Race Time Predictor

Enter one recent race result and get predicted times for a 5K, 10K, half marathon, and marathon using Riegel's formula.

How it works

This tool uses Riegel's formula, a well-known way to estimate one race time from another: predicted time equals your known time multiplied by the ratio of the two distances raised to the power of 1.06. That exponent is slightly above 1, which reflects the fact that you can't hold your 5K pace for a marathon. As the distance goes up, the pace you can sustain has to come down, and 1.06 is the average slowdown researchers have found across a large number of runners and distances.

Worked example: run a 5K in 30:00 (1,800 seconds), and the formula predicts a 10K time of about 3,753 seconds, or 1:02:33. Chain that 10K prediction forward to a marathon and you get an estimate in the high 4-hour range. Notice how much rougher that second jump feels. Going from a 5K to a 10K is a small, well-supported extrapolation. Going from a 10K estimate to a marathon estimate compounds a lot more uncertainty, because marathon performance depends heavily on long-run training and fueling that a 10K doesn't test at all.

FAQ

Can I really predict a marathon time from a 5K?

You can get a rough number, but treat it as a ceiling, not a promise. Riegel's formula assumes you've put in the long runs and fueling practice a marathon actually demands. A fast 5K tells you about your speed and fitness, not whether your legs and gut are ready for 26.2 miles.

Why does the prediction get less accurate for bigger distance jumps?

The formula was built from average slowdown patterns across many runners, but individual variation grows the further apart the two distances are. Someone who trains mostly short and fast will underperform this formula's marathon prediction, while a dedicated long-distance runner might beat it.

What race time should I actually enter?

Use your most recent honest race effort, not a training run or a time-trial you weren't fully rested for. A recent result within the last few months reflects your current fitness better than a personal best from a year ago.

Should I use this to set my marathon pace on race day?

Use it as a starting estimate, then adjust based on your actual long-run training paces. If your longest training runs feel much harder than this prediction implies, plan for a more conservative pace on race day.

For more on building toward these distances, see how to train for your first 5K, a beginner's 10K training plan, and how long it takes to train for a 5K.