How to Convert km/h to m/s (and m/s to km/h): Divide by 3.6

The short answer

To convert km/h to m/s, divide by 3.6. To go the other way, multiply m/s by 3.6. In fraction form, multiply by 5/18 to get m/s and by 18/5 to get km/h. So 72 km/h is 72 ÷ 3.6 = 20 m/s, and 25 m/s is 25 × 3.6 = 90 km/h. The free Speed Converter does this instantly and also shows mph, knots, feet per second and Mach.

Where 3.6 comes from

Speed is distance divided by time, so you convert both parts:

• 1 kilometre = 1,000 metres
• 1 hour = 60 × 60 = 3,600 seconds

So 1 km/h = 1,000 m ÷ 3,600 s = 0.2778 m/s, which is 1/3.6 m/s. Dividing by 3.6 and multiplying by 1,000/3,600 are the same thing, and 1,000/3,600 simplifies to 5/18.

Knowing the reason helps you avoid the most common error: multiplying when you should divide. A speed in m/s is always the *smaller* number, because a metre is a much smaller distance than a kilometre.

Quick conversion table

These values come from the Speed Converter.

km/h	m/s
5	1.38888889
18	5
36	10
54	15
72	20
90	25
100	27.77777778
120	33.33333333

Notice that multiples of 18 give whole numbers in m/s. Exam questions often use 18, 36, 54, 72 or 90 km/h for exactly that reason.

Worked examples

Example 1: A car travels at 54 km/h. What is its speed in m/s?

54 × 5/18 = 270/18 = 15 m/s.

Example 2: A sprinter runs at 10 m/s. What is that in km/h?

10 × 18/5 = 180/5 = 36 km/h.

Example 3: A train at 90 km/h crosses a 250 m bridge. How long does the front of the train take to cross it?

First convert: 90 ÷ 3.6 = 25 m/s. Then time = distance ÷ speed = 250 ÷ 25 = 10 seconds. (If the question includes the train's length, add it to the bridge length first.)

Example 4: A cyclist covers 9 km in 30 minutes. What is the speed in m/s?

Speed = 9 km ÷ 0.5 h = 18 km/h. Then 18 ÷ 3.6 = 5 m/s.

How to use the Speed Converter

Enter the speed in Value to convert.

Choose the unit it is in from the From list. It starts on km/h.

Choose the target unit. It starts on miles per hour; change it to metres/second for this conversion.

Read the answer in the green box, or look at the All Speed conversions table to see every unit at once.

Click any row in the table to copy that value, or use ⇄ to swap the units.

The converter uses km/h as its base and these factors:

Unit	Equal to
1 metre/second	3.6 km/h
1 mile/hour	1.60934 km/h
1 knot	1.852 km/h
1 foot/second	1.09728 km/h
1 Mach (sea level)	1,234.8 km/h

Other speed conversions people need

Conversion	Rule	Example (converter result)
km/h to mph	divide by 1.609	100 km/h = 62.14 mph
mph to km/h	multiply by 1.609	60 mph = 96.56 km/h
mph to m/s	multiply by 0.447	1 mph = 0.447 m/s
knots to km/h	multiply by 1.852	20 knots = 37.04 km/h
km/h to knots	divide by 1.852	100 km/h = 54.0 knots
km/h to ft/s	multiply by 0.911	100 km/h = 91.13 ft/s

The mile is defined as exactly 1.609344 km. The converter uses 1.60934, so results in mph can differ from an exact calculation in the fourth or fifth significant figure. For everyday use and exams that is not noticeable.

About Mach: Mach 1 is the speed of sound, which depends on air temperature. The converter uses 343 m/s (1,234.8 km/h), a common value for sea-level air at about room temperature, so treat Mach results as approximate.

Doing it in your head

You rarely need a calculator for these:

• km/h to m/s: divide by 4, then add about 10% of the result. 100 km/h ÷ 4 = 25, plus 2.5 = 27.5 m/s (exact: 27.78).
• m/s to km/h: multiply by 4, then subtract 10%. 20 m/s × 4 = 80, minus 8 = 72 km/h (exact).
• Multiples of 18: every 18 km/h is exactly 5 m/s, so 36 km/h is 10 m/s and 108 km/h is 30 m/s.

The divide-by-4 shortcut works because 3.6 is close to 4; the 10% adjustment corrects most of the gap.

Which unit to use when

• Physics and engineering use metres per second, the SI unit. Convert to m/s before using formulas such as distance = speed × time or kinetic energy = ½mv².
• Road speeds and speed limits are given in km/h in India and most countries, and in mph in the United States and United Kingdom.
• Ships and aircraft use knots. One knot is one nautical mile (1.852 km) per hour.
• Wind speeds appear in km/h, m/s or knots depending on the source, so check the unit before comparing forecasts.

Common mistakes

• Multiplying instead of dividing. If your m/s answer is bigger than the km/h number, you went the wrong way.
• Converting only distance or only time. Changing km to m without changing hours to seconds gives an answer 3,600 times too big.
• Using 60 instead of 3,600. An hour has 3,600 seconds, not 60.
• Rounding too early. In multi-step problems, keep fractions such as 5/18 until the final step.
• Mixing units in one formula. If speed is in m/s, distance must be in metres and time in seconds.

Related tools

Distances in mixed units are easier with the Length Converter, and for timing laps or splits you can use the Stopwatch. If you are converting fuel economy rather than speed, see the Fuel Efficiency Converter.