What pace can you run for a marathon if you just ran a 25:00 minute 5k? What about for the half marathon?
That’s a question almost every runner has at least a few times in their training cycle.
As you may well know, it can be tricky to figure out what’s a realistic goal for an upcoming race, but fortunately, conversion charts can make this process a little easier.
Most runners are familiar with race conversion and equivalence charts or calculators, and we have our own performance calculator that will tell you your predicted race times at various distances based off a recent race or training run.
But when a chart says that a 20:00 5k converts to a 41:20 10k, what is the significance of this?
Today, we’re going to take a closer look at how these calculations are made, and under what conditions they are valid.
Understanding race conversions
Broadly speaking, there are two different ways to look at how one race distance converts to another.
One is by calculating the physiological “expected outcome” of a runner of a given fitness level running a particular race distance; the other is calculating the equivalent performance from a competitiveness perspective.
Option 1: Expected fitness level
In the first case, the basic question that’s being asked is this: “Among runners who are currently in, say, 18:00 5k shape, what is their average time when they run a 10k, half marathon, etc.?”
While it’s pretty easy to look at either a list of lifetime PRs or season-best performances over a range of distances and determine a formula to convert from one race to another, there are several assumptions that go into this sort of project.
- First among these is the race course and conditions: a flat, fast road 5k will overestimate your performance in a rainy, windy, hilly 10k. Since the most easily accessible databases of lifetime or seasonal running performances are from elite runners, times in the 5k and 10k usually come from the track—which is, of course, much faster than any road course.
- Second, the predictions made by a conversion formula are only averages. Some runners fare better at one distance than another. Even a conversion that is very accurate in the statistical sense can be pretty vague in the real world: a 2% difference in a 5k time, for example, could be over 30 seconds!
Another major assumption is that you are equally well-prepared for the two distances you are converting to and from. While it isn’t too hard to be in shape for both the 5k and the 10k, you can’t always say the same about the 5k and the marathon!
Because of this, a converted time doesn’t always mean “you could run this time right now” — it might mean something more along the lines of “you might be able to run this if you train for this event.”
Additionally, conversions become less valid the further you get from the race distance you’ve been training for.
For example, a lot of high school runners can hit five minutes for the mile, but very few can run under 2:50 in the marathon—even though this is an “equivalent” performance!
Option 2: Equivalent performance calculation
Calculating an equivalent performance is a slightly different undertaking.
These sorts of charts attempt to answer the question of “how good is my time?” by looking at the relative competitiveness of that race performance compared to another event. The best way to picture this is in terms of “world ranking.”
It’s a bit absurd to think about your world ranking if you’re a 25-minute 5k runner, but this doesn’t mean the statistics don’t have merit.
There are sets of equivalent performances in the 10k, marathon, and so on.
This is also the same concept behind age-grading percentiles: these tell you “how good” a certain time is for a certain age group.
While performance equivalences work reasonably well as conversion charts too, they do have some shortcomings.
- By their nature, they overestimate your performance in less-common race distances. Using our world-ranking analogy, it’s easy to see why: far more runners have recorded times in the 5k or the 10k than more exotic races like the 25k or quarter-marathon (yes, those do exist!).
- And a performance equivalence chart is going to be less accurate for slower times, since these charts are almost universally made by referencing elite times—running a 13-minute 5k become a very different event than a 20-minute 5k.
Final thoughts
Conversion and equivalence charts can be very useful training tools, but don’t put too much stock into them.
They are best used for getting a rough idea of what you can run for an unfamiliar race distance, or figuring out which race distance is your best.
Specifically, you must factor in weather and course conditions and, more importantly, whether you’re training for the specific demands of that event.
To help you understand exactly how you should be training for the specific demands of your goal race, we’re conducting a series of webinars over the next few weeks. If you’re interested, you can see the available times below:
Half marathon specific workouts
Finally, you shouldn’t let a conversion chart beat up your ego. If you can’t run an “equivalent” performance, it might be for any number of reasons: you may not be training for that particular event, you may have run your first performance in better conditions, or you might just be better at shorter or longer races!
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4 Responses
I also believe that running economy factors in very strongly at longer races, like the marathon. This may well be why marathon race time predictions based on say a 10K PB may not work for many runners.
Great point Tasos. The longer the distance, the more important your running form becomes. If you are inefficient, you will waste more energy that becomes critical towards the end of a marathon. Thanks for your feedback!
For me, I don’t think the calculations do enough to account for speed. For example, I have a 17:00 5k PR, and the calculations predict me running a 5:06 mile, even though I once went out with a 5:01 in a 5k (yeah I know I went out too fast that race). Still, I think I can run under 5 easily, and probably even like a 4:50
Conversion calculators have an element of “fun” to them. Fun in terms of what you in theory could run based on another distance. But there are a lot of factors to think of. I think the further you to try and extrapolate the less accurate the prediction. Doubling your 800M time may be somewhat accurate for 1500M for example, but trying to predict marathon time from a mile PR is far less accurate.