Online pace calculators provide some great insights into comparable performances at various distances. But like any useful tool, you have to know how to use the tool if you want it to work for you. The following two examples, both using a half marathon time, show how a pace calculator can be misleading, and a detriment to your training and racing.
Let's say you run a half marathon and you feel pretty good about your time—perhaps you even ran a PR. You enjoy your post-race refreshment, get your medal, and go home. That night you put your half-marathon time in a pace calculator and it spits out a marathon time that you should be able to run. But can you run that time? The answer, like so much in life: It depends.
Pace Calculator Problem #1: Using 26.2 to Predict 13.1
Taking a half-marathon PR and assuming you can run the comparable marathon time is problematic because to run a good marathon, you have to be able to utilize fat as a fuel source. The body has enough stored glycogen to run roughly 18 miles. When you run a half marathon, the limiting factor is not fuel, but rather fitness. The marathon, however, is a different beast because you have to fuel throughout the 26.2-mile race.
Yes, you can take sports drinks and gels along the way, but most runners don't practice the skill of taking in sugars during their long runs, and then they bonk during the marathon somewhere after the 20-mile mark.
This is the problem: The calculator told you that you could run ___ for a marathon based on your half-marathon time, yet you haven't done the long-long runs to be able to run that projected time. You probably also haven't done enough running at marathon pace to be prepared properly to run the marathon at this projected time.
The calculator is accurate if you do proper marathon training, but it's not accurate if you haven't done the work necessary to run a good marathon.
Pace Calculator Problem #2: Using 13.1 to Predict 3.1
Using a half-marathon time to predict a 5K time becomes problematic because the race-specific training for these distances is so different. For instance, when you prepare for a half marathon, your training should include long runs and threshold runs, which are key workouts to improve fitness, but they're not the 6 x 1,000m with 400m recovery workouts you should complete to get ready for a 5K. This type of workout does two things. First, it gets you grooving at race pace, and, for many runners, 5K race pace feels frenetic because it's faster than their easy, long run and threshold pace.
Second, those 5K-specific workouts hurt, and a 5K hurts differently than a half marathon. The 5K is less than a quarter of the distance of a half marathon, but the race-pace effort is a type of hurt that is intense, but ends quickly. You have to practice being "5K uncomfortable" to be able to run the projected time the calculator tells you.
Final point with pace calculators: The projected times the calculator spits out assumes you don't run dumb. Runners run dumb often. They go out at a pace they can't maintain and then they die a horrible death in the final quarter of the race. Don't run dumb. Run even or negative splits if you want the calculator's projections to work.
In a nutshell, you have to be specifically prepared for each race distance for pace calculators to be useful.race.