Solve for any one of the three variables given the other two. Supports all common speed and distance units — mph, km/h, m/s, knots, Mach, and more.
The three variables are linked by a single relationship: Speed = Distance ÷ Time. Rearranging gives you Distance = Speed × Time and Time = Distance ÷ Speed. This triangle relationship is one of the most fundamental in physics and everyday life.
Average speed is total distance divided by total time — not the average of your speeds. If you drive 100 km at 50 km/h and another 100 km at 100 km/h, your average speed is 200 km ÷ 3 hours = 66.7 km/h, not (50+100) ÷ 2 = 75 km/h. The slower leg takes proportionally more time and dominates the average.
Mach is the ratio of an object's speed to the local speed of sound. Mach 1 at sea level and 20°C is approximately 1,235 km/h (767 mph). It changes with altitude and temperature — at cruise altitude where commercial aircraft fly, the speed of sound is closer to 1,062 km/h, which is why aircraft quote airspeed in both km/h and Mach number.
A light-year is the distance light travels in one year: about 9.461 trillion kilometres. At a commercial aircraft cruise speed of 900 km/h it would take roughly 1.2 million years. At the speed of Voyager 1 (the fastest human-made object, about 61,500 km/h), it would still take around 17,500 years. Use the calculator above with light-years as your distance unit to explore this.