Question 1

How accurate is a race time predictor?

Accepted Answer

It is reliable between neighbouring distances: predicting a 10K from a 5K gives very usable numbers. The error grows with big jumps, typically 5K to marathon, where distance-specific preparation matters more than raw speed. Use a recent result over a distance as close as possible to your target race.

Question 2

Can I predict my marathon with my Garmin watch?

Accepted Answer

Garmin, Polar and Coros race predictors use your estimated VO2 max and training history, which makes them more contextual. They still tend to be optimistic for runners short on long runs. Cross-checking a watch estimate against a Riegel calculation gives you a more honest range than either source alone.

Question 3

Which race should I use as my reference?

Accepted Answer

The one closest to your goal distance, and the most recent. To plan a marathon, an all-out half marathon run within the last six weeks is the best input you can give. Anything older than three months no longer reflects your current fitness.

Question 4

Why does my marathon prediction look too fast?

Accepted Answer

This is the most common complaint, and it almost always comes down to the exponent. If you are quick over 5K or 10K but your weekly mileage is modest, raise the exponent towards 1.10 and the projection becomes realistic. The marathon punishes a lack of miles far more than a lack of speed.

Race time predictor

Your recent performance

Predicted times

Other useful tools

Pace calculator

Heart rate zones

VO₂max estimation

Calories burned

Split planner