1 Inches to Centimeters

One inch is 2.54 centimeters, exactly

One inch is 2.54 centimeters, and no rounding is hiding in that number. The 1959 yard and pound agreement fixed the international yard at 0.9144 m, which makes a foot 0.3048 m and an inch 0.0254 m. NIST publishes the chain on its survey foot page, and the meter behind it comes from the speed of light in the BIPM SI brochure. Because the inch is defined by the meter, the 2.54 is exact to as many decimals as you care to write.

That single number covers most of the conversions people search for. One inch is 25.4 millimeters, 0.0254 meters, and 1/12 of a foot, or 0.08333 ft. It is 1/36 of a yard. A ruler with inch marks and a ruler with centimeter marks describe the same length, which is why 2.54 shows up on the back of every school ruler and on the scale bar of most maps printed in the US.

The reason an inch feels like a big unit is that 2.54 cm is a wide gap on a centimeter ruler. A 1 cm mark is 0.394 in, so an inch is two and a half centimeters, and a 10 cm object is 3.94 in, just under 4 in. That mismatch is the source of nearly every size surprise when a metric pattern meets an imperial ruler.

Where the inch sits among feet, yards and millimeters

The inch is the smallest unit in the US customary chain that people still use by name. Twelve inches make a foot, three feet make a yard, and 5,280 feet make a mile. In metric terms one inch is 25.4 mm, and that is the number machinists and engineers reach for, because a millimeter is a workable shop unit and 25.4 is easy to halve and quarter.

Print work still runs on the inch even in metric countries, because the desktop publishing point is 1/72 in, a definition that predates the switch. A 12 pt font is 12/72 in tall, or 0.167 in, which is 4.23 mm. Page margins in a word processor default to 1 in, or 2.54 cm, in every locale, and that default is why a document set up in Chicago and one set up in Berlin line up when printed.

One inch of rain and one inch of snow

Weather is where a single inch turns into a real amount of water. A rain gauge that catches 1 in has caught 2.54 cm, and a forecast of 25 mm is 0.98 in, close enough that most people treat the two as the same storm. On a roof, the volume adds up fast: 1 in of rain over 1,000 sq ft is 83.33 cu ft of water, and at 7.48 gal per cu ft that is 623 gal running into the gutters. A 2,000 sq ft roof doubles it to 1,246 gal, and a 50 gal rain barrel is full after about 0.08 in of rain on that 1,000 sq ft roof.

Snow is the opposite problem, because the inch on the ground is not the inch of water. Forecasters often use a 10 to 1 ratio, so 1 in of liquid water becomes about 10 in of snow, and 2.54 cm of water becomes 25.4 cm of snow. Wet snow near freezing can run 5 to 1, which puts 5 in on the driveway from the same water, while dry powder in cold air can run 20 to 1 and pile up 20 in. That spread is why a 10 in snow forecast can arrive as 5 in or as 20 in and still be technically correct.

For shoveling, the water content is what matters to your back. Ten inches of wet snow at 5 to 1 holds 2 in of water, roughly 10.4 lb per sq ft, while 20 in of powder at 20 to 1 holds the same 1 in of water and about 5.2 lb per sq ft. The depth on the news is a poor guide to the weight on the roof.

Lumber and pipe that carry an inch in the name

A 1 in board is not 1 in thick. Dimensional lumber is sold by nominal size and planed down after drying, so a 1x4 is 3/4 in thick and 3 1/2 in wide, which is 1.905 cm by 8.89 cm. A 1x6 is 3/4 in by 5 1/2 in, or 1.905 cm by 13.97 cm. The 1 in in the name survives from the rough green board, and the 1/4 in that planing removes is the most common source of error in a first woodworking project.

Pipe is worse, because the inch in the name is neither the outside nor the inside diameter. A 1 in nominal steel pipe has a 1.315 in outside diameter, or 3.34 cm, and a schedule 40 wall of 0.133 in, which is 3.38 mm thick. That leaves an inside diameter of 1.049 in, or 2.66 cm. Copper tube follows a different rule: 1 in nominal copper has a 1.125 in outside diameter, or 2.86 cm, and the wall thickness depends on whether it is type L or type M.

The one place the inch is honest is fasteners. A 1 in wood screw is 1 in long from the tip to the underside of the head, which is 25.4 mm, and a 1 in washer has a 1 in inside diameter. When a spec sheet says 1 in, check which dimension it means before you order, because lumber, pipe and bolts all mean something different by it.

Screens, camera sensors and photo prints

Screen sizes are diagonal inches, so a 6.1 in phone has a 15.49 cm diagonal, and a 27 in monitor is 68.58 cm across the corner. The bezel and the aspect ratio decide the width and height, not the diagonal, so a 6.1 in phone at 19.5 by 9 is about 14.1 cm tall and 6.5 cm wide. That is the number that matters when you check whether it fits a pocket or a car mount.

Camera sensors are the trap. A so-called 1 inch sensor is not 25.4 mm across. The label comes from old vidicon tube sizes, and a modern 1 in sensor measures about 13.2 by 8.8 mm, with a 15.9 mm diagonal. Its area is roughly 116 sq mm, or 0.18 sq in, well short of the 1 sq in the name suggests. Buyers who compare a 1 in sensor to a micro four thirds sensor are comparing 116 sq mm to 225 sq mm, and the label tells them nothing useful.

Prints are simpler. A 2 by 2 in passport photo is 5.08 by 5.08 cm, a 4 by 6 in print is 10.16 by 15.24 cm, and a 5 by 7 in print is 12.7 by 17.78 cm. Photo frames sold in Europe are sized in centimeters, so a 10 by 15 cm frame takes a 4 by 6 in print with a sliver of crop on the long edge, since 6 in is 15.24 cm.

Fractions of an inch in centimeters

Tape measures and drill charts use fractions, and the metric equivalent of each one is a fixed number. Halving an inch gives 1.27 cm, halving again gives 0.635 cm, and the sixteenths run down to 0.159 cm. The table below is the one to keep on a phone for a hardware store run.

An eighth of an inch is the smallest mark on most carpenter tapes, and it is 3.175 mm, close to the 3 mm a metric drawing would call out. That gap of 0.175 mm sounds tiny until you stack it eight times across a 1 in board and find the row of holes 1.4 mm off. Drill bits are where this bites hardest: a 1/4 in bit is 6.35 mm, and the nearest metric bit is 6.5 mm, which drills a hole 0.15 mm oversize.

Square inches and cubic inches

Area multiplies the factor twice. One square inch is 6.4516 square centimeters, not 2.54, and a 1 sq in patch of skin or a 1 sq in vent is 6.45 cm2. Volume multiplies it three times: one cubic inch is 16.3871 cubic centimeters, which is the same as 16.3871 milliliters. That is why engine displacement, which US buyers read in cubic inches, converts to liters with a factor near 16.39.

A 350 cu in V8 is 5,735 cc, or 5.7 L. A 122 cu in four cylinder is 1,999 cc, or 2.0 L. The rule of thumb is 61 cu in per liter, and it is accurate to about half a percent across the range car makers use. A 1 cu in model engine is 16.39 cc, small enough that the fuel tank is measured in milliliters, and a 1 cu in cube of water weighs 16.39 g, or 0.0361 lb.

Square inches also show up in pressure and in material specs. A 1 sq in piston with 100 psi behind it pushes 100 lb, and a 1 sq in cross section of mild steel can carry about 36,000 lb before it yields. Those numbers only work in square inches, so a metric drawing that gives the area in cm2 needs the 6.4516 factor before the load math makes sense.

Sewing, hems and paper margins

Sewing patterns sold in the US use a 5/8 in seam allowance, which is 1.588 cm, and a 1 in hem is 2.54 cm. A pattern that calls for a 1 in hem on a metric machine is asking for 25 mm, and the difference between 25 mm and 25.4 mm is small enough to ignore on a curtain but not on a tailored jacket. Bias tape comes in 1/2 in and 1 in widths, or 1.27 cm and 2.54 cm, and the fold takes up half of that.

Paper is the other place the inch refuses to leave. US Letter is 8.5 by 11 in, or 21.59 by 27.94 cm, while A4 is 21.0 by 29.7 cm. The two are close enough to look alike and different enough that an A4 page printed on Letter loses 1.76 cm off the bottom, and a Letter page printed on A4 gains 1.76 cm of white space. A 1 in margin on Letter is 2.54 cm, and the same 2.54 cm margin on A4 leaves a wider text block because the page is taller.

Body measurements follow the same conversion. A 1 in change at the waist is 2.54 cm, a 1 in heel is 2.54 cm of lift, and a 1 in cuff is 25.4 mm of fabric turned back. Tailors who work between the two systems keep a 2.54 times table on the wall, because guessing at 2.5 cm per inch builds a 0.4 cm error into every 10 in of a garment, and that error shows at the shoulder seam.

Hem allowances and seam widths come in eighths on US patterns, so the same allowance in centimeters is worth keeping on one card while you sew:

Each centimeter cell is the decimal inch times 2.54, so the 5/8 in seam most patterns assume measures 1.588 cm and a metric pattern that asks for 1.5 cm is about 1/16 in narrower.