Content

• To step
• Method 1 of 2: Convert millimeters to inches
• Tips

There are many tools for converting from millimeters to inches (mm to inches), but most teachers would like you to show how you did the conversion. All you need for a random conversion is the "conversion factor" and some simple math. The same standard process can be used for all kinds of mathematics and arithmetic tasks, and in daily life.

To step

Method 1 of 2: Convert millimeters to inches

1. Learn the conversion factor. One inch equals 25.4 millimeters. Write this "conversion factor" as a fraction: ${ displaystyle { frac {1inch} {25.4mm}}}$Understand the conversion factor. The numerator and denominator of the fraction have the same value, but written in different units. This means that the fraction is equal to 1, just like the fractions ${ displaystyle { frac {3} {3}}}$Write down the math problem. Write down all units of your measurement. For example, write the following to convert 16 millimeters (mm) to inches (in):
   • 16 mm x ${ displaystyle { frac {1in} {25.4mm}}}$Get rid of the units. If the same unit is in the numerator and denominator, you can cancel them out. If you have written down the calculation correctly, the millimeter will be eliminated and you will only be left with the inch.
     • 16 mm x ${ displaystyle { frac {1in} {25.4mm}}}$Solve the problem. Complete the math problem by hand or with a calculator.
       • 16 x ${ displaystyle { frac {1in} {25.4}}}$Memorize useful conversions. If you can remember that 1 mm = ${ displaystyle { frac {1} {25.4}}}$Convert millimeters to centimeters. This is easy to memorize. Just divide the number of millimeters by 10. All you have to do is move the decimal one position to the left. If you do not understand why this is possible, look at the following example:
         • 430 mm
           = 430 mm x ${ displaystyle { frac {1cm} {10mm}}}$Estimate the centimeter to inch conversion. The conversion 1 cm = 0.4 inch is accurate enough for most purposes. But how do you multiply by 0.4 in your head? Following are two different ways of doing this, using the same example:
           • First divide by two and then subtract 1/10 from the number of cm:
             43 cm ÷ 2 = 21.5
             → 1/10 or 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
             → 4.3x4 = (4x4) + (0.3x4) = 16 + 1.2 = 17.2 inches.
         • Understand that this is an estimate. Being able to do this is useful for quick guesses in real situations, where the exact answer is usually less important. It's not a good idea for homework assignments that require an exact answer, but you can use it to check your answer to make sure you're on the right track.
           • In the example, the actual answer is 16.921 inches. The estimate is less than a third of an inch off, which is particularly good for mental arithmetic.

Tips

• When calculating an area, you have to convert from square millimeters (mm) to square inches (in). So multiply it by ${ displaystyle { frac {1in ^ {2}} {645mm ^ {2}}}}$ (or ${ displaystyle { frac {1in ^ {2}} {645.16mm ^ {2}}}}$ if your measurement is very precise). Note that 645.16 = 25.4.
• For volume calculations, convert from cubic millimeters (mm) to cubic inches (in). Multiply by ${ displaystyle { frac {1in ^ {3}} {15800mm ^ {3}}}}$, or with ${ displaystyle { frac {1in ^ {3}} {15806.42mm ^ {3}}}}$ for an exact answer. Note that 15806.42 = 25.4.
• Some sources will state that you should multiply by 0.0393701 instead of the above. This also works because ${ displaystyle { frac {1in} {25.4mm}}}$ = 0.0393701 in / mm.