Understanding Speed Units

Speed is distance divided by time. The SI unit is metres per second (m/s), but for everyday life, kilometres per hour (km/h) and miles per hour (mph) dominate. Two conversions worth memorising: to convert km/h to mph, multiply by 0.6214 (or roughly divide by 1.6). To convert m/s to km/h, multiply by 3.6 — because there are 3,600 seconds in an hour and 1,000 metres in a kilometre.

Speed Benchmarks

Human walking pace: ~5 km/h (3 mph). Usain Bolt's world record sprint reached 44.72 km/h (27.8 mph). Highway driving typically sits at 100–130 km/h (62–81 mph). A commercial aircraft cruises at around 900 km/h (560 mph). The speed of sound at sea level (Mach 1) is 1,235 km/h (767 mph) — though this varies with temperature and altitude. The speed of light is 299,792 km/s, fast enough to circle the Earth more than seven times in a single second.

Knots — and Why They Still Exist

A knot is one nautical mile per hour, equal to 1.852 km/h or about 1.151 mph. It's used in aviation and maritime navigation because nautical miles are tied to Earth's geometry: one nautical mile equals one minute of arc of latitude. This makes chart calculations and position fixes much simpler. A ship travelling at 20 knots is covering about 37 km/h — not fast by road standards, but the unit choice makes the navigation maths cleaner.

Terminal Velocity and Record Freefalls

A human in a belly-to-earth skydiving position reaches terminal velocity at roughly 195 km/h (120 mph), where air resistance balances gravitational acceleration. Head-down dives can push this past 300 km/h. In 2012, Felix Baumgartner jumped from a helium balloon at 39 km altitude and reached 1,357 km/h (Mach 1.25) during freefall — breaking the sound barrier without an aircraft, a feat made possible by the thin air at that altitude offering far less drag.