In length measurements, the single prime symbol (′) means feet and the double prime symbol (″) means inches. So when you see something written as 5′10″, it means 5 feet, 10 inches. One foot equals 12 inches, so these two units work together to describe heights, room dimensions, lumber sizes, and countless other everyday measurements.
How the Symbols Work
The single prime (′) always comes after the number of feet, and the double prime (″) always comes after the number of inches. A few examples:
- 6′2″ = 6 feet, 2 inches (a person’s height)
- 8′ × 10′ = 8 feet by 10 feet (a room or rug)
- 2″ × 4″ = 2 inches by 4 inches (lumber dimensions)
- 5′11″ = 5 feet, 11 inches
You can use one without the other. A ceiling described as 9′ is simply 9 feet tall. A screw listed at 3″ is 3 inches long. When both appear together, feet always come first because they’re the larger unit.
Primes, Not Quotation Marks
Technically, these symbols are called “prime” (′) and “double prime” (″), and they’re distinct from apostrophes, quotation marks, and the straight tick marks on your keyboard. The Chicago Manual of Style specifically notes that the inch symbol is the double prime, not double quotation marks or straight quotes. In practice, though, most people type a keyboard apostrophe for feet and a keyboard double quote for inches, and everyone understands what’s meant. You’ll only run into trouble in formal typography or professional publishing, where using the correct prime characters matters for polish.
If you need the true symbols, they have their own Unicode characters: U+2032 for the single prime and U+2033 for the double prime. On most systems you can find them through a special character menu. But for a text message, email, or casual document, the apostrophe and quotation mark on your keyboard work fine.
Where These Symbols Are Used
Feet and inches are the standard length units in the United States, and the prime notation shows up constantly in construction, real estate, interior design, and everyday conversation. But the U.S. isn’t the only place you’ll encounter them.
In the United Kingdom, road signs display height and width restrictions in feet and inches, and government documents sometimes list body height in imperial units alongside metric. Canada uses feet and inches heavily in construction, home renovation, and recipes, and many English-speaking Canadians describe their height that way on a daily basis. Ireland, Hong Kong, India, and Australia all use feet and inches informally for things like personal height and property listings, even though their official systems are metric. Real estate in particular tends to stick with square feet across much of the English-speaking world.
So if you’re reading a property listing from London, a building plan from Toronto, or a height description from Dublin, you’ll likely see the same prime notation.
The Same Symbols Mean Something Else in Angles
Here’s where it can get confusing: the prime and double prime symbols are also used for angular measurements, which come up in geography, navigation, and astronomy. In that context, the single prime means arcminutes (1/60 of a degree) and the double prime means arcseconds (1/60 of an arcminute).
A GPS coordinate written as 40° 26′ 46″ N means 40 degrees, 26 arcminutes, and 46 arcseconds north. The symbols look identical to the feet and inches notation, but the context makes the meaning clear. If there’s a degree symbol (°) in front, you’re looking at angles. If not, you’re looking at length.
Other Uses of the Prime Symbol
Outside of measurements, you might spot the single prime in math class. Mathematicians use it to indicate a derivative (as in f′(x), read “f prime of x”), to label transformed coordinates, or to mark related but distinct points on a graph (like point A versus point A′). These uses have nothing to do with feet or inches, and you’ll only encounter them in mathematical notation. The double prime shows up in math too, typically for second derivatives (f″(x)).
In everyday life, though, the overwhelmingly common meaning is the one you probably searched for: single prime means feet, double prime means inches.