How to Calculate Cubic Inches and Square Inches (Plus What ‘ and ” Mean)

A single tick mark (‘) means feet. A double tick mark (“) means inches. So 5’3” means 5 feet, 3 inches — not “5 minutes 3 seconds” or anything else.

This shorthand comes from the prime and double-prime symbols used in geometry and navigation. Both fields picked them for the same reason: a fast, compact way to mark a unit without spelling out the full word.

What the ‘ and ” symbols mean, and where else they show up

The prime symbol (‘) and double-prime symbol (“) aren’t exclusive to length. They do double duty across a few fields. Context tells you which meaning applies:

• In length measurements: ‘ = feet, ” = inches. 6’2″ is six feet two inches.
• In geographic coordinates: ‘ = minutes of arc, ” = seconds of arc. A latitude like 40°26’46” means 40 degrees, 26 minutes, 46 seconds.
• In time notation (rare, mostly historical): ‘ = minutes, ” = seconds. “min” and “sec” have mostly replaced this usage in everyday writing.

If you see 5’8″ on a spec sheet, height chart, or lumber listing, read it as feet and inches. The geographic and time usages only show up next to a degree symbol or a clock-related context. The surrounding text usually settles any ambiguity.

Other single-letter measurement abbreviations

A few other short abbreviations cause similar confusion.

• “m” for meters: In the metric system, a lowercase “m” is the standard SI abbreviation for meters, as in 1.8m for 1.8 meters. People sometimes confuse it with “mi” for miles or “mm” for millimeters. Watch for the full unit label when precision matters — “5m” and “5mm” differ by a factor of 1,000.
• “d” for diameter: In plumbing, machining, and engineering notation, a lowercase “d” (or the Ø symbol) commonly denotes diameter. That’s the distance straight across a circle through its center. A spec reading “d = 12 in” means a 12-inch diameter, not a 12-inch radius.
• “d” for penny (nail sizes): A separate, older use of “d” shows up in nail sizing, where it stands for “penny.” That’s a holdover from a centuries-old English pricing system. A “16d nail” runs about 3.5 inches long. It has nothing to do with diameter or cost today; it’s a legacy sizing label that hardware stores and lumber specs still carry forward.

Calculating square inches

Square inches measure area. Multiply length by width, both in inches.

square inches = length (in) × width (in)

A photo measuring 4 inches by 6 inches covers 24 square inches (4 × 6). A standard sheet of printer paper, 8.5″ × 11″, covers 93.5 square inches (8.5 × 11).

Calculating cubic inches

Cubic inches measure volume. Multiply length, width, and height, all in inches.

cubic inches = length (in) × width (in) × height (in)

A box 10″ × 8″ × 6″ holds 480 cubic inches (10 × 8 × 6). Cubic inches are the standard unit for engine displacement in classic American car specs. A “350 cubic inch” engine, for example, runs about 5.7 liters. Coolers, moving boxes, and other small storage volumes get measured the same way.

Worked example: converting a real box to cubic feet

Shipping and storage specs sometimes need cubic feet instead of cubic inches. Take a moving box measuring 18″ × 14″ × 12″:

1. Volume in cubic inches = 18 × 14 × 12 = 3,024 cubic inches
2. Since 1 cubic foot = 1,728 cubic inches, divide: 3,024 ÷ 1,728 = 1.75 cubic feet.

That’s how a moving company converts a box’s dimensions into the cubic footage it uses for truck-space estimates.

Quick reference: notation and conversions

Notation / unit	Meaning	Common conversion
5′	5 feet	1 ft = 12 in
8″	8 inches	1 in = 2.54 cm
5’8″	5 feet, 8 inches	= 68 inches total
40°26’46”	Degrees, minutes, seconds	Coordinate notation, not length
24 sq in	Square inches	144 sq in = 1 sq ft
480 cu in	Cubic inches	1,728 cu in = 1 cu ft
m	Meters	1 m = 39.37 in
d (diameter)	Distance across a circle	radius × 2 = d

Reading feet-and-inches notation without a ruler in hand

Read a measurement like 5’8″ left to right. The number before the single tick is feet. The number before the double tick is inches.

The inches number always stays under 12. If you ever see something like 5’14”, that’s a formatting error. Fourteen inches should roll into the feet count as 1’2″ added on top, making it 6’2″.

Measuring inches without a ruler

In a pinch, an average adult’s thumb runs roughly 1 inch across at its widest point. Measure from the first knuckle. A standard credit card measures 3.37 inches long.

Both work as rough field references, not replacements for an actual measuring tool when precision matters. A US dollar bill runs 6.14 inches long — a handy reference for longer rough estimates. A sheet of printer paper measures 11 inches on its long edge.

Frequently Asked Questions

What does the ‘ symbol mean in measurements?
Feet, when used after a number in a length context — 5′ means 5 feet. It’s also used for “minutes” in geographic coordinates and, less commonly, in older time notation. Context tells you which.

What does the ” symbol mean in measurements?
Inches, when used after a number in a length context — 8″ means 8 inches. It’s also used for “seconds” in geographic coordinate notation, again distinguished by the surrounding context.

What does m stand for in measurement?
Meters, the base unit of length in the metric system. A lowercase “m” is the standard SI abbreviation. Don’t confuse it with “mi” (miles) or “mm” (millimeters) — those represent very different distances.

What does d mean in measurements?
Most commonly, diameter — the distance across a circle through its center, used heavily in plumbing, machining, and engineering specs. In nail sizes specifically, “d” instead stands for “penny,” an old English sizing convention unrelated to diameter.

How many square inches are in a square foot?
144 square inches. Since 1 foot equals 12 inches, a square foot works out to 12 × 12 = 144 square inches.

How many cubic inches are in a cubic foot?
1,728 cubic inches. Since 1 foot equals 12 inches, a cubic foot works out to 12 × 12 × 12 = 1,728 cubic inches.

How do I measure inches with just my fingers?
An average adult thumb, measured across the base of the nail, comes close to 1 inch. The span from thumb tip to index fingertip, stretched wide, runs roughly 8 to 9 inches for an average adult hand. Neither is precise, but both work for quick, rough field estimates when a ruler isn’t handy.