Enter any two of speed, distance and time. It returns the third in several units, and converts speed into running pace.
Speed is distance divided by time, and the three quantities rearrange freely: v = d / t, d = v × t, t = d / v. Knowing any two fixes the third, which is what this calculator does in whichever direction you need.
The only real trap is unit consistency. Kilometres divided by hours gives km/h; metres divided by seconds gives m/s. Mixing them silently produces a number that looks reasonable and is wrong by a factor of 3.6, which is the conversion between the two. This calculator normalises internally and reports several units at once, so the comparison is always visible.
A marathon is 42.195 km. Covering it in exactly 4 hours means 42.195 ÷ 4 = 10.549 km/h, or 6.555 mph, or 2.93 m/s. As pace — which is how runners actually think — that is 5:41 per km or about 9:09 per mile.
Pace is the inverse of speed, and inverting it is why a small speed change feels larger than it looks. Going from 10 km/h to 11 km/h sounds like a 10% improvement; in pace terms it moves 6:00/km to 5:27/km, which over a marathon is more than twenty minutes.
This catches people constantly. Drive 60 km at 60 km/h, then 60 km at 30 km/h. The average of 60 and 30 is 45, but the actual average speed is not 45 km/h. The first leg takes one hour and the second takes two, so you covered 120 km in 3 hours — an average of 40 km/h.
Average speed is always total distance divided by total time. When equal distances are covered at different speeds, the correct average is the harmonic mean, which is always lower than the arithmetic one. The slow section dominates because you spend longer in it.
| km/h | mph | m/s | |
|---|---|---|---|
| Walking | 5 | 3.1 | 1.4 |
| Recreational running | 10 | 6.2 | 2.8 |
| Usain Bolt, 100 m peak | 44.7 | 27.8 | 12.4 |
| Urban speed limit (typical) | 50 | 31 | 13.9 |
| Motorway cruising | 110 | 68 | 30.6 |
| Airliner cruise | 900 | 560 | 250 |
| Speed of sound (sea level) | 1235 | 767 | 343 |
A knot is one nautical mile per hour, and a nautical mile is 1852 metres — defined as one minute of latitude along a meridian. That definition is the reason it survives in marine and aviation use: it makes chart navigation arithmetic straightforward, because one minute of latitude on the chart is one nautical mile of distance regardless of where you are. A knot is about 1.852 km/h or 1.151 mph.
Kinetic energy rises with the square of speed, so a car at 60 km/h carries four times the energy it does at 30, not twice. Braking distance follows the same square relationship. This is the single most useful thing physics has to say about road safety, and it is why a modest increase in speed produces a disproportionate increase in both stopping distance and collision severity.
Divide by 3.6. A kilometre is 1000 metres and an hour is 3600 seconds, so 1 km/h is 1000/3600 = 0.2778 m/s. Going the other way, multiply m/s by 3.6. It is worth memorising because the two units dominate different contexts — road speeds are quoted in km/h or mph, while physics problems and sports science almost always use m/s.
Because you spend more time at the slower speed, so it carries more weight. Drive 60 km at 60 km/h then 60 km at 30 km/h: that is one hour plus two hours for 120 km, giving 40 km/h, not the 45 you would get by averaging the two numbers. Average speed is always total distance divided by total time. For equal distances at different speeds the correct figure is the harmonic mean, which is always the lower of the two.
Speed is a scalar — just a magnitude. Velocity is a vector, carrying both magnitude and direction. A car going round a roundabout at a constant 30 km/h has constant speed but continuously changing velocity, because its direction keeps changing. That change is an acceleration, which is why you feel pushed sideways. For everyday distance-and-time questions the distinction rarely matters; in physics it always does.
One knot is one nautical mile per hour, where a nautical mile is 1852 metres — one minute of latitude along a meridian. That definition is exactly why it persists at sea and in the air: on a chart, one minute of latitude equals one nautical mile of distance anywhere on the globe, which makes navigation arithmetic simple. A knot is about 1.852 km/h or 1.151 mph.
Pace is the inverse of speed — time per unit distance rather than distance per unit time. Divide 60 by your speed in km/h to get minutes per kilometre: 10 km/h gives 6.0, which is 6:00 per km. The calculator does this conversion automatically. Runners prefer pace because it maps directly onto what a watch shows during a run, and because it makes finishing times easy to project.
Related: Pace Calculator · Time Calculator · Fuel Cost Calculator · Unit Converter Vehicle-specific tools: the Gas Mileage Calculator, the Horsepower Calculator, and the Tire Size Calculator — which also shows the speedometer error a size change causes.