Speed Converter

Speed Limits, Road Signs, and Why Americans Still Can't Figure Out Kilometers

I'll never forget my first week driving in Canada. I'm cruising down the highway, speedometer reading 90, thinking I'm a responsible driver staying under the limit. Then I glance at a road sign—100. Wait, am I going too slow? Turns out Canadian speed limits are in kilometers per hour, not miles. That 100 on the sign meant roughly 62 mph, and I was actually doing about 56. Meanwhile, everyone's zooming past me like I'm standing still. That's when speed conversion became very real, very fast. Literally.

The Two Main Systems Everyone Actually Uses

Most of the world uses kilometers per hour. Makes sense—it lines up with the metric system everyone else adopted decades ago. The United States? Nope, we're sticking with miles per hour because apparently we enjoy being difficult. The UK does this weird hybrid thing where they use miles per hour for roads but meters for athletics. To convert between them, remember that 1 mile equals about 1.6 kilometers. So 60 mph is roughly 100 km/h—a nice round conversion that actually helps when you're driving and don't want to do math in your head. Going the other way, just multiply kilometers by 0.6 to get a rough mph figure. Not exact, but close enough when you're trying to figure out if that 130 km/h speed limit means you can floor it or not.

When Boats and Planes Decided They're Too Cool for Regular Speed Units

Then maritime and aviation folks had to go and create their own unit: the knot. One knot equals one nautical mile per hour, which is based on the actual size of the Earth's surface rather than some arbitrary measurement. A nautical mile is about 1.15 regular miles or 1.85 kilometers, making knots slightly faster than regular miles per hour. Ships and planes use knots because it makes navigation calculations easier when you're plotting courses across oceans or through the sky. Commercial jets cruise around 450-500 knots, which sounds way more impressive than saying 575 mph, even though they're the same thing. Pilots and sailors learn to think in knots so naturally that asking them to convert to miles or kilometers probably feels as weird as us trying to visualize speeds in parsecs per fortnight.

Going Supersonic and Other Ridiculous Speeds

When things get really fast, we start using Mach numbers. Mach 1 is the speed of sound—about 767 mph or 1,235 km/h at sea level. The actual speed changes based on temperature and altitude, but that's the baseline everyone uses. Fighter jets hit Mach 2 or Mach 3, meaning two or three times the speed of sound. The Concorde cruised at Mach 2.04, which is why you could fly from New York to London in under three hours. These days, only military aircraft regularly go supersonic because of the whole sonic boom thing that really annoys people on the ground. And don't even get me started on relativistic speeds—once you're talking about significant fractions of the speed of light, normal speed units become meaningless anyway. Spacecraft headed to Mars travel around 58,000 km/h, which sounds insanely fast until you realize that's barely 0.005% the speed of light.

Real Talk About Speeding Tickets and Speed Cameras

Here's where speed conversion gets expensive. Speed cameras don't care if you're confused about units—60 mph in a 50 km/h zone will get you a ticket every single time. In places where tourists commonly mix up speed limits, locals know the conversion cold. That 50 km/h residential limit? It's 31 mph, not 50 mph. I learned this the hard way when a friend visiting from the US racked up three speeding tickets in one day because he thought all those 50 signs meant he could do 50 mph. Cost him about $400 in fines and a whole lot of embarrassment. GPS systems usually let you toggle between units, which helps, but road signs don't come with conversion charts. When in doubt, watch what the locals do and match their speed.

Quick Mental Math for When Your Phone Dies

Want to convert mph to km/h in your head? Multiply by 1.6, or just add half the number and then add it again. So 60 mph: half is 30, add them together for 90, close enough to the actual 96.56 km/h. Going from km/h to mph, multiply by 0.6, or just take about 60% of the number. 100 km/h times 0.6 equals 60 mph—actual answer is 62.14, but 60 gets you in the ballpark. For everyday driving, being within 5% is plenty accurate. Nobody needs to know that 88 mph (the speed needed to time travel in Back to the Future) equals exactly 141.622 km/h. Just knowing it's roughly 140 km/h works fine. Unless you're actually trying to time travel, in which case you probably have bigger concerns than speed conversions.