How To Convert Mm To Inches

I don’t always write articles about mm to inches, but when I do it’s just because I feel like converting mm to inches. Today is one of those days, so here is an article about mm to inches. Also, if you are curious about how many yards there are in an inch or how many inches there are in an yard, you might enjoy this article.

There are many online tools that convert millimeters to inches (mm to in), but most teachers require you to show your work. All you need for any conversion is the “conversion factor”[1] and basic arithmetic. This same basic process will come in handy for all math and science classes, as well as everyday life.

Table of Contents

Method 1 Converting Millimeters to Inches Download Article

  1. 1 Learn the conversion factor. One inch is the same length as 25.4 millimeters. You can write this “conversion factor” as a fraction: 1inch25.4mm{\displaystyle {\frac {1inch}{25.4mm}}}{\frac  {1inch}{25.4mm}}.
    • Any two units have a special conversion factor for converting between them. If you don’t know what it is, you can look it up online or in a textbook.
  2. 2 Understand the conversion factor. The top and bottom of the fraction are the same length, just written in different units. This means the fraction is equal to 1, just like the fractions 33{\displaystyle {\frac {3}{3}}}{\frac  {3}{3}} or 88{\displaystyle {\frac {8}{8}}}{\frac  {8}{8}} simplify to 1.
    • Remember, multiplying a number by 1 doesn’t change its value. That means you can multiply your measurement by this factor and the length will be the same. The only difference is the units: your answer will be written in inches instead of millimeters.[2]
  3. 3 Write down the multiplication problem. Make sure to write down all units in your measurement. For example, here’s what you would write down to convert 16 millimeters (mm) into inches (in):
    • 16 mm x 1in25.4mm{\displaystyle {\frac {1in}{25.4mm}}}{\frac  {1in}{25.4mm}} = ?
  4. 4 Cancel out the units. If the same unit appears on the top and bottom of a fraction, they cancel out. If you’ve set up your problem correctly, the millimeters will cancel out and you’ll be left with just inches.
    • 16 mm x 1in25.4mm{\displaystyle {\frac {1in}{25.4mm}}}{\frac  {1in}{25.4mm}} = ?
    • Cancel out the millimeters:
      16 x 1in25.4{\displaystyle {\frac {1in}{25.4}}}{\frac  {1in}{25.4}} = ?
    • Since the only unit left is inches, your answer will be in inches.
  5. 5 Solve the problem. Finish the math problem by hand or on a calculator.
    • 16 x 1in25.4{\displaystyle {\frac {1in}{25.4}}}{\frac  {1in}{25.4}}
    • = 16 in ÷ 25.4
    • = about 0.63 inches

Method 2 Estimating in Your Head Download Article

  1. 1 Memorize useful conversions. If you can remember that 1 mm = 125.4{\displaystyle {\frac {1}{25.4}}}{\frac  {1}{25.4}} inches, that’s great. But this conversion doesn’t come up very often. It’s more useful to memorize these common conversions:[3]
    • 1 cm = about 0.4 inches
    • 1 cm = 10 mm
    • 1 inch = exactly 2.54 cm (you don’t need this for this conversion, but it’s a good one to know)
  2. 2 Convert millimeters to centimeters. This is easy to do in your head. Just divide the millimeter measurement by 10. All you have to do is move the decimal point one digit to the left. If you’re not sure why this works, look at this example:
    • 430 mm
      = 430 mm x 1cm10mm{\displaystyle {\frac {1cm}{10mm}}}{\frac  {1cm}{10mm}}
      = 43 cm.
  3. 3 Estimate the centimeter to inch conversion. The 1 cm = 0.4 in is close enough for most purposes. But how do you multiply by 0.4 in your head? Here are two different ways to do it, continuing with the same example:
    • Divide by two, then subtract 1/10 of the cm value:
      43 cm ÷ 2 = 21.5
      → 1/10 of 43 is 4.3
      → subtract the second number from the first: 21.5 – 4.3 = 17.2 in.
    • Or divide by ten, then multiply by four:
      43 cm ÷ 10 = 4.3
      → 4.3 x 4 = (4 x 4) + (0.3 x 4) = 16 + 1.2 = 17.2 in.
  4. 4 Understand that this is an estimate. Learning how to do this will come in handy for quick real-life estimates, where the exact answer is usually not important. It’s not a good idea for homework problems that require an exact answer, but you can use it to double check your answer and make sure it’s in the right ballpark.
    • In the example, the exact answer is actually 16.921 inches. The estimate is less than a third of an inch off, which is pretty good for mental math.

What Is a Millimeter?

To measure the length of the very small objects less than a centimeter, a millimeter is a suitable unit. The millimeter with abbreviation as mm is a unit of displacement or distance for a small object.

The mm is equivalent to 0.001 meters. Also, there are about 25.4 millimeters in one linear inch. In this topic, the student will learn about millimeters and their conversions.

Definition of Millimeter

The millimeter is a unit of length in the metric system, equivalent to one-thousandth of a meter (the SI base unit of length). In the standard metric system, power-of-10 prefix multipliers facilitate the derivation of other.

Formally, one millimeter is the distance travelled by a ray of electromagnetic (EM) energy through a vacuum in a time span of 3.33564095 x 10-12 seconds.

Thus, the millimeter and its kin are also used to specify the wavelengths of electromagnetic fields, especially at ultra-high and microwave radio frequencies.

Further, such radio spectrum occupies an informally defined range of wavelengths from roughly 1 mm to several tens of kilometers. The speed of EM-field propagation in a vacuum is 2.99792458 x 1011 millimeters per second.

One simple way to measure the centimeter is by using everyday objects that are close to 1 centimeter in size, like a pencil, pen or highlighter. Millimeter gives more precision value to such small objects.

Moreover, a millimeter is a unit of length in the metric system which is equal to \(\frac{1}{1000} th of a meter.

History of a Millimeter

The metric system of weights and measures was defined and adopted in France in 1795. John Wilkins was the first person to publicly recognize the need for universal length measurement.

So as to carry out a measurement locally to avoid having to have a definition that refers to a stored reference unit.

Thus, he proposed using the length of a “second’s pendulum” which made half an oscillation per second. But later, it was found that there is a slight difference in this length around the world.

Therefore, the Academy of Sciences devised a new measure, 1/10000000 of the distance from the Equator to the North Pole.

Moreover, they put to use the new value to create a metal bar that stores at the BIPM’s headquarters and known as “international prototype meter”. This standard has since been superseded and further defined the meter as “1650763.73 wavelengths of the orange-red emission line of a krypton-86 atom in a vacuum”.

Most recently, in 1983, the meter defines as “the distance travelled by light in a vacuum in 1/299792458 of a second”. From this, we derive the centimeter as the distance travelled by light in a vacuum in 1/2997924580 of a second.

What Is an Inch?

inch, unit of British Imperial and United States Customary measure equal to 1/36 of a yard. The unit derives from the Old English ince, or ynce, which in turn came from the Latin unit uncia, which was “one-twelfth” of a Roman foot, or pes. (The Latin word uncia was the source of the name of another English unit, the ounce.) The old English ynce was defined by King David I of Scotland about 1150 as the breadth of a man’s thumb at the base of the nail. To help maintain consistency of the unit, the measure was usually achieved by adding the thumb breadth of three men—one small, one medium, and one large—and then dividing the figure by three. During the reign of King Edward II, in the early 14th century, the inch was defined as “three grains of barley, dry and round, placed end to end lengthwise.” At various times the inch has also been defined as the combined lengths of 12 poppyseeds. Since 1959 the inch has been defined officially as 2.54 cm.

Millimeter to Inch Conversion Table

MillimetersInches (decimal)Inches (fraction)
1 mm0.03937″3/64″
2 mm0.07874″5/64″
3 mm0.11811″1/8″
4 mm0.15748″5/32″
5 mm0.19685″13/64″
6 mm0.23622″15/64″
7 mm0.275591″9/32″
8 mm0.314961″5/16″
9 mm0.354331″23/64″
10 mm0.393701″25/64″
11 mm0.433071″7/16″
12 mm0.472441″15/32″
13 mm0.511811″33/64″
14 mm0.551181″35/64″
15 mm0.590551″19/32″
16 mm0.629921″5/8″
17 mm0.669291″43/64″
18 mm0.708661″45/64″
19 mm0.748031″3/4″
20 mm0.787402″25/32″
21 mm0.826772″53/64″
22 mm0.866142″55/64″
23 mm0.905512″29/32″
24 mm0.944882″15/16″
25 mm0.984252″63/64″
26 mm1.0236″1 1/32″
27 mm1.063″1 1/16″
28 mm1.1024″1 7/64″
29 mm1.1417″1 9/64″
30 mm1.1811″1 3/16″
31 mm1.2205″1 7/32″
32 mm1.2598″1 17/64″
33 mm1.2992″1 19/64″
34 mm1.3386″1 11/32″
35 mm1.378″1 3/8″
36 mm1.4173″1 27/64″
37 mm1.4567″1 29/64″
38 mm1.4961″1 1/2″
39 mm1.5354″1 17/32″
40 mm1.5748″1 37/64″


A piece of artwork, any existing building or even a manufactured product can be measured in various ways. The unit for this measurement normally depends on the region. The other thing that should be taken into consideration is that different nations and regions use different measurements and scales to measure various types of objects and products which will then affect the metric conversion.

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