Ask two runners what they did this morning and you’ll hear “5:20 pace, nice and easy” — not “11.25 kilometers per hour.” Both describe the same physics, but they answer different questions, and confusing them is behind a remarkable number of wrong treadmill settings and misread race plans.

The definitions, and why they’re inverses

Pace answers “how long for each unit of distance?” — minutes per kilometer (or per mile). Speed answers “how much distance per unit of time?” — kilometers or miles per hour. Because they are reciprocals of each other, converting is one division:

• speed (km/h) = 3600 ÷ pace (seconds per km)
• pace (s/km) = 3600 ÷ speed (km/h)

The mile versions use the same shape — mph = 3600 ÷ pace (s/mi) — and since a mile is exactly 1.609344 km, moving between the metric and imperial pairs is a second fixed conversion, never an estimate. That is why the calculator can promise exactness: a 50:00 10K is 5:00/km, 8:02.8/mile, 12 km/h and 7.456 mph, all at once, all exact.

A lookup table worth knowing cold

min/km	km/h	min/mi	mph	What it looks like
3:00	20.0	4:50	12.4	world-class marathon pace
4:00	15.0	6:26	9.3	fast club runner
5:00	12.0	8:03	7.5	solid recreational tempo
6:00	10.0	9:39	6.2	easy jog / median 5K pace
7:00	8.6	11:16	5.3	recovery or run-walk
8:00	7.5	12:53	4.7	brisk walk / shuffle

Notice the nonlinear feel: at the fast end, 20 seconds per km is a huge speed jump (3:00 → 3:20/km is 20 → 18 km/h); at the slow end it barely registers (7:00 → 7:20/km is 8.57 → 8.31 km/h). Pace compresses the differences that matter to runners into the seconds-per-km range where we can feel them; speed spreads them out. That compression is the real reason the sport standardized on pace — it puts race-relevant precision into units your watch already measures well.

Where each unit wins

Pace is the language of execution. Hold 5:00/km translates directly to lap times (2:00 per 400 m), split clocks and effort feel. Interval prescriptions — “6×1 km at 4:10” — are meaningless in km/h.

Speed is the language of machines and general audiences. Treadmills, bike computers and headlines use km/h or mph because those units describe motion for everyone, not just distance runners. It also wins for comparing across sports: 20 km/h is a fast run, a relaxed cycle and a slow drive, and only speed makes that sentence possible.

The classic mistakes

1. Reading 12.0 as “12 minutes.” A treadmill at 12 km/h is not a 12-minute pace — it’s 5:00/km. Decimal hours and sexagesimal pace don’t mix in your head; use the table.
2. Forgetting the mile gap. 8:00/mile is faster than 8:00/km — it is 4:58/km. The two pace systems look identical in writing but differ by 61%.
3. Trusting either number blindly. GPS pace under-reads in forests and tall cities; treadmill speed drifts with belt tension. When the two disagree by 1–3%, both can be “right” within their own error.

For the deep mechanics of racing on these numbers — predicting a finish from a pace or splitting a race plan into per-km targets — the calculator handles the arithmetic, and the Riegel article explains how pace at one distance predicts pace at another.

Frequently asked questions

Is 10 km/h a good running speed?

10 km/h is 6:00/km — a comfortable jog for a regular runner and a typical easy-run speed. It corresponds to a 30:00 5K and a 1:00:00 10K, which puts a recreational runner right around the middle of most race fields. Whether it's good depends on the distance you hold it for and who you compare against — see the honest breakdown.

Why do treadmills show km/h instead of min/km?

Because treadmills are general fitness machines, not runner-specific ones — km/h reads the same for walkers, joggers and runners, and it matches what cars and cyclists already know. Runners think in pace because race plans are written that way. The speed mode on the calculator converts treadmill numbers to running pace in both units.

How do I convert min/km to km/h in my head?

The exact rule is km/h = 60 ÷ pace-in-decimal-minutes, so 5:00/km → 60 ÷ 5 = 12 km/h and 6:00/km → 10 km/h. A quick approximation for typical running paces: subtract from 15 — 4:00/km ≈ 15−4 = ~15 km/h (exact 15), 6:00/km ≈ 10 (exact). It only works because the numbers happen to land on round values; for anything in between use the exact formula.

Does GPS pace equal treadmill speed?

Approximately, but both lie a little. GPS smooths your actual path — it under-reads on twisty routes and tree cover by up to a few percent. Treadmill belts drift with wear and your weight, so a displayed 12 km/h can be 11.7–12.3 in reality. Treat either as ±2% and they agree.