Pace is a simple division, but predicting a race time at a different distance from a time you already have is more nuanced than most people expect — and it's not a straight-line multiplication. Here's the math for both.
Pace: time per unit distance
Worked example: running 10km in 50 minutes:
| Measure | Calculation | Result |
|---|---|---|
| Pace per km | 50 ÷ 10 | 5:00 /km |
| Pace per mile | 5:00 × 1.60934 | 8:03 /mile |
| Speed | 10 ÷ (50÷60) | 12 km/h |
Pace vs. speed: these measure the same run from opposite directions. Pace (minutes per km) gets smaller as you get faster. Speed (km per hour) gets larger as you get faster. Watches and apps show one or the other depending on convention — runners generally think in pace, cyclists generally think in speed.
Predicting a race time at a different distance
A common question: "I ran a 10K in 50 minutes — what marathon time could I run?" The naive approach — multiplying by 4.2 (the ratio of a marathon to a 10K) — significantly overestimates performance, because pace naturally slows over longer distances even for well-trained runners.
Worked example: predicting a marathon (42.195km) time from a 10K time of 50 minutes:
| Calculation | Result |
|---|---|
| 50 × (42.195 ÷ 10)^1.06 | ≈230 minutes |
| In hours and minutes | ≈3h 50m |
Compare that to the naive straight multiplication (50 × 4.2195 ≈ 3h 31m) — the exponent-based formula predicts a noticeably slower marathon time, which more accurately reflects how endurance and pacing actually degrade over a much longer distance.
Why the exponent matters
The 1.06 exponent (rather than 1.0, which would be a straight linear scale) captures the fact that maintaining a given pace gets progressively harder as distance increases — a runner's pace at 10K is very rarely sustainable, unchanged, for a full marathon. This formula (commonly attributed to Pete Riegel) is a widely used estimate in the running community, not an exact prediction: individual factors like specific endurance training, pacing strategy, terrain, and weather on race day all shift the real result.
Common mistakes
- Predicting a longer race with a straight linear multiplication instead of accounting for the pace decline over distance.
- Confusing pace and speed when comparing two runs or two people — remember lower pace numbers and higher speed numbers both mean faster.
- Treating a race prediction as a guarantee. It's a reasonable planning estimate based on a formula, not an outcome specific training and race-day conditions can't change.
Try the calculator
The running pace calculator converts a distance and time into pace per kilometer, pace per mile, and speed instantly.
Frequently asked questions
How do you calculate running pace?
Pace = Time ÷ Distance, typically expressed as minutes per kilometer or minutes per mile. For example, running 10km in 50 minutes gives a pace of exactly 5:00 per kilometer.
Can you predict a marathon time from a 5K or 10K time?
Approximately, using formulas like Riegel's, which scale time by distance raised to roughly the 1.06 power rather than a straight multiplication — but these are estimates that don't account for training specific to longer distances, pacing strategy, or race-day conditions.
Why isn't a marathon prediction just 4.2 times a 10K time?
Because fatigue increases disproportionately over longer distances — running economy declines and pace naturally slows as distance increases, even for well-trained runners. A simple linear scale-up underestimates how much slower per-kilometer pace becomes over a much longer race.
What's the difference between pace and speed?
Pace is time per unit distance (e.g., minutes per kilometer) — smaller numbers mean faster. Speed is distance per unit time (e.g., kilometers per hour) — larger numbers mean faster. They measure the same thing from opposite directions.