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Trim Marks

Fraction and Unit Converter

Decimals to fractions, and every typographic unit.

Measurement

Decimal or a fraction — 0.375, 3/8 and 3 3/8 all work.

Result

Exact — this value lands on the fraction with nothing left over.

The same measurement, everywhere else

Points here are PostScript points — 72 to the inch — which is what every design application has meant by a point since the 1980s. The older American point is 1/72.27 inch, a 0.4% difference that only shows up on long measures.

Convert a decimal to the nearest fraction at whatever precision you are working to, and see the same measurement in points, picas, inches, millimetres, centimetres and pixels. The rounding error is reported rather than hidden, because at eighths on a trim measurement it can be nearly two hundredths of an inch.

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Why use this tool?

Six units at once

Points, picas, inches, millimetres, centimetres and pixels, all from one value.

Any precision

Halves through sixty-fourths, reduced to lowest terms — 6/16 comes back as 3/8.

Shows the error

Tells you how far the fraction is from the value you entered, and when it is exact.

Reads fractions too

Type 3 3/8 or 3/8 and it parses them.

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How this fraction and unit converter works

A decimal is multiplied by the denominator, rounded to the nearest whole, then reduced to lowest terms — so 6/16 comes back as 3/8, because that is what anyone would actually write on a drawing. A remainder that rounds up to a full unit carries into the whole number rather than producing something like 2 8/8.

The rounding error is reported rather than swallowed. At sixteenths on an inch measurement the worst case is about three thousandths of an inch, which nobody cares about. At halves it is a quarter of an inch, which on a trim is the difference between a clean edge and a reprint. The tool tells you which situation you are in.

Unit conversion runs through points internally. Everything is defined against the PostScript point — exactly 1/72 inch — because that is what design software has meant by a point since the 1980s. The traditional American point of 1/72.27 inch and the continental Didot point both still exist in the trade, and a converter that does not tell you which one it means is not much use.

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How to use it

  1. Step 1: Enter a value

    A decimal or a fraction. Both directions work.

  2. Step 2: Choose your precision

    Whatever your ruler or spec actually uses.

  3. Step 3: Read across the units

    Every conversion is one click to copy.

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Example usage

Reading an American spec
A drawing dimensioned in sixteenths, converted to millimetres for a European supplier.
Working from a ruler
A physical measurement taken in fractions, turned into the decimal a layout application wants.
Setting a column
A width in picas checked against its millimetre equivalent before it goes into a template.
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Where a unit error actually costs money

Most rounding errors are harmless. The ones that are not cluster around the edge of the page, because that is where a small measurement error stops being invisible and starts being a visible white line down the side of a printed job. Bleed is the usual culprit: artwork has to extend past the trim so that the inevitable drift in cutting still lands on image rather than on paper.

The conventional allowance is 3mm, or an eighth of an inch, and those two are not the same number — 3mm is 0.118in, an eighth is 0.125in. The gap is small enough to ignore and large enough to matter if it compounds with a rounding at the other end. This is exactly the situation where a converter that reports its error is worth more than one that quietly gives you a tidy figure.

The other reliable source of expensive confusion is a mixed-unit supply chain: a designer working in millimetres, a client specifying in inches, and a printer quoting in whichever the press was set up in. The habit worth having is to fix one unit as authoritative for the job, convert everything to it once at the start, and treat every other figure on the spec as a reference note rather than a measurement.

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Frequently asked questions

How many points in a pica?

Twelve, and six picas to an inch. So a pica is 1/6 inch and a point is 1/72 inch — using PostScript points, which is what your design software means.

Which point does this use?

The PostScript point, exactly 1/72 inch. The traditional American point is 1/72.27 inch and the Didot point different again. The 0.4% gap is invisible on a caption and adds up over a page depth.

Why is 96 the pixel figure?

Because CSS defines a pixel as 1/96 inch, which makes 1px exactly 0.75pt. That is a definition rather than a property of any actual screen — physical displays vary enormously.

What is bleed and how much do I need?

Artwork extended past the trim line so that drift in the cutting still lands on image rather than on white paper. 3mm is the usual allowance in metric markets and an eighth of an inch in the US. They are not identical — 3mm is 0.118in — so pick the one your printer specified rather than converting between them.

Should I work in millimetres or inches?

Whichever your printer works in, and then consistently. Millimetres are standard almost everywhere outside the US and avoid fractions entirely, which removes a whole category of error. The important part is not which you choose but that you fix one as authoritative rather than converting back and forth through the job.

Why do my millimetre conversions have such long decimals?

Because an inch is exactly 25.4mm, so most fractional inch values do not land on tidy millimetre figures. An eighth of an inch is 3.175mm. Rounding it to 3.2mm is almost always fine; rounding it repeatedly through several conversions is how a measurement drifts far enough to notice.

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Related tools

Browse every tool in Layout & Measurement Tools.