Inches to Centimeters Conversion for Patternmaking

One inch is exactly 2.54 centimeters: it is an exact mathematical conversion, not an approximation, fixed by international agreement since 1959. Every fraction of an inch that appears in patterns and CAD programs of North American origin (1/8", 1/4", 1/2", 5/8", among others) comes from that number, each with its exact equivalent in centimeters. Knowing that chart and checking which unit a file is set to before importing a DXF keeps a pattern from ending up at the wrong scale.

Modeltex production guide. Updated on Sep 8, 2026.

How many centimeters are in an inch, exactly

The value is not an approximation: since 1959, by international agreement among the United States, the United Kingdom and other English-speaking countries, 1 inch (in, or the " symbol) is defined as exactly 2.54 centimeters. It is not 2.5 or 2.55: it is exactly 2.54, and every other conversion between the two systems comes from it.

The formula works both ways: to go from inches to centimeters, multiply by 2.54; to go from centimeters to inches, divide by 2.54. A 60-inch fabric width is 60 × 2.54 = 152.4 cm; a 150 cm plotter roll is 150 ÷ 2.54 = 59.06 inches.

Chart of inch fractions used in sewing

In sewing patterns of North American origin, small measurements are almost never written as decimal inches but as fractions whose denominator is a power of two: eighths and sixteenths, found in seam allowances, in the spacing between notches or in the height of a hem. The chart gives the equivalent in centimeters, rounded to the hundredth; the exact value of 1/8", for example, is 0.3175 cm, rounded to 0.32 cm.

For finer fractions the same logic works: 1/16" is 0.0625 inches, which multiplied by 2.54 gives 0.16 cm. It is enough to convert the fraction to a decimal inch and multiply by 2.54, with no need to memorize a longer chart.

Why fractional inches are still alive in North American patterns and CAD

The United States never completed the switch to the metric system in its everyday industry and commerce, and apparel was no exception: rulers, tape measures, size charts and CAD programs of North American origin remain in inches out of habit and for compatibility with decades of patternmaking and documentation already done in that unit.

CAD programs of that origin let you set the file to inches or fractions of an inch as the native unit, and a large share of commercial patterns, magazines and English-language courses still publish their measurements that way, including the standard home-sewing seam allowance, almost always in eighths of an inch.

Why most pattern shops outside the United States work in centimeters and millimeters

Most of the world works in the metric system, and the garment industry that follows it (tape measures, tailor's rulers, size charts, patternmaking schools and publishers) works in centimeters and millimeters. It is what a patternmaking school teaches in most countries and the default in the technical sheets Modeltex produces, which come from more than 18 years in the Argentine textile industry.

That matters when you mix suppliers or origins: a pattern made in a metric country, a pattern from a US magazine and a DXF exported from a CAD set to inches can sit side by side in the same workshop without anything telling you, at a glance, which unit each one is in.

How to convert without losing precision: practical tips

The first rule is to check the unit set in your CAD before importing a DXF, not after: Optitex, Audaces, Gerber and Lectra let you set the working unit, and if the file is in inches while the system expects millimeters, the whole pattern comes in 25.4 times too small (2.54 times too small if the system expects centimeters) and the program gives no warning, because to the software they are just numbers.

Convert first with the exact formula and round only at the end, to about half a millimeter, the precision you can realistically mark and cut to: a 5/8" allowance is exactly 1.5875 cm, which is marked as 1.6 cm, and a 24-inch length is 60.96 cm, which rounds to 61 cm. Apply that rounding once, at the end: rounding fraction by fraction accumulates more error.

Check the scale after importing a DXF in inches

Before making a marker or cutting from a converted file, run a simple test: measure in the program a piece whose real measurement you already know, for example the waist girth of the base size, and compare it with that figure. If it comes out 2.54 or 25.4 times smaller or larger than expected, the import unit was set wrong and has to be redone before going on, not corrected by hand piece by piece.

The check matters even more when the DXF passes through more than one program, because every format conversion is one more chance for the unit to get lost along the way. Recording the file's original unit in the pattern's tech pack saves you from guessing the next time it is opened.

Patterns that already come in the unit your workshop uses

For a workshop that works in centimeters and millimeters, avoiding this conversion is as simple as starting from patterns that already come in those units. Modeltex digital patterns are delivered as A4 PDF and plotter PDF with measurements in centimeters, and also as DXF/AAMA, PDS, MRK and ADS for Optitex, Audaces, Gerber and Lectra, with the unit set explicitly.

Conversion becomes necessary mainly with an imported pattern, an English-language magazine or a CAD file that came from a supplier in another country. Keeping the chart and the formula at hand, and the habit of checking the unit before importing, keeps that pattern from ending up at the wrong scale.

Frequently asked questions

How many centimeters is exactly one inch?

One inch is exactly 2.54 centimeters, by the international definition in force since 1959, with no approximation. To convert any measurement from inches to centimeters, multiply it by 2.54; to go the other way, divide by 2.54.

What is the difference between the metric system and fractional inches in patternmaking?

The metric system measures in centimeters and millimeters with decimal divisions, and it is used in most of the world. The fractional inch system, used mainly in the United States, divides the inch into successive halves (1/2, 1/4, 1/8, 1/16) instead of decimals, and appears in patterns, magazines and CAD programs of North American origin. The two convert into each other with the same exact constant, 2.54 cm per inch, but it is best not to mix them inside one pattern without putting everything in a single unit.

How do I convert 1/4 inch to centimeters?

1/4 inch is 0.25 inches; multiplied by 2.54 it gives 0.635 cm, which rounds to 0.64 cm. The same procedure works for any fraction: convert it to a decimal inch and multiply by 2.54.

Why do American sewing patterns use inches instead of centimeters?

Because the United States never completed the switch to the metric system in its everyday industry and commerce, and the patterns and CAD programs of that origin stay in that unit out of habit and for compatibility with decades of patterns already published. The home-sewing seam allowance in that country, for example, is usually 5/8 inch, or 1.59 cm.

What happens if I import a DXF in inches into a program set to millimeters?

The pattern comes in 25.4 times too small (or too large, if the mix-up goes the other way) with no warning from the program, because to the software they are just numbers with no implied unit; with a program set to centimeters the error is 2.54 times. Check and set the import unit before opening the file, then confirm by measuring a piece whose real measurement you already know.

Should I round inch-to-centimeter conversions?

Yes, but at the end of the calculation, not at each intermediate step: convert with the exact formula and only then round to about half a millimeter, the precision you can realistically mark and cut to. Rounding several fractions separately before adding them accumulates more error than rounding the final result once.

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