Training 5 min
How accurate are marathon time predictors?
Marathon time predictors are usually accurate to within a few percent for experienced runners racing in mild conditions on a fair course—but the error grows fast for first-time marathoners, hot or hilly races, and predictions built from a single short race. The method matters less than the inputs: a predictor is only as good as the training and racing data you feed it.
How do marathon time predictors actually work?
Most predictors use some version of the Riegel formula, which scales a known race time to a longer distance using a fatigue exponent (typically around 1.06). Plug in a recent 10K or half marathon time, and it projects a marathon finish. More sophisticated tools also weigh training volume, long-run pace, and recent workouts rather than relying on a single race result, which is why app-based predictions can differ from a plain formula calculator.
Why is a prediction from a 5K or 10K less reliable than one from a half marathon?
The marathon tests fuelling, pacing discipline and endurance in ways a 5K simply doesn't touch. A fast 5K time tells you about your speed, not whether you can hold pace for three-plus hours without bonking. The shorter the reference race, the more the formula has to extrapolate—and the more it assumes your aerobic endurance scales the same way your speed does, which isn't true for everyone. A half marathon time, run reasonably close to marathon effort, is a far sturdier base.
What makes a prediction more accurate?
- A reference race or time trial run in the last 4–8 weeks, not months old
- Training that includes long runs at or near race pace, not just easy mileage
- A realistic weekly mileage in the lead-up—predictors assume you've actually put in marathon-specific endurance work
- Conditions on the day similar to when the reference time was set
- Using a recent trend of workouts rather than a single best-ever result
What throws a prediction off on race day?
Heat is the biggest single factor—pace that felt sustainable in training can become unsustainable once the temperature climbs a few degrees. Hilly or technical courses also erode a flat-course prediction, since net elevation gain costs more time than most runners expect. And going out at the predicted pace from the gun, rather than easing into it, turns a reasonable target into an early red line that the back half can't sustain.
Should you trust the predicted time over how you feel on the day?
Treat a prediction as a planning tool, not a promise. It's most useful for setting your early pacing bands and a sensible goal range, not as a number to chase no matter what. If you're cooked by mile 18 and the prediction says you should be fine, your legs are the more reliable source. The best use of a predictor is to stop you starting too fast, not to override race-day judgement.
How does RunV keep its predictions current?
RunV recalculates your projected finish time weekly from your actual training—long runs, tempo sessions and overall load—rather than freezing it after one race result. That means the number moves as your fitness does, and it's flagged as a range rather than a false-precision single figure when your recent training doesn't give it much to go on.
FAQ
- Are marathon predictors accurate for first-time marathoners?
- Less so. Most formulas are calibrated on runners who already know how their endurance holds up over the distance. A first marathon introduces unknowns—fuelling, pacing discipline, how your legs feel after 20 miles—that no formula can account for from a 10K time alone. Treat the number as a rough guide and build in a buffer.
- Does the Riegel formula work for everyone?
- It works reasonably well on average but not for any individual runner in particular. The fatigue exponent it uses is a population average; some runners fade faster over distance than the formula assumes, others slower. It's a useful starting point, not a guarantee.
- Can weather really change a predicted marathon time by much?
- Yes. Running in noticeably warmer conditions than you trained in can cost several minutes over a marathon, even for well-prepared runners, because heart rate drifts higher at the same pace. If race-day forecasts look hot, it's more sensible to adjust your target down than to chase a prediction set on a cool training day.
Train smarter
RunV turns this thinking into your plan — adaptive coaching that rebuilds after every run.
Keep reading
