Speed Converter

What this tool is used for

• Converting a vehicle speed limit between imperial and metric units for a trip.
• Reading a wind speed given in knots into km/h or m/s.
• Converting a metric lab figure in m/s into something intuitive.
• Comparing performance figures quoted in different units.
• Converting a pace or a speed for equipment calibrated in another unit.

Frequently Asked Questions

What are the common conversion factors?

1 m/s = 3.6 km/h exactly (3600 seconds per hour ÷ 1000 metres per km). 1 mph = 1.609344 km/h, and 1 knot = 1.852 km/h exactly, since a nautical mile is defined as 1852 m.

Why is a knot not a statute mile per hour?

Because it is one NAUTICAL mile per hour, and a nautical mile is one minute of latitude — 1852 m against the statute mile's 1609.344 m. Converting knots with the statute factor understates speed by about 15%, which matters at sea and in the air.

How do I convert m/s to km/h in my head?

Multiply by 3.6, or double and add 80% for a quick estimate. 10 m/s is 36 km/h — a useful anchor, since it is roughly the top speed of a fast sprinter.

What is Mach and why does it vary?

A ratio to the local speed of sound, not a fixed speed. Sound travels at about 343 m/s at 20°C at sea level but only ~295 m/s in the stratosphere, so Mach 1 is a different absolute speed depending on altitude and temperature.

Are speed limits converted exactly?

Never. 70 mph is 112.65 km/h but is signed as 110 or 120, because limits must be round numbers. Converting a posted limit gives the equivalent speed, not the limit that would actually be posted.

Why is a knot defined the way it is?

One nautical mile per hour, and a nautical mile is one minute of latitude — so a speed in knots relates directly to position on a chart. That is why marine and aviation navigation kept it.

How do pace and speed relate?

They are reciprocals with a unit change: min/km is 60 divided by km/h. That is why halving your pace doubles your speed, and why the two cannot be averaged the same way.

Common errors and gotchas

• Confusing knots with km/h, a difference of about 85% that appears constantly in marine and aviation data.
• Converting a speed with a distance factor only, forgetting the time unit is part of the quantity.
• Mixing pace and speed, which are reciprocals rather than the same quantity in different units.
• Rounding a small speed heavily, where the absolute error is proportionally large.
• Assuming a knot is a distance. It is a speed, so 'knots per hour' is a unit error.