Pace vs Speed: Reading min/km, min/mi, and mph Together
I’m a Korean medicine doctor in Vancouver by day, a software engineer the rest of the time, and a runner in whatever hours are left over — Ryutility is a small side project I build under R-Lab, part of Ryu Clinic Inc. A patient once told me they’d “sped up” from 10 min/mi to 5:30/km and were confused why their watch didn’t seem to agree. It wasn’t their watch — 10 min/mi and 5:30/km are almost the same pace. They hadn’t sped up at all.
Pace and speed carry the same information — how fast you’re covering ground — but they’re built for different jobs, and converting between them by memory is where most of these mix-ups happen.
What each one actually measures
Pace is time per unit distance: minutes and seconds per kilometer or mile. It’s the unit runners plan by, because a training plan says “run 6:00/km for 40 minutes,” not “run at 10 km/h for 40 minutes.” Pace also matches how splits work — you can read a pace straight off a split table and know exactly what each kilometer should feel like.
Speed is distance per unit time: kilometers or miles per hour. It’s the unit treadmills default to, because a motor’s belt runs at a fixed rate — km/h or mph is what the machine controls directly. Pace, on that same treadmill, is a number the console has to compute for you (if it bothers to show it at all).
They’re reciprocals of each other, scaled for units:
Speed (km/h) = 60 / pace (min/km)
Pace (min/km) = 60 / speed (km/h)
A worked example
Say your watch shows a pace of 5:30 per km. Convert the pace to decimal minutes first: 5:30 = 5.5 minutes.
Speed = 60 / 5.5 ≈ 10.9 km/h
Convert that to mph by multiplying by 0.621371 (1 km = 0.621371 mi):
10.9 × 0.621371 ≈ 6.8 mph
And in miles instead of km, 5:30/km becomes roughly 8:51/mi — divide the km pace by 0.621371, since a mile is the longer unit and takes proportionally longer to cover: 5.5 / 0.621371 ≈ 8.85 minutes, i.e. 8:51/mi.
So the same effort reads as any of: 5:30/km, 8:51/mi, 10.9 km/h, or 6.8 mph — four different-looking numbers describing one identical running speed.
Where the confusion actually comes from
Rounding compounds across conversions. A treadmill display showing “6.8 mph” has already been rounded from a more precise speed. Convert that rounded 6.8 back to pace and you’ll land on 8:49/mi rather than the original 8:51/mi — a 2-second discrepancy that’s meaningless per rep, but adds up if you’re back-calculating a full workout’s total time from a treadmill’s speed display.
km and mile paces don’t move in a straight line relative to each other. People assume a “10 second” pace change means the same thing in both units, but a 10 sec/km change is only about a 16 sec/mi change, not 10. If you’re used to thinking in miles and a plan is written in km (or vice versa), doing the adjustment in your head mid-run is where a lot of GPS-watch-driven pace errors come from — better to convert the whole session’s target paces before you start, not mid-stride.
“Speeding up” isn’t linear in either unit near easy paces. Going from a 12:00/mi jog to an 11:00/mi jog is a smaller absolute change in km/h than going from a 7:00/mi tempo to a 6:00/mi tempo, even though both are “one minute per mile faster.” If you’re using speed (km/h or mph) to judge how big a pace change feels, the same mph jump feels very different depending on which end of the range you’re starting from — because effort scales with speed roughly quadratically, not with pace linearly.
Using it in practice
If your training plan is written in pace but your treadmill only shows speed (or the reverse), don’t convert by hand mid-workout — that’s exactly where the rounding and unit-direction mistakes above creep in. Use the Running Pace Calculator to get an exact pace, time, or distance from any two known values, in both km and miles, and cross-check it against the training zones from the Training Zone Calculator so you know which pace band you’re actually supposed to be running in before you start the watch.
This guide is part of R-Lab, the side-project arm of Ryu Clinic Inc. — I write these between clinic hours and code commits, from what I’ve seen work (and not work) in 14 years of treating runners and a lot longer than that of being one.