What is a Millimeter? — Definition & Conversion
What is a millimeter? Learn to measure millimeters, how many millimeters in a meter and other units of measurement, and explore examples of millimeter conversions.
What is a Millimeter?
A millimeter is a unit of length measurement in the Standard International (SI) or “metric” system. A millimeter is the smallest unit of measurement of length on a standard ruler in the metric system. It is abbreviated mm. There are 10 mm in 1 centimeter.
Millimeters are the smallest unit of length.
Why are Millimeters Used?
Like the decimal system, the metric system is based on multiples of 10. In an attempt to standardize measurement systems around the world, the metric system was created in the late 18th century in France. All metric measurements of mass, volume, and length are based on a ratio used to measure water: a milliliter is equivalent to 1 cubic centimeter of water at its highest density, which weighs 1 gram. So, a millimeter is one tenth of the length of one side of 1 gram of water in cube form.
Millimeters are useful for measuring very small things such as the width of a guitar string or the clipper size for cutting hair.
Millimeter Measurement
A metric ruler or measuring tape is required to measure in millimeters. Millimeters are the smallest increment on a metric ruler. Usually the numbers on a tape measure or meter stick mark the centimeters (the numbers 1, 2. 3 are marked for the centimeters). The millimeters are so small that they are not labeled. Instead, they are only indicated by the shortest mark as shown in the image below.
Metric ruler with cm numbered. Mm are the smallest marks.
Note that there are 10 smaller marked spaces before and after each centimeter number. Therefore, when finding the measurement of an object in mm, every whole number increment on the ruler is 10 mm. For example, an object that measures 3 centimeters also measures 30 millimeters.
One advantage of using millimeters is the precision of the measurement. Every space between the smallest marks is 1 mm. For measuring a length in millimeters, follow these steps:
- Place the left end of the object on the 0 mark of the ruler.
- Count the spaces (not the lines) from the 0 to the end of the object or just before the end of the object.
- If the object is long, to count the spaces more quickly, the number of centimeter markings before the end of the object can be multiplied by 10
Example 1
A 4 mm line segment
The 0 mark of the ruler is placed at the left end of the line segment. There are 4 spaces to the end of the segment, so it is 4 mm long.
Example 2
The diameter of the tack is 10 mm.
The middle of the tack is lined up at the 0 on the ruler. The right center is at the 1, which means it is 10 mm long.
Example 3
The staple is 12 mm long.
The left end of the staple is in line with the 0 on the ruler. Since the end of the staple is 2 spaces past the 1 on the ruler, the length is 10 mm + 2 mm or 12 mm long.
How Many Millimeters in a Meter?
The metric system is based on units of length that differ by multiples of 10. The progression of metric units from smallest to largest are
- millimeter (mm)
- centimeter (cm)
- decimeter (dm)
- meter (m)
- decameter (dam)
- hectometer (hm)
- kilometer (km)
Each unit is made of 10 of the previous units. For example, 1 cm = 10mm, 1 dm = 10 cm, and 100 centimeters = 1 meter. The most common measurements of length are millimeter, centimeter, meter, and kilometer. Therefore it is more common to convert between these units. One method of converting measurements is to use a conversion factor. A conversion factor is a number that can be used to change one unit into another unit. The quantity in one unit is multiplied or divided by the conversion factor to get the quantity in the other unit. To convert a larger unit to a smaller, multiply by the conversion factor. To convert from a smaller unit to a larger, divide by the conversion unit. Millimeter conversion, which means to convert from millimeters to another unit of length, is necessary to change units within the metric system, or from metric units to customary units.
Since the metric system is based on powers of 10, the conversion factor between units will be powers of 10. The power of 10 can be determined by the number of steps between the units on the metric scale. The scale when written from smallest to largest is shown below:
Metric scale with abbreviations
Example 1
Convert 3 cm to mm.
- Since there is 1 step between cm and mm on the scale, the conversion factor is 10.
- Moving from a larger unit to a smaller unit, multiply by 10.
3 cm * 10 = 30 mm
Example 2
Convert 4 m to mm
- There are 3 steps between millimeters and meters, so the conversion factor is 10 * 10 * 10 = 1000.
- Converting from a larger unit to a smaller, multiply by 1000.
4 m * 1000 = 4000 mm
Example 3
Change 43 mm to cm.
- Since mm is 1 step from cm on the metric scale, the conversion factor is 10.
- Converting from a smaller unit to a larger, divide by the conversion factor.
43/10 = 4.3 cm
