Speed Calculator

Speed tells you how quickly something covers ground. It is a scalar quantity: it has a size but no direction. Velocity is the related vector quantity that includes direction, so a car circling a track at a steady 100 km/h has a constant speed but a constantly changing velocity. The SI unit of speed is the meter per second; road traffic uses kilometers or miles per hour, ships and aircraft use knots, which are nautical miles per hour.

Speed, distance, and time

Speed, distance and time are linked by a single relationship: speed equals distance divided by time. Knowing any two lets you find the third, which is what the calculator above does. Runners use it to work out pace, drivers to estimate arrival times, and pilots and sailors to plan fuel and routes.

The result is an average speed over the whole distance. If a runner covers 100 meters in 20 seconds, the average is 5 m/s, even though they were slower out of the blocks and faster in the middle. Instantaneous speed, what a speedometer shows, is the speed over a vanishingly short interval.

Average speeds over trips with different legs are a common trap. Driving 60 miles at 30 mph and then 60 miles at 60 mph does not average 45 mph; it takes 2 hours plus 1 hour to cover 120 miles, so the average is 40 mph. Always divide the total distance by the total time.

Unit conversions often cause mistakes too. To turn km/h into m/s divide by 3.6; to turn mph into km/h multiply by 1.609344; one knot is exactly 1.852 km/h. The table puts some familiar speeds side by side.

speed = distance ÷ time · distance = speed × time · time = distance ÷ speed

Use consistent units: meters with seconds give m/s, miles with hours give mph.

When solving for time, the calculator shows the answer in hours, minutes and seconds as well as decimal units. A trip of 250 miles at 55 mph takes 250 ÷ 55 = 4.545 hours, which is 4 hours 32 minutes 44 seconds; converting the decimal part (0.545 h × 60) to minutes is the step most often forgotten.

Solving for distance works the same way in reverse. A cyclist holding 18 mph for 2 hours 30 minutes covers 18 × 2.5 = 45 miles. Light years appear in the distance list for fun: even at the speed of light, the trip to the nearest star beyond the Sun, Proxima Centauri, about 4.24 light years away, would take 4.24 years.

Frequently asked questions