Points, millimetres, and two different inches

The two inches

There are two competing definitions of a “unit” in graphics, and both are correct in their own world.

• The PostScript point. 72 points to the inch. This is the unit of typesetting, of PDF, and of everything that goes to a press. A 12 pt type size is one sixth of an inch of em, and a 3 mm bleed is 8.504 pt.
• The CSS pixel. 96 to the inch. This is the unit of the browser and therefore of SVG when no units are given.

Neither has anything to do with the dots on a printer. A vector file has no dpi at all. When somebody asks for a “300 dpi logo”, they are asking a question with no answer; the answer they need is a finished size in millimetres.

The arithmetic that catches everyone

Inkscape before version 0.92 used 90 units per inch. From 0.92 onwards it uses 96, to match the SVG standard. Old files that stored bare numbers with no unit suffix were therefore reinterpreted.

Draw a 100 mm square in Inkscape 0.91. The file stores:

100 mm ÷ 25.4 mm/inch × 90 units/inch = 354.33 units

Open it in Inkscape 1.x, which reads those same 354.33 units at 96 to the inch:

354.33 ÷ 96 × 25.4 = 93.75 mm

The square is now 93.75 mm. Nothing is corrupt. Two programs disagreed about what a unit is, and the file never said. Inkscape 1.x offers a scaling fix on open for exactly this reason, and if you inherit old artwork you should measure a known dimension before trusting anything.

The same class of error explains why an EPS from a 1990s archive sometimes opens at 133% in a modern application, and why a DXF for a cutting machine can arrive in inches when the shop works in millimetres. Always measure one known edge after importing.

Set the document up properly, once

Decide the unit from the output, not the software default.

• Print of any kind: millimetres. Set the document units to mm and the page to trim size. A visiting card is 90 × 54 mm in India, 85 × 55 mm in most of Europe, 89 × 51 mm in the US. Ask; do not assume.
• Screen and icons: pixels, and a viewBox that is a round number.
• Signage and cutting: millimetres, and build at 1:1 if the machine can take it. Large-format software usually can. If you must work at a scale, put it in the filename: hoarding-1-to-10.svg.

Where to set it:

• Inkscape: File ▸ Document Properties. Set Display units and the page size, and check Scale is 1.
• Illustrator: File ▸ Document Setup ▸ Units, and Edit ▸ Preferences ▸ Units for the rulers.
• CorelDRAW: the units dropdown on the property bar with nothing selected.
• Scribus: File ▸ Document Setup ▸ Document ▸ Units.

Stroke weights live in the same trap

A 1 pt rule is 0.353 mm. A 1 px rule at 96 dpi is 0.265 mm. A 0.5 pt rule is 0.176 mm, which is about as thin as offset litho reliably holds. If you draw in pixels and print in millimetres, every hairline in the job is 25% thinner than you think it is.

Get in the habit of quoting line weights in millimetres when you talk to a press. “Point-two-five millimetre keyline” is unambiguous. “Half a point” invites the question “half of whose point?”

The one number worth memorising

25.4. Millimetres to the inch. Every other conversion in this subject is that number with a 72 or a 96 beside it:

• mm → pt: × 72 ÷ 25.4 (so 1 mm = 2.835 pt)
• mm → px at 96: × 96 ÷ 25.4 (so 1 mm = 3.780 px)
• pt → px: × 96 ÷ 72 (so 1 pt = 1.333 px)

A 3 mm bleed is 8.5 pt or 11.34 px. An A4 page is 210 × 297 mm, 595 × 842 pt, 794 × 1123 px. You will use these enough to stop looking them up.

One more habit worth forming: when a client or a shop sends a size, write back with the millimetres. “You said A5: that is 148 × 210 mm trim, and I will supply 154 × 216 with 3 mm bleed.” Half the size disputes in print are two people using the same word for different measurements, and one sentence settles it before any drawing happens.