Speed Calculator

Average Speed Hides Your Stops

Your average speed on a road trip includes all the time you spent stopped at gas stations, traffic lights, and bathroom breaks. So if GPS says 5-hour drive but you take 6 hours, your average speed was lower than the posted speed limit. That's normal.

Use Consistent Units

If you're calculating speed in mph, your distance needs to be in miles and time in hours. Sounds obvious but people constantly try to mix kilometers with hours and wonder why the answer looks wrong. Pick your units and stick with them.

Speed vs Velocity (Physics 101)

Speed is just the number (60 mph). Velocity includes direction (60 mph northeast). For everyday use, most people call it 'speed' even when they technically mean 'velocity.' Only physicists care about the difference.

Mixing Units

Someone enters 50 kilometers for distance and 30 minutes for time, forgets to convert minutes to hours (0.5), and gets confused when the calculator says they were going 100 km/h instead of the real answer (which is also 100 km/h... bad example, but you get the point).

Why it happens: People think in different units (minutes feel more natural than decimal hours).
How to avoid: Convert everything first. 30 minutes = 0.5 hours. 15 minutes = 0.25 hours. Get the units straight before you calculate.

Confusing Average and Instantaneous Speed

This calculator gives you average speed. That's different from how fast you were going at any particular moment. You might have hit 80 mph on the highway, sat in traffic at 10 mph, but your average was 55 mph over the whole trip.

Real consequence: People use their max speed instead of average when estimating arrival times and show up late.
Fix: Use this calculator's average speed, not your peak speed, when planning trips.

Forgetting About Stops

GPS says "4 hours, 240 miles" so you think you'll average 60 mph. But you need gas, bathroom, food. In reality, you're driving for 4 hours at 70 mph, but the trip takes 5 hours total. Your true average speed? 48 mph.

How to avoid: Add 15-20% to your estimated time for long trips. Or calculate based on actual elapsed time (including stops), not driving time.

How do you calculate speed from distance and time?

Speed equals distance divided by time. If you drive 100 miles in 2 hours, your speed is 50 mph (100 ÷ 2 = 50). Same formula works for any units - km/h, m/s, whatever. Just make sure your units match (don't mix miles with meters).

What's the difference between speed and velocity?

Speed is how fast you're going (scalar). Velocity is speed with direction (vector). '60 mph' is speed. '60 mph north' is velocity. For most everyday calculations (like road trips), people use 'speed' and 'velocity' interchangeably, but physics nerds will correct you.

What is average speed vs instantaneous speed?

Average speed is total distance divided by total time (what this calculator gives you). Instantaneous speed is how fast you're going at one specific moment (what your car's speedometer shows). Your average speed on a 3-hour road trip might be 55 mph, even though you hit 75 mph at times.

How do you convert between mph and km/h?

Multiply mph by 1.609 to get km/h. Or divide km/h by 1.609 to get mph. Example: 60 mph = 96.5 km/h. Quick rough estimate: multiply mph by 1.6. Close enough for most purposes.

Can you calculate distance if you know speed and time?

Yes. Distance = Speed × Time. If you're driving at 60 mph for 3 hours, you'll cover 180 miles (60 × 3 = 180). This is just rearranging the speed formula to solve for distance instead.