The standard inches abbreviation is in., two lowercase letters followed by a period. The same short form covers one inch and many inches, so you write 1 in. and 12 in. alike. In tight spaces, such as a drawing, a chart or a tape measure, people also use the inch mark, a double prime (″) that most keyboards replace with a straight double quote (").
Both carry one meaning, so pick by setting: in. reads better in sentences, product listings and forms, while the mark saves room in measurements like 5′ 10″ for height or 2″ × 4″ for a board.
Quick facts
Abbreviation
in.Symbol
″ (double prime), often typed as "Plural
in. (no s added)Period
Yes, in US style, so it is not read as the word inSquare inches
sq. in. or in²Cubic inches
cu. in. or in³Related unit
Feet: ft. or ′ (feet abbreviation)
The inches abbreviation and when to use it
Use in. when the measurement sits in running text or on a label people read like a sentence.
- "The shelf is 30 in. wide."
- "Cut the fabric into 6 in. squares."
- "Screen size: 15.6 in."
Notice the space between the number and the letters. Also notice that it never turns into "ins." The abbreviation for inch is the same in the singular and the plural, a pattern shared by many US units, as the common abbreviations section shows with lb., oz. and ft.
Why the period matters
Most unit abbreviations in American writing drop the period: ft, lb and oz often appear without one. Inch is the main exception. Without a dot, "in" looks exactly like the everyday word, so a sentence such as "Add 2 in to the length" can trip a reader. Style guides that skip periods on other units still keep it here for that reason.
If the measurement ends your sentence, one period does both jobs: "The gap is half an inch, or 0.5 in." You do not add a second dot.
Using the inch mark (″)
The inch mark is a double prime, two short slanted strokes. It follows the number with no space: 8″. The foot mark is a single prime: 6′. Together they give the familiar height format 5′ 4″, read "five feet, four inches."
On most keyboards you will type a straight quotation mark (") instead, and that is accepted almost everywhere. What to avoid is the curly closing quote (”) that word processors add on their own. It looks like the end of a quotation rather than a unit.
On a lumber tag: "2″ × 6″ × 8′." This means a board two inches by six inches, eight feet long. In practice the board is a bit smaller than its name, but the label uses the marks to stay short.
Square and cubic inches
| Measure | Words | Short forms |
|---|---|---|
| Length | 12 inches | 12 in. or 12″ |
| Area | 40 square inches | 40 sq. in. or 40 in² |
| Volume | 350 cubic inches | 350 cu. in. or 350 in³ |
Engine sizes in older American cars were often given in cubic inches, often shortened to CID (cubic inch displacement). For area, sq. in. is friendlier in a product listing, and in² suits math homework. Larger spaces such as rooms are measured in square feet instead.
What is the abbreviation for inches on forms and in school?
Forms that ask for height often have two boxes labeled ft and in, so you write 5 and 7 rather than any marks. Medical charts and driver license records may list height as 5-07, meaning five feet seven inches. Clothing tags give waist and inseam in plain numbers, such as 32 × 30, with inches understood.
In math class, teachers usually accept in. or the mark, but they want the unit every time. Writing "12" alone for a side of a square can cost points, since it could be inches, feet or centimeters.
Mistakes to avoid
- Adding an s: write 10 in., not 10 ins.
- Mixing styles: pick in. or ″ and keep it the same through one document.
- Confusing marks: one tick is feet, two ticks is inches.
- Dropping the space: 4 in. has a space; 4″ does not.
- Mixing up metric: one inch is exactly 2.54 centimeters, so do not swap cm and in. as if they were equal.
For the rules on shortening other words, see how to abbreviate.
Questions people ask
Which abbreviation for inches is correct?
Use in., with a period. The inch mark (″, or a typed ") is used in tight spaces, such as drawings and height measurements.
Is the inch abbreviation the same for one inch?
Yes. Write 1 in. and 20 in. the same way. Unit abbreviations in US style do not add an s for the plural.
Does in. need a period?
In American usage, yes. The period keeps the abbreviation from being read as the word "in." Some technical fields drop it when the meaning is clear.
Which mark means inches, ′ or ″?
The double mark (″) means inches and the single mark (′) means feet. So 6′ 2″ is six feet, two inches.
How do you write square inches?
Use sq. in. in everyday writing or in² in math and science. For example, a 3 by 4 inch card covers 12 sq. in.