Speed Converter

What this converter answers

How fast a speed is in another unit, and whether that answer is exact or rounded. Type one speed and you see it in kilometers per hour, meters per second, miles per hour, feet per second and knots together. Next to the answer you also get the figure to 2 decimal places, a mental shortcut with its exact error, the Beaufort wind force the speed would be, the time it takes to cover 400 m, 1 km and 1 mile, and a ladder of round speeds around yours.

How to use it

• Speed: the number to convert, in the unit chosen under From. It is a size without a direction, so it is 0 or more. You can type 1,250 with a thousands comma, or 3e8 for 300,000,000.
• From and To: the unit on your sign, spec sheet or forecast, and the one you need. kph and km/h are the same unit; fps is feet per second; knots are nautical miles per hour.
• Significant figures: Auto shows up to 12, so exact factors such as 1.609344 appear in full. Pick 3 to 12 for a shorter answer; the 2-decimal tile stays as it is.

Swap, under the answer, keeps your number and trades the units, so 100 km/h in mph becomes 100 mph in km/h. Pick a unit in the table to see your speed in that unit as the headline instead. The Try buttons load a 65 mph speed limit in km/h, a boat speed, a treadmill setting, a wind reading and the 88 ft/s physics fact.

How to convert mph to km/h (and km/h to mph)

Multiply miles per hour by 1.609344 to get kilometers per hour, and divide kilometers per hour by 1.609344 to get miles per hour. The factor is exact because a mile is defined as exactly 1,609.344 m and both units count per hour, so the hours cancel.

For 65 mph: 65 × 1.609344 = 104.60736 km/h exactly, or 104.61 km/h to 2 decimal places. Going the other way almost never gives a short decimal, because 1 km/h is 0.621371192237… mph and the digits don’t end. That is why each value on the page carries an Exact or Rounded label.

In your head, × 1.6 and ÷ 1.6 are about 0.58% off: 65 × 1.6 = 104 km/h, 0.61 km/h short of the exact figure.

Worked example: 100 km/h in mph

1. Kilometers per hour to miles per hour: 100 ÷ 1.609344 = 62.1371192237 mph. The division doesn’t end, so this is rounded to 12 significant figures.
2. The same result through meters per second: 100 ÷ 3.6 ≈ 27.7777777778 m/s, and 27.7777777778 ÷ 0.44704 ≈ 62.1371192237 mph. The m/s figure is rounded (the 7s never end), so the steps write ≈; divide the rounded figure yourself and the last digit comes out 8, while the exact speed still gives 7.
3. To 2 decimal places: 62.14 mph.
4. In your head: 100 ÷ 1.6 = 62.5 mph, which is 0.363 mph (0.58%) more than the exact value.
5. Time to cover 1 km: 1,000 m ÷ 27.7777777778 m/s ≈ 36 s, exactly 36 s with the unrounded speed (1,000 × 3.6 ÷ 100). For 1 mile: 1,609.344 m ÷ 27.7777777778 m/s ≈ 57.936384 s, shown as 57.9 s.
6. As a wind: 100 km/h is 53.9956803456 knots, which rounds to 54 kn, so it would be force 10 (storm) on the Beaufort scale.

The conversion table under the result gives the same speed as 91.1344415281 ft/s.

How to convert m/s to km/h and mph

Multiply meters per second by 3.6 to get kilometers per hour, exactly: an hour has 3,600 seconds and a kilometer 1,000 meters, and 3,600 ÷ 1,000 = 3.6. To get miles per hour, divide by 0.44704, which is the same as multiplying by about 2.23694.

A wind of 20 m/s is 20 × 3.6 = 72 km/h, 20 ÷ 0.44704 = 44.74 mph and 38.88 knots, which is a gale (force 8). The Try button 20 m/s wind shows every step.

Knots: speed at sea and in the air

A knot is one nautical mile per hour. The international nautical mile is exactly 1,852 m, so 1 knot is exactly 1.852 km/h and about 1.15078 mph. Nautical charts and flight navigation work in latitude and longitude, and a nautical mile is one minute of latitude, which is why ships and aircraft keep using knots.

A boat at 10 knots moves at 18.52 km/h, or 11.51 mph. Because a nautical mile is about 15% longer than a statute mile, knots and miles per hour can’t be swapped one for one.

Exact conversion factors

Every unit here is a fixed number of meters per second, $f$, so a speed $v$ converts as

where $f$ is 1 for m/s, 1,000/3,600 for km/h, 0.44704 for mph, 1,852/3,600 for knots and 0.3048 for ft/s. One unit in each of the others, with the exact factors in bold and the rest rounded to 6 significant figures:

The rounded ones are still exact fractions, just not short decimals: 1 km/h is 5/18 m/s and 1 mph is 22/15 ft/s. That is why 88 ft/s comes out as exactly 60 mph while 1 km/h never ends in mph.

Common speed limits in mph and km/h

Speed-limit signs use round numbers in one unit, so a limit from one country rarely lands on a round number in the other unit. Here are round values converted to 2 decimal places:

For other values, enter the limit and read the Round speeds near table under the result, which steps through nearby round numbers in your unit.

Why Mach isn’t a fixed speed

A Mach number is a speed divided by the speed of sound in the air around it, and that speed of sound depends on the air’s temperature. NASA gives it as the square root of γRT, where T is the absolute temperature, and puts it at about 761 mph (about 1,225 km/h or 340 m/s) at sea level on a standard day. In colder air sound travels slower, so there Mach 1 is fewer miles per hour. Because Mach 1 has no single value in mph or km/h, this converter doesn’t list Mach as a unit.

Reading the result

• The headline is the speed in the To unit. The line under it repeats your speed and marks the result exact (65 mph = 104.60736 km/h) or rounded to 12 significant figures (100 km/h = 62.1371192237 mph). Next comes 1 unit in the other, both ways.
• The tiles show the result to 2 decimal places, a mental shortcut for this pair of units with its error at your speed, and the Beaufort force the speed would be as a wind.
• How this was calculated shows the factor used, the same conversion through meters per second, the time for 1 km and 1 mile, and the Beaufort rounding.
• The conversion table gives your speed in km/h, m/s, mph, ft/s and knots, with an Exact or Rounded label on each row. Download CSV and Copy table export those five rows.
• Time to cover a distance gives the time for 400 m, 1 km and 1 mile at a steady speed, which is what runners call pace. Under a minute it is in seconds; from a minute up it is minutes and seconds. At a speed of 0 it says Never instead of an error.
• Round speeds near your value lists eleven round speeds in your unit with their conversions, and marks your own.
• Save for comparison holds up to three speeds, such as a limit at home and two abroad, and shows how much faster or slower each is than the first, in meters per second.

Common mistakes

• Using 1.6 instead of 1.609344 at highway speeds. 120 km/h ÷ 1.6 = 75 mph, but the exact figure is 74.56 mph. Fine for a glance at the speedometer, not for a report.
• Reading knots as miles per hour. 20 knots is 23.02 mph, so taking it as 20 mph understates the speed by 13%.
• Dividing by 3.6 when you should multiply. Meters per second to kilometers per hour is × 3.6: 20 m/s is 72 km/h. Dividing gives 5.56, which is far too slow.
• Averaging two speeds over equal distances. Driving one leg at 60 km/h and an equally long leg at 40 km/h averages 48 km/h, not 50, because the slow leg takes longer. Divide the total distance by the total time instead.
• Converting back from a rounded result. 62.14 mph × 1.609344 = 100.00463616 km/h, not 100. Convert from the number you started with.
• Typing minutes per kilometer as a speed. 6 minutes per kilometer is 60 ÷ 6 = 10 km/h, not 6 km/h.

Assumptions and limitations

• The factors are exact definitions (the mile, foot and international nautical mile in meters, and 3,600 seconds in an hour). Only the figures on screen are rounded, as the Significant figures setting says.
• The Speed field takes 0 up to 10¹⁵ in the From unit. Results of 10¹⁶ or more, or below 0.000001, switch to scientific notation. Negative velocities are not accepted: convert the size and keep the direction yourself.
• A converted speed is no more precise than the reading it came from. A speedometer showing 100 km/h is not accurate to the 12 figures of 62.1371192237 mph.
• Times assume the same speed the whole way. The Beaufort force is approximate: it applies the table’s knot ranges to the speed rounded to the nearest whole knot, and it describes wind, not a vehicle.
• Knots are international knots. Mach numbers, light speed and less common units such as feet per minute are not included.