Inch (in)
An inch is an imperial and US customary unit defined as exactly 25.4 millimeters. Twelve inches make one foot.
- Symbol
- in
- System
- Imperial
- Meter value
- 0.0254 m
- Singular
- inch
- Plural
- inches
- Aliases
- inch, inches
What a inch measures
The inch is suited to short customary measurements. Rulers often divide it into halves, quarters, eighths, or sixteenths rather than decimal tenths.
- One inch equals exactly 25.4 millimeters.
- Twelve inches make exactly 1 foot.
- Thirty-six inches make exactly 1 yard.
- A quarter inch is 0.25 inch and an eighth inch is 0.125 inch.
Where inches are used
Definition and standardization
The international inch is defined as exactly 25.4 millimeters, providing a fixed bridge between customary and metric length.
The accepted text symbol is in. A double-prime mark is also common in dimensions but can be confused with quotation marks.
Fractional inch notation remains common in trades, so the measurement resolution depends on the ruler divisions.
Tools used to measure inches
- Customary ruler for short dimensions
- Tape measure for construction and household work
- Caliper for decimal-inch or millimeter readings
Common mistakes
- Reading the longest nearby tick instead of counting fractional divisions
- Treating 1.5 inches as 1 inch 5/8
- Using a rounded 2.5 centimeters per inch when accuracy matters
Inch questions
How many millimeters are in an inch?
One inch equals exactly 25.4 millimeters.
How many inches are in a foot?
There are exactly 12 inches in 1 foot.
How do I read sixteenths of an inch?
Between two whole-inch marks, count the 16 equal intervals; each smallest interval represents 1/16 inch.