Running Pace Calculator

Five modes, one tool

Pace turns a distance plus a finish time into the number runners actually quote — minutes per kilometer — alongside min/mile, km/h, mph and a 400-meter track lap. Finish time runs it forward: give a pace and a race distance (5K, 10K, half, marathon or custom) and get the predicted finish plus a full splits table. Race prediction applies the Riegel formula — T2 = T1 × (D2/D1)1.06 — to translate a result at one distance into another, with an honest accuracy window instead of a single confident number. Speed → pace converts treadmill and bike-computer units both directions. Training zones derive easy, long, marathon, tempo, interval and repetition paces from a recent race result.

The math, stated plainly

• Pace: pace = time ÷ distance — 10 km in 50:00 is 300 s ÷ 1 km = 5:00/km; the same effort is 8:02.8 per mile because a mile is exactly 1.609344 km.
• Speed: speed = distance ÷ time — 10 km in 0.8333 h = 12 km/h = 7.46 mph. Pace and speed are inverses: pace (s/km) = 3600 ÷ km/h.
• Riegel (1977): T2 = T1 × (D2/D1)^1.06 — the 1.06 exponent is an empirical fatigue factor fit to race results. We show a ±4–16% window that widens with the distance ratio because the formula knows nothing about your training.
• Zones: anchor = Riegel-equivalent 10K pace; bands at 78–84% (repetition), 86–92% (interval), 95–100% (tempo), 104–110% (marathon), 113–120% (long run), 125–138% (easy) of that anchor pace.

Go deeper

• Pace vs speed: the math runners mix up — why the two units cause so many wrong treadmill numbers.
• The Riegel formula and its honest limits — where the 1.06 exponent comes from and when not to trust it.
• Negative splits, and how to actually run them — why almost every record is even-or-negative and how to pace it.
• What is a good running pace? — honest numbers by level, not by Instagram.

Everything runs in your browser — your times are never transmitted, logged or stored.

Frequently asked questions

What is the difference between pace and speed?

Pace is time per distance — minutes per kilometer or per mile — while speed is distance per time — km/h or mph. They are the same measurement read in opposite directions: a 5:00/km pace is exactly 12 km/h. Runners prefer pace because a race plan reads naturally in it (hold 5:00/km for 42 km), while treadmills and bike computers report speed. The pace-vs-speed article has the conversion math.

How does the race prediction mode work?

It uses the Riegel formula: T2 = T1 × (D2/D1)1.06, published by engineer Peter Riegel in 1977. Your known result at distance A is scaled by the distance ratio raised to the 1.06 power — a fatigue factor that makes predicted pace decay about 4.3% per doubling of distance. The tool shows the prediction plus an honest accuracy window that widens as the two distances get further apart. Details and limits: the Riegel article.

Is a 5:00/km pace good?

It depends entirely on who is running and over what distance. For a club runner it is a solid 10K pace; for an elite marathoner it is a recovery jog. Rough population anchors: the median recreational 5K runner finishes near 30–34 minutes (6:00–6:50/km), while top age-group marathoners hold 4:00–4:40/km. What counts as a good pace breaks it down by level honestly.

How are the training zones computed?

Your race result is converted to a Riegel-equivalent 10K pace, which acts as the anchor — a rough proxy for threshold pace. Each zone is then a published-style percentage band of that anchor: easy 125–138%, long run 113–120%, marathon 104–110%, tempo 95–100%, interval 86–92%, repetition 78–84%. These are coaching heuristics, not lab measurements — treat the edges as soft.

Why does the finish-time table show a shorter last split?

Because real race distances are not round numbers in your pace unit. A half marathon is 21.0975 km, so after 21 full kilometers a final 97.5-meter split closes the distance — at 5:00/km that last piece takes about 29 seconds, not a full 5 minutes.

Do the numbers I enter get sent anywhere?

No. All five modes are plain JavaScript running in this tab — there is no server doing the math, nothing is logged, and the page keeps working if you go offline after it loads. The privacy policy has the details.