Pace and speed are two ways of describing exactly the same run. Pace says how much time each unit of distance takes, like 5:00 per kilometer. Speed says how much distance each unit of time covers, like 12 km/h. They are inverses of each other, which is why a smaller pace number and a bigger speed number both mean running faster.
The exact relationship
If pace is p seconds per kilometer, speed in km/h is 3600 divided by p. Going the other way, speed s in km/h corresponds to a pace of 3600 / s seconds per kilometer. For miles, divide the km/h figure by 1.609344 to get mph, or multiply a per-kilometer pace by 1.609344 to get a per-mile pace.
The 10 km in 50:00 example makes this concrete: pace is exactly 5:00 per km, which is 8:03 per mile after conversion, and speed is 12.00 km/h or 7.46 mph. The pace calculator shows all four values at once so you never have to convert by hand mid-conversation.
Where each unit shows up
Pace dominates in running because race logistics are time-based: pace bands, track laps, and split tables all answer "when do I reach this point?" Speed dominates on treadmills, which are usually labeled in km/h or mph, and in cycling, where power-to-speed thinking carried over. If you train indoors and race outdoors you will inevitably translate between the two.
A useful trick for quick mental math: at 12 km/h you cover one kilometer every 5 minutes, so km/h times 5 gives a rough seconds-per-km pace for speeds near that range. It is a shortcut, not a conversion — use the calculator when the number matters.
Why rounding matters
Pace displays usually round to the whole second, and speed displays to one or two decimals. Over a marathon, a second per kilometer is roughly 42 seconds of finish time, so tiny display differences can look meaningful when they are not. RunPaceKit always calculates with exact values and rounds only for display, which is why its split tables keep summing exactly to your target time.
What this changes for your planning
Nothing about your fitness changes when the unit changes — the run is the run. But mixing units carelessly can corrupt a plan: entering a treadmill mph value into a pace field, for example, produces a plan that is off by a large factor. Pick one unit per plan, convert once at the start, and let the tools do the arithmetic.