Speed Converter for mph, km/h, m/s, ft/s, Knots and Mach

Six units for one speed, and where each one comes from

A speed converter looks like a small tool and carries a pile of history. Miles per hour belongs to road signs in the United States and the United Kingdom. Kilometers per hour belongs to most of the rest of the world. Meters per second is the SI unit, so it is what physics and engineering use. Feet per second shows up in airflow, ballistics and building design. Knots belong to ships, aircraft and weather reports, and Mach belongs to anything that flies near or past the speed of sound.

All six describe the same physical thing, distance covered per unit of time, and each one exists because a group of people needed a number they could read at a glance. This page converts between them and stops there. It does not solve for distance or for time, which is a different calculation, and it has nothing to do with internet throughput.

The practical difference is mostly a rounding habit. An American driver reads a speedometer in miles per hour, a Canadian sign is in km/h, and a marine forecast is in knots, so the same 25 mph wind is 40 km/h on one map and 22 knots on the next.

The exact factors behind every conversion

The NIST Special Publication 811 lists the factors that every reference work repeats. The mile is exactly 1,609.344 meters, so 1 mile per hour is 1609.344 / 3600 = 0.44704 m/s, a number with five decimals and no rounding inside it. The foot is exactly 0.3048 meters, so 1 ft/s is 0.3048 m/s. The nautical mile is exactly 1,852 meters, so 1 knot is 1852 / 3600 = 0.514444 m/s.

Because the definitions are exact, a conversion is only approximate when you round it. Do the arithmetic first and round at the end, and the error stays below the last digit you print. That is the whole reason 0.44704 and 0.45 are not interchangeable once you are converting long distances.

One more identity is worth memorizing: 1 mph is exactly 1.609344 km/h, and 1 knot is exactly 1.852 km/h. Those two come from the same three definitions and cover most of the questions people ask.

Miles per hour, kilometers per hour and the sign on the road

Almost every speed question in the United States starts with a posted limit. A 30 mph city street is 48.28 km/h, a 70 mph rural interstate is 112.65 km/h, and the highest limit posted anywhere in the country is 85 mph on a few Texas toll roads, which is 136.79 km/h.

The conversion in the other direction is the one travelers need. Canada's standard rural limit of 100 km/h is 100 / 1.609344 = 62.14 mph, so a Canadian 100 sign is a little slower than a US 65 mph sign, which is 104.61 km/h. Germany has no general limit on the Autobahn, but the recommended speed of 130 km/h is 80.78 mph.

Limits are usually written in multiples of 5 or 10 on purpose, so a driver can do the conversion in their head. The rough rule for km/h to mph is to take 60 percent and add a little: 100 km/h is 62 mph, not 60, and the 2 mph gap is the difference between a warning and a ticket in a strict enforcement zone.

Knots: the unit ships, planes and weather all share

A knot is one nautical mile per hour, and a nautical mile is 1,852 meters, which is one minute of latitude on a chart. That is why sailors and pilots ended up with the same unit: both navigate by position on a globe, and a minute of latitude is the same distance everywhere on it. The word comes from the chip log, a line with knots tied in it that was trailed behind a ship and timed against a sandglass.

The identity to remember is 1 knot = 1.15078 mph, which is also 1.852 km/h. A boat making 20 knots is doing 23.02 mph, since 20 x 1.15078 = 23.0156, and a jetliner cruising at 450 knots is doing 517.85 mph. Wind forecasts use knots for the same reason ship logs do, so a gale warning means the same thing to a freighter and to a sailboat.

That shared unit is why marine weather, aviation weather and hurricane advisories all quote knots while the evening news converts to mph. The numbers are not interchangeable in one step, but the factor is short enough to keep in your head: add 15 percent to a knot figure and you have mph within a rounding error.

Mach is a ratio, not a unit

Mach 1 means the speed of sound in the air around the aircraft, so the same Mach number is a different speed at a different altitude and temperature. The NASA Glenn Research Center puts the sea level standard at about 761 mph, which is 340.3 m/s with the air at 15 degrees Celsius.

An example shows why the altitude matters. At the sea level standard, Mach 0.8 is 0.8 x 340.3 = 272.24 m/s, which is 272.24 / 0.44704 = 608.98 mph. At 11,000 m the speed of sound drops to about 295 m/s, so the same Mach 0.8 comes out near 236 m/s, or about 528 mph.

This converter uses 340.3 m/s for Mach, the sea level value, and says so on the result panel. For a flight at altitude, convert to knots or mph first and treat the Mach number as a label rather than a fixed speed.

Speedometers, tire sizes and the small print

A speedometer does not measure speed. It counts how fast the wheels turn and multiplies by the tire circumference the factory assumed. Fit a larger tire and the wheel turns fewer times for each real mile, so the needle reads low. With a stock 26.5 inch tire and a 27.5 inch replacement the circumference grows 3.8 percent, and a needle showing 60 mph means a true speed of 60 x 27.5 / 26.5 = 62.26 mph.

That is why the same model is sold with different wheel sizes per trim, and why a car on 20 inch wheels can read lower than the same car on 17 inch wheels even though both are within the factory spec. The error runs in one direction: bigger tires make the speedometer optimistic about distance and pessimistic about speed.

Police tolerance is usually written as a percentage, so a 3.8 percent tire error can matter on a road where enforcement is strict. A GPS reading does not have the problem, because it measures motion against satellites rather than against wheel rotation. The NIST SI unit tables give the exact inch and foot factors if you want to redo the arithmetic for your own tire size.

Reading the wind on the Beaufort scale

The Beaufort scale, published by the National Weather Service, grades wind by what it does to trees and water rather than by an instrument reading. Mariners still use the numbers, and they are the thresholds behind small craft, gale and storm warnings.

Converting a forecast is a good check on whether a number makes sense. 25 knots is 28.77 mph, which sits in the strong breeze band, and a small craft advisory begins in the low 20s of knots, with the exact threshold set by region. A hurricane force wind of 64 knots is 73.65 mph, and that is where the damage categories begin.

Beaufort numbers are also how a ship log can be checked against a forecast: the log records the sea state, the forecast gives a speed, and the two should land on the same line of the table. When they do not, the forecast is usually right and the observer is estimating from a sheltered deck.

What this converter does and does not do

This page converts a speed from one unit to another, shows the same speed in all six units at once, and adds the time needed to cover a distance you choose. It does not solve for distance traveled in a given time or for the speed needed to arrive on schedule, which is a different problem with a different formula, and it does not measure internet throughput.

The distance line is a convenience, not a second calculator. Enter 65 mph and 300 miles and the panel reports 4.6154 hours, because 300 / 65 = 4.62 when rounded, which is 4 hours and 37 minutes. Change the speed to 55 mph and the same trip takes about 5.45 hours.

Two habits keep the numbers honest. Convert once and then work in the unit you need, rather than converting back and forth and stacking rounding errors. And keep the factor to the last digit, since 0.44704 and 0.45 differ by more than half a percent, which is more than a mile in every 200 miles.

For the distance and time version of the same question, the speed, distance and time calculator is the page to open. For boats and aircraft, knots to mph goes deeper on the nautical mile.