these 3 calculators for me. My last marathon was somewhere right in the middle.

  6. I haven’t figured out what type of distance runner I am. My DNA tests (23andme) indicate more fast-twitch muscles so perhaps I should be sprinter? Yet my 5k time in fall 2014, before I finally started running more consistently in 2015, was much slower relatively than my 10k PRs. And my 10k times would have predicted a faster half marathon last fall, but there’s the “age factor,” the injury factor and the training schedules that go awry….trying to predict what this senior masters runner pace might be in any distance seems not very fruitful. For me the calculators are interesting from a theoretical perspective but as a former math nerd (until sophomore year in college when political philosophy won out over five-page long calculus formulae homework), that’s really what they are–theoretical but not subjective–which running seems to be!

  7. Oh my gosh! Thank you for this post. (Long-time listener, first-time caller!) I am very much like you in being almost the same speed at the super short stuff as I am at the long stuff. I find those prediction calculators to be very disheartening. I used to race ironman triathlons exclusively because I went the same pace as in half ironmans or shorter, and am trying to cross over to standalone marathons. Also I was excited to see you start blogging on Salty Running! I just discovered your personal blog, Miles to the Trials, recently.

  8. The calculators vary big time. I found one that includes 5 different equations including age grade, but even those times were up to 2 minutes different for the half marathon. After a recent half marathon, it predicted that I should be able to run a 5K much faster than I had been. I did, about a month later, but most of my 5Ks were in training, en route to the half marathon. It’s especially hard to “race” any other distance when you’re in marathon training.

    The marathon is a huge jump for those calculators, because 26.2 miles is a long time for things to go wrong or everything to go perfectly. I’ll plug my half time into the calculator but I don’t trust what it would spit out for a marathon. You are definitely a different case though, as most people have the opposite problem with the calculators.

    I honestly don’t think ANY runner who is running a half marathon or marathon for the first time should look at the calculators to set goal times or get expectations- they should just enjoy the race. I only ran one marathon, and I ran a 4:15. At the time, I had a half marathon PR of 1:49… and stupidly thought I would surely be sub-4.

    1. Yea, that’s a great point to make! I agree that first time marathoners shouldn’t have time expectations. It’s just impossible to predict what will happen in that first one. Another reason to ignore the calculators!

  9. I don’t put too much faith in any of these calculators – I am primarily a marathoner and based on my marathon time, all of my shorter distance PBs should be a whole heck of a lot faster than they really are. The big joke is that I routinely set shorter distance PBs while racing at longer distances (and setting PBs in the longer distances too). I think this is mostly because I train to peak for the marathon, and don’t usually run a lot of shorter races.

    1. Oh yea, I’m the same way! My 10K split in a recent half is faster than my official 10K PR. Until recently, my fastest 5K was from a ten mile race. I only ever run those shorter races in the off season or beginning of the season. The marathon gets all my love and attention!