Exact conversions (1 inch = 2.54 cm, by definition) and real-world size guides: every length is shown next to objects you can actually hold, see or buy — so the number means something.
An inch is exactly 2.54 centimeters. That is 25.4 millimeters. It has been the legal definition since 1959, in the US and in every country that uses inches.
Each size guide shows one length with a dozen real objects, drawn to scale. Start with 5 inches, 6 inches or 8 inches. New sizes are added every week.
Need a number fast? Use the inches to cm converter or the cm to inches converter. Each one has a full chart from 1 to 100.
3/4 inch is bigger than 5/8 inch. 3/4 is 0.750 inch and 5/8 is 0.625 inch, so 3/4 is larger by exactly 1/8 inch — 0.125 inch, or 3.175 mm. In percentage terms, 3/4 is 20% larger than 5 The…
The most accurate inch reference in an average pocket is a US penny: its diameter is exactly 0.750 inch (19.05 mm), so four pennies in a row measure exactly 3.000 inches. A credit card is better still for longer spans,…
A4 is 210 × 297 mm, which is 8.27 × 11.69 inches. US Letter is 8.5 × 11 inches, which is 215.9 × 279.4 mm. Letter is 0.232 inch (5.9 mm) wider; A4 is 0.693 inch (17.6 mm) taller. Neither…
A US men’s size 9 fits a foot 10.33 inches (26.2 cm) long; a US women’s size 8 fits a foot 9.67 inches (24.6 cm) long. Both scales move in steps of one third of an inch — 0.333 in,…
A US size 7 ring has an inside diameter of 0.681 inch (17.30 mm). The whole US scale is built on a single rule: every full size adds 2.55 mm of inside circumference, which works out at 0.812 mm of…
9 inches is 22.86 centimeters, 228.6 millimeters, or exactly 0.75 feet: three quarters of a foot. It is the size of a standard pie dish, a dinner-size paper plate and the paint roller you use on walls. 9 inches in…
20 cm is 7.87 inches. On a US tape measure, that is the 7 7⁄8 inch mark, just 3.2 mm short of 8 inches. In other units, 20 cm is 200 mm, 0.2 m or 0.66 feet. Centimeters (cm)=Inches (in)…
A standard Pokémon card is 63 × 88 mm, or 2.48 × 3.46 inches (6.3 × 8.8 cm). Most people round it to 2.5 × 3.5 inches, the same size as Magic: The Gathering cards, baseball cards and poker cards.…
An Instagram post is 1080 pixels wide, so its size in inches depends on the resolution: 3.6 inches wide at 300 dpi, 7.2 inches at 150 dpi and 15 inches at 72 dpi. The standard portrait post, 1080 × 1350…
30 cm is 11.81 inches, or 0.98 feet (300 mm). On a US tape measure, that is just past the 11 ¾ inch mark, about 11 13⁄16 inches. It is the length of a standard school ruler, and only 4.8…
The inch is the basic unit of length in the US customary system. Its abbreviation is in. Its symbol is the double prime (″), which most people type as a straight quote: 6" means 6 inches. A single prime (′) means feet, so 5′ 9″ reads as 5 feet 9 inches.
Since July 1, 1959, one inch has been exactly 25.4 millimeters. That date comes from the International Yard and Pound Agreement, signed by the US, the UK, Canada, Australia, New Zealand and South Africa. Before it, the American inch was a hair longer (1/39.37 of a meter, about 25.4000508 mm). The gap is tiny, but it added up over long distances, so land surveys kept the old "survey foot" until it was retired at the end of 2022.
The word comes from the Latin uncia, "one twelfth". It is the same root as "ounce". An inch is still one twelfth of a foot. Many languages name it after the thumb: pouce in French, duim in Dutch. An adult thumb is about an inch wide at the knuckle.
A medieval English statute set the inch at three grains of barley, dry and round, laid end to end. The barleycorn survives in one place: US shoe sizes. One full size up adds a third of an inch.
A US ruler splits each inch in half, then in half again, down to sixteenths. The longer the line, the bigger the fraction. Learn the five line lengths and you can read any tape.
| Mark | Decimal (in) | What the line looks like | How many per inch |
|---|---|---|---|
| 1 inch | 1.0 | The longest line, with a number | Every inch |
| 1/2 inch | 0.5 | The longest line between two numbers | 1 per inch |
| 1/4 inch | 0.25 | Slightly shorter, halfway to the 1/2 line | 2 per inch (1/4, 3/4) |
| 1/8 inch | 0.125 | Shorter again | 4 per inch (1/8, 3/8, 5/8, 7/8) |
| 1/16 inch | 0.0625 | The shortest line on most US tapes | 8 per inch (1/16, 3/16 … 15/16) |
Common fractions are worth knowing by heart: 1/8 inch is 3.2 mm, 3/8 inch is 9.5 mm, 5/8 inch is 15.9 mm and 3/4 inch is 19.1 mm. For every other fraction, the inches to cm chart lists the cm and mm values.
The US is one of very few countries that still measures daily life in inches, feet and miles. Yet the inch is defined by the meter, and has been since 1893. Congress made metric conversion voluntary in 1975. In 1988 it named metric the preferred system for trade. Daily life never fully switched.
So Americans use both, often in the same aisle. Soda comes in 2-liter bottles. Nutrition labels list grams. Medicine is dosed in milligrams and milliliters. Engine sizes are given in liters. Lumber, screens, pizza and people are still measured in inches. That is why "cm to inches" gets more than a million US searches a month. Our cm to inches converter does it in one step.
The same unit can measure very different things. Before you compare two sizes, check what was measured.
| What you measure | What the inches describe | Example |
|---|---|---|
| TV and monitor screens | The diagonal of the screen | A 55-inch 16:9 TV is about 47.9 × 27.0 inches |
| Pizza and cake pans | The diameter, edge to edge | A 16-inch pizza has 4 times the area of an 8-inch one |
| Lumber | A nominal size, cut before drying and planing | A 2 × 4 is really 1½ × 3½ inches |
| Height | Feet plus inches | 5 ft 9 in = 69 inches = 175.26 cm |
| Rain and snow | Depth collected on a flat surface | 1 inch of rain is 2.54 cm of water; as snow it is often about 10 inches, but the ratio varies |
| Shorts | The inseam, crotch to hem | A 5-inch inseam is 2 inches shorter than the classic 7-inch cut |
One more case: screens. Your phone's "inches" is the diagonal of the display, and its sharpness is in pixels per inch. Web design uses a fixed rule of 96 CSS pixels per inch, while photo prints usually use 300 dots per inch. For a worked example, see Instagram post size in inches.
Each object size comes from a published standard or the maker's own spec. Our images are drawn to scale above a ruler. If we can't verify a size, we leave the object out.
The symbol is the double prime (″), usually typed as a straight quote: 12" means 12 inches. The abbreviation is "in". Feet use a single prime (′).
There are 12 inches in a foot and 36 inches in a yard. A foot is 30.48 cm and a yard is 91.44 cm.
3/8 inch is 0.375 inch, or 9.5 mm (0.95 cm). It equals 6/16, so on a tape measure you count six lines past the whole inch.
5/8 inch is 0.625 inch, or 15.9 mm (1.59 cm). It sits one 1/8 mark past the 1/2-inch line.
A US quarter is 0.955 inch across, just under an inch. The top joint of an adult thumb is also about an inch wide.
On the web, 1 inch is 96 CSS pixels by definition. A real screen has its own density, from about 100 pixels per inch on a desktop monitor to more than 400 on many phones.