Centimeters and millimeters are units of distance used in the metric system. The word “centi” means one hundredth so there are 100 centimeters in every meter. The word “milli” means one thousandth so there are 1,000 millimeters in every meter. Because these two units are similar, people often change from one unit to the other. There are 10 millimeters in each centimeter, so to convert units, multiply the number in centimeters by 10. Keep in mind that the metric system is an orderly system so you can use a simple comma (decimal) trick to make conversions quickly, without having to do any math. With practice, you can change a quantity from one unit to another without any problems.
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Method 1 of 3: Performing Conversion Calculations
Step 1. Find out the number or quantity you want to convert in centimeters
For example, when you want to solve a problem, read the question carefully to find the required units. Make sure the length is in centimeters (cm) and the question asks you to convert it to millimeters (mm). If you need to measure the length of an object yourself, make sure you measure it in centimeters. Measurements in millimeters are more difficult to do because they are so small. However, you can easily convert a measurement from centimeters to millimeters.
For example the question: “The width of a table is 58.75 centimeters. How wide is the table in millimeters?”
Step 2. Multiply the measurement in centimeters by 10 to convert it to millimeters
One centimeter is equal to 10 millimeters. This means that you can easily convert any quantity through simple calculations. Regardless of the number or quantity you want to convert, always multiply the number (in centimeters) by 10.
- For example, 58.75 cm x 10 = 587.5 mm.
- A millimeter is a unit smaller than a centimeter, even though they both have the word "meter". You should always use multiplication to convert larger units to smaller units.
Step 3. Divide the number or quantity in millimeters by 10 to convert it back to centimeters
There are 10 millimeters for every 1 centimeter. This means that converting millimeters to centimeters can be done by reversing the calculation. Measure the length of the object in centimeters, then perform the basic calculations. If you've previously converted from centimeters to millimeters, compare the results with the initial measurements to check your answers.
- For example the question: “The height of a door is 1,780, 9 millimeters. Find the height of the door in centimeters." The answer is “178.09 cm” because 1780, 9 mm / 10 = 178.09.
- Keep in mind that centimeters are larger units than millimeters so when converting smaller units to larger units you must divide the initial number/quantity.
Method 2 of 3: Moving Commas (Decimal)
Step 1. Find the position of the comma in the number you want to convert
When you want to answer a math problem, first identify the magnitude/number in centimeters. If you must measure the measurements yourself, make sure you take measurements in centimeters. Remember or observe the position of the comma. For numbers that do not have a comma, assume the comma is at the end or end of the number.
- For example, you may be asked to find the width of a television screen that is 32.4 centimeters in millimeters. The position of the comma is important information and can be used to convert numbers/quantities quickly, without additional mathematical calculations.
- For integers such as 32 centimeters, a comma is placed after the last digit. You can write it as 32.0 cm.
Step 2. Move the comma one digit to the right to convert the quantity/number to millimeters
Moving commas in this way is like multiplying a number/quantity by 10. Because of the regular workings of the metric system, you don't even need a calculator. One centimeter equals 10 millimeters, and this equation can be proven by moving the comma in the centimeter number one digit to the right.
- For example, 32.4 cm becomes 324.0 mm when you move the comma one digit to the right. You can prove it through multiplication because 32.4 x 10 = 324, 0.
- For integers like 32, write the number first, insert a comma after it, and add the number 0. After that, move the comma one digit to the right. For example, 32, 0 x 10 = 320, 0.
Step 3. Slide the comma one digit to the left to return the measurement to centimeters
If you need to convert from millimeters to centimeters or check the initial conversion, simply move the comma back one digit to the left. 10 millimeters is equal to one centimeter. This equation can be proved when the comma is shifted one digit to the left. Check the results by moving the comma one digit back (left) or performing basic calculations.
- For example, you might get a math problem like this: “The height of a chair is 958.3 millimeters. Find the seat height in centimeters!” All you need to do is shift the comma one digit to the left so you will get the value 95.83 cm.
- To check work, divide the initial number by 10 (the number of millimeters in a centimeter). For example, 958, 3 / 10 = 95, 83 cm.
Method 3 of 3: Practice Conversion
Step 1. Convert 184 centimeters to millimeters
This problem requires you to remember how to complete the conversion. There are two ways to follow. You can multiply the number/quantity in centimeters by 10, or move the comma one digit to the right. Both of these strategies yield the same answer.
- To solve the problem mathematically: 184 cm x 10 = 1,840 mm.
- To solve the problem through decimal shift, write "184, 0 cm" first on the book/paper. After that, move the comma one digit to the right so you get 1840, 0 mm.
Step 2. Convert 90.5 millimeters to centimeters
Keep in mind that actually, this problem starts with centimeters, not millimeters. If you know how to convert from centimeters to millimeters, you'll understand how to do the reverse conversion. One way that can be followed is to divide the number/quantity by 10. Alternatively, move the comma one digit to the left of the existing number/quantity.
- Mathematically, you can solve the problem with an answer like this: 90.5 mm / 10 = 9.05 cm.
- For decimal shift, start by writing “90.5 mm” on the book/paper. Move the comma one digit to the right so you get 9.05 cm.
Step 3. Convert 72.6 centimeters into millimeters
This easy conversion can be done using one of the two techniques discussed earlier. One centimeter equals 10 millimeters so simply multiply the number/quantity by 10 to get the right answer. For the no-calculated method, shift the comma one digit to the right.
- To change units by calculation, write the answer as follows: 72.6 cm x 10 = 726 mm.
- To use the decimal shift method, note the position of the comma on the 72.6 cm. Move the comma one digit to the right to get 726 mm.
Step 4. Convert 315 millimeters into centimeters
Note that this problem begins with a measurement in millimeters. However, be careful when you want to change the magnitude. Since 10 millimeters equals 1 centimeter, divide the quantity/number by 10 to convert it to centimeters. If you are using the comma displacement technique, move the comma one digit to the left.
- For example, 315 mm / 10 = 31.5 cm.
- To solve the problem with the decimal shift technique, write "315.0 mm" first on the book/paper. After that, move the comma one digit to the left so that you get 31.5 cm.
Tips
- The technique of converting from centimeters to millimeters can also be applied to other units in the metric system, such as meters and kilometers.
- One meter is equal to 100 centimeters and 1,000 millimeters. Since they have the word “meter”, the three can be confusing if you don't read the questions carefully.
- If you need help converting numbers/quantities, search the internet for calculator features. There are various features that allow you to quickly convert from centimeters to millimeters.
