A millimeter is a metric unit of length equal to one thousandth of a meter. It is used for small dimensions, thickness, tolerances, hardware, engineering drawings, product specifications, medical measurements, and any situation where centimeters are too coarse.
Quick facts
- Unit name
- millimeter
- Plural
- millimeters
- Symbol
- mm
- Quantity
- length
- System
- metric system, SI-compatible derived decimal unit
- Exact relation
- 1 mm = 0.001 m
- Centimeter relation
- 10 mm = 1 cm
- Inch relation
- 1 in = 25.4 mm exactly
Visual scale
One millimeter is one tenth of a centimeter. On a metric ruler, the small marks between centimeter numbers usually represent millimeters.
Definition and exact value
The millimeter is defined through the meter. The prefix milli- means one thousandth, so one millimeter is exactly one thousandth of a meter:
This decimal relationship is exact. No rounding is involved when converting between millimeters, centimeters, and meters.
Symbol and writing rules
The standard symbol for millimeter is mm. Unit symbols are not pluralized, so write 1 mm and 12 mm, not 12 mms. A space is normally placed between the number and symbol in technical writing.
How big is one millimeter?
One millimeter is a small but visible unit. It is useful when a measurement needs more detail than centimeters but can still be read with common tools such as rulers, calipers, and measuring tapes.
Ruler scale
The small marks between centimeter numbers on a metric ruler usually represent 1 mm each.
Object scale
Millimeters are useful for thickness, gaps, small parts, screws, cards, cases, and product details.
Technical scale
Engineering drawings often use millimeters because they avoid decimals for many small dimensions.
Millimeter in the metric hierarchy
The millimeter is one of the most practical metric subunits of the meter. It keeps small dimensions readable while staying connected to the decimal metric system.
| Unit | Symbol | Relationship | Typical use |
|---|---|---|---|
| millimeter | mm | 1 mm = 0.001 m | small parts, thickness, tolerances |
| centimeter | cm | 1 cm = 10 mm | everyday object dimensions |
| meter | m | 1 m = 1,000 mm | rooms, height, construction, science |
| kilometer | km | 1 km = 1,000,000 mm | roads, geography, travel distance |
Millimeter vs centimeter
Millimeters and centimeters are both metric length units, but they serve different levels of detail. A centimeter is 10 times longer than a millimeter.
Visual comparison
Use millimeters for small parts, thickness, fit, clearance, and tolerances. Use centimeters for more readable everyday dimensions such as body measurements, classroom geometry, and product sizes.
Millimeter vs inch
The millimeter is metric and decimal. The inch is customary and often used with fractions. Their relationship is exact because one inch equals exactly 25.4 millimeters.
Millimeters are often better for engineering drawings and exact product dimensions. Inches are often used in US product descriptions, screen sizes, hardware labels, and construction contexts.
Common uses
- Mechanical parts, fasteners, screw sizes, washers, and hardware dimensions.
- Thickness of cards, sheets, panels, glass, plastic, metal, and packaging.
- Engineering drawings, CAD, manufacturing tolerances, and product specifications.
- Medical measurements, rainfall, small anatomy measurements, and clinical records.
- Camera lenses, watch sizes, phone dimensions, cases, and precision consumer products.
How to measure in millimeters
To measure in millimeters, use the small marks on a metric ruler or a more precise tool such as calipers. On most rulers, each centimeter is divided into 10 millimeter marks.
- Align the zero mark with the starting edge of the object.
- Read the last full centimeter before the endpoint.
- Count the millimeter marks after that centimeter.
- Convert if needed: 2 cm plus 7 mm equals 27 mm.
Precision and tolerances
Millimeters are common in technical work because they make small dimensions easy to write without long decimals. A dimension such as 2.5 mm is easier to read than 0.25 cm and often more natural than 0.098425 in.
Everyday precision
A ruler can usually support whole-millimeter readings. This is enough for many household, school, and craft measurements.
Technical precision
For fits, tolerances, machining, and quality control, use calipers or specified measuring tools and record decimal millimeters when required.
Conversion formulas
Millimeter conversion table
| Millimeters | Centimeters | Meters | Inches | Feet |
|---|---|---|---|---|
| 1 mm | 0.1 cm | 0.001 m | 0.0393700787 in | 0.0032808399 ft |
| 5 mm | 0.5 cm | 0.005 m | 0.1968503937 in | 0.0164041995 ft |
| 10 mm | 1 cm | 0.01 m | 0.3937007874 in | 0.032808399 ft |
| 25.4 mm | 2.54 cm | 0.0254 m | 1 in | 0.0833333333 ft |
| 1,000 mm | 100 cm | 1 m | 39.37007874 in | 3.280839895 ft |
Common mistakes
- Confusing mm and cm: 1 cm equals 10 mm, so this mistake creates a factor-of-10 error.
- Confusing mm and m: 1 m equals 1,000 mm, so the scale error can be very large.
- Using 25 instead of 25.4 for inches: 1 inch equals exactly 25.4 mm.
- Dropping decimal millimeters: 2.5 mm is not the same as 25 mm.
- Mixing length and area: mm measures length, while mm2 measures area.
Related Xunits pages
Millimeter converters
Length references
FAQ
How many millimeters are in one centimeter?
One centimeter equals exactly 10 millimeters.
How many millimeters are in one meter?
One meter equals exactly 1,000 millimeters.
How many millimeters are in one inch?
One inch equals exactly 25.4 millimeters.
What is the symbol for millimeter?
The standard symbol is mm.
Is millimeter an SI unit?
The SI base unit for length is the meter. The millimeter is a decimal metric unit derived from the meter through the milli- prefix.
Should I use mm or cm?
Use millimeters for small details, thickness, and tolerances. Use centimeters for everyday object dimensions that do not need fine detail.
Is 10 mm the same as 1 cm?
Yes. 10 millimeters equal exactly 1 centimeter.
Is 1000 mm the same as 1 m?
Yes. 1,000 millimeters equal exactly 1 meter.
Can millimeters be decimal?
Yes. Decimal millimeters such as 2.5 mm are common in technical specifications and precise measurements.
What is the difference between mm and mm2?
mm measures length. mm2, written as square millimeters in plain text, measures area.
Reference notes
This guide uses the exact metric relationship of 1 mm = 0.001 m, the exact centimeter relationship of 10 mm = 1 cm, and the exact international inch relationship of 1 in = 25.4 mm.
- Milli- means one thousandth.
- One millimeter equals exactly 0.001 meter.
- One centimeter equals exactly 10 millimeters.
- One inch equals exactly 25.4 millimeters.