A millimeter is a unit of length that is part of the standard measure in the metric system. One millimeter is equal to 1/1,000 of a meter. There are several ways to calculate millimeters. The easiest and simplest way is to use a metric ruler, which is already labeled with millimeter markers. The second way is to use basic math to convert it from another unit of measure, such as centimeters, kilometers, inches, or yards.
Method 1 of 3: Using the Metric Ruler
Step 1. Look at the unmarked line on the metric ruler
There are two separate units in the standard metric ruler: centimeters and millimeters. Numbered lines indicate centimeters, while unmarked lines represent millimeters. If you look closely, you'll notice that 10 millimeters is equal to 1 centimeter.
- The medium-sized line at the midpoint between each numbered line represents half a centimeter, aka 5 millimeters.
- This labeling system is also used on other longer metric measuring devices, such as meter sticks and tape measure.
Step 2. Align the tip of the ruler with the object you want to measure
More specifically, place a line marked “0” with the edge of the object's end. Make sure the ruler is completely straight and aligned to the starting point of your measurement.
- If you want to find the length of your phone in millimeters, adjust the ruler so that the "0" line is parallel to the horizontal end of the device.
- Not all rulers have a line numbered “0”. If your ruler looks like this, assume that the end of the ruler to the left of the line numbered “1” is the point “0 mm”.
Step 3. Multiply the number of centimeters immediately before the end of the object being measured by 10
Record the last rounded centimeter number on a ruler. Multiply this number by 10 to convert it to millimeters and show the length of the object in millimeters up to that point.
If the last centimeter is 1, multiply by 10 to get 10 because 1 cm = 10 mm
One quick and easy way to multiply an integer by 10 is to put a "0" behind it.
Step 4. Add up the number of lines after the last centimeter number
Now, count the number of final millimeter lines that go up to the end of the object. This step is necessary because the number of millimeter lines remaining is not enough to make up 1 full centimeter. Just use the centimeter measure to quickly calculate the length of an object in millimeters to save time.
- If the object being measured is 1.5 centimeters long, multiply 1 by 10 to get 10, and add 5 so that the total length is 15 mm.
- If it's easier, you can also measure one centimeter past the end of the object and subtract the number of millimeters in between. 2 centimeters (20 millimeters) minus 5 millimeters is 15 mm.
Method 2 of 3: Changing to Another Size
Step 1. Learn how metric measures work to be able to calculate millimeters easily
As you can see, there are 10 millimeters in a centimeter. Similarly, there are 1,000 millimeters in 1 meter and 1,000,000 millimeters in 1 kilometer, which is also equal to 1,000 meters. Once you understand the math, it's easy to change to another metric measure.
The prefix “centi” means “hundred,” which means that 1 centimeter is 1/100 of a meter. Similarly, "milli" means "thousand", so 1 millimeter is 1/1,000 of a meter
Step 2. Multiply inches by 25.4 to find the length in millimeters
You will need a calculator for this one. Start by entering inches to two digits behind the decimal (for example, 6, 25). Then, press the “x” key and enter the number “25.4” because 1 inch equals 25.4 millimeters. Press the “=” key, and you will get the corresponding measurements, but in millimeters.
- Using the example discussed above, 6.25 inches equals 158.75 millimeters.
- Converting inches to millimeters is a bit more difficult than other conversions because inches are imperial units and millimeters are metric units.
Rounding a decimal number to an integer should suffice. If you want extra accuracy, round up to two digits after the comma.
Step 3. Multiply the measurement in feet by 304, 8
The basic idea is the same as converting inches to millimeters. There are approximately 304.8 millimeters in a foot so multiply the measurement in feet by 304.8 to find the size of a measurement in the smallest metric unit for length, which is millimeters.
If you are 5 feet tall, your height in millimeters is 1,524 millimeters. Your height will appear bigger
Step 4. Use the conversion factor of 914, 4 to convert yards to millimeters
Nothing is different here. 1 yard is equal to 914.4 millimeters. As a result, multiply 1 yard by 914.4 to convert it to millimeters.
- The same basic principle for converting inches and feet to millimeters applies here as well. One foot equals 12 inches so 12 x 25, 4 = 304, 8; 1 yard equals 3 feet so 304, 8 x 3 = 914, 4, and so on.
- For example, the sports field of American Football is 100 yards. If converted, this size is equal to 91,440 millimeters. Imagine if you measured it using a ruler!
Method 3 of 3: Measuring Millimeters with a Credit Card
Step 1. Take a regular credit card
Most credit cards (and other types of plastic cards) are 30 mil thick, which is about 0.76 millimeters (or 0.762 mm to be exact). It's not the most accurate gauge, but it's sufficient if you only need a rough estimate of the length of something in millimeters
- If you don't have a credit card, stack 10 sheets of 22 cm x 28 cm printer paper to get 1 millimeter thick. However, this method can be more difficult than plastic cards.
- A “mile” is an imperial unit which is 1/1,000 of an inch, and should not be confused with a millimeter.
Since this method only produces a rough estimate, don't apply it if you need a very accurate measure.
Step 2. Stand the card on a piece of paper next to the object being measured
Align the outer edge of the card with the selected starting point on the object. Think of this card as a ruler, and the edges are 0 mm lines.
For this method, you're essentially adding up 1 millimeter each time to find the size of the corresponding object
Step 3. Use a pen or pencil to draw a thin line along the inner edge of the card
Pull the tip of your writing utensil along the edge of the card to create a straight line that is long enough to be clearly visible. This line represents a distance of 0.762 millimeters between the end of the object and the first line.
You will be drawing a number of lines that are close together so press lightly and make the lines as thin as possible. Sharpen a pencil or use a very pointed pen to make the job easier
Step 4. Move the card next to the drawn line and repeat the process
This line will mark a point 1.52 millimeters from the starting point of the measurement. Move the card to the other side of the last line, and draw another line. Continue measuring and marking little by little until you reach the end of the object, then count the number of each gap between each line.
- Make sure you count the space between the lines and not the lines themselves because the number of lines will be over 1.
- For added accuracy, count every 4 lines as 3 millimeters. This trick will help with your measurements because credit card thickness is not exactly 1 millimeter.
- Knowing how to measure in millimeters is a useful skill. Sizes of general products and specialty items, usually expressed in millimeters, include building tools and materials, electrical components, eyeglass lenses, and jewelry.
- The metric system is currently known by another name: the International System of Units (shortened to SI). However, these two names refer to the same unit of measurement.