{"course":{"slug":"vector-and-print","title":"Vectors, Logos and Things That Get Printed","url":"https://addaly.com/learn/vector-and-print","level":"beginner","track":"create"},"lesson":{"slug":"units-and-the-dpi-that-is-not-there","title":"Points, millimetres, and two different inches","url":"https://addaly.com/learn/vector-and-print/units-and-the-dpi-that-is-not-there","position":3,"of":73,"minutes":9,"answer":"A vector file stores unitless numbers plus a rule for how many of them make an inch, and because PostScript says 72 and the web says 96.","keyIdea":"A vector file stores unitless numbers plus a rule for how many of them make an inch, and because PostScript says 72 and the web says 96, a file that does not state its units can open at the wrong size in perfectly working software."},"body":{"format":"markdown","text":"## The two inches\n\nThere are two competing definitions of a \"unit\" in graphics, and both are correct in their own world.\n\n- **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.\n- **The CSS pixel.** 96 to the inch. This is the unit of the browser and therefore of SVG when no units are given.\n\nNeither 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.\n\n## The arithmetic that catches everyone\n\nInkscape 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.\n\nDraw a 100 mm square in Inkscape 0.91. The file stores:\n\n```\n100 mm ÷ 25.4 mm/inch × 90 units/inch = 354.33 units\n```\n\nOpen it in Inkscape 1.x, which reads those same 354.33 units at 96 to the inch:\n\n```\n354.33 ÷ 96 × 25.4 = 93.75 mm\n```\n\nThe 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.\n\n> Figure: A 100 mm square that opens at 93.75 mm. Steps: 1. Draw 100 mm (Inkscape 0.91, at 90 units to the inch); 2. Stores 354.33 (100 divided by 25.4, times 90); 3. Open in 1.x (Which reads units at 96 to the inch); 4. Reads 93.75 mm (354.33 divided by 96, times 25.4). Nothing is corrupt at any stage. The file stored a bare number and never said what a unit was, so two programs answered the question differently. Measure one known edge after every import.\n\nThe 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.**\n\n## Set the document up properly, once\n\nDecide the unit from the output, not the software default.\n\n- **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.\n- **Screen and icons**: pixels, and a viewBox that is a round number.\n- **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`.\n\nWhere to set it:\n\n- **Inkscape**: File ▸ Document Properties. Set *Display units* and the page size, and check *Scale* is 1.\n- **Illustrator**: File ▸ Document Setup ▸ Units, and Edit ▸ Preferences ▸ Units for the rulers.\n- **CorelDRAW**: the units dropdown on the property bar with nothing selected.\n- **Scribus**: File ▸ Document Setup ▸ Document ▸ Units.\n\n## Stroke weights live in the same trap\n\nA 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.\n\nGet 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?\"\n\n## The one number worth memorising\n\n**25.4.** Millimetres to the inch. Every other conversion in this subject is that number with a 72 or a 96 beside it:\n\n- mm → pt: `× 72 ÷ 25.4` (so 1 mm = 2.835 pt)\n- mm → px at 96: `× 96 ÷ 25.4` (so 1 mm = 3.780 px)\n- pt → px: `× 96 ÷ 72` (so 1 pt = 1.333 px)\n\nA 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.\n\nOne 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."},"check":{"question":"A 100 mm square drawn in Inkscape 0.91 is opened in Inkscape 1.x and now measures 93.75 mm. The file is not damaged and both programs are behaving correctly. What happened?","options":[{"text":"The older version saved in points and the newer one reads millimetres, so the square was converted using 72 rather than 25.4.","correct":false,"tempting":"Mixes up the two conversions. Points and millimetres would give a far larger discrepancy than 6.25%, and the file's units were never points."},{"text":"The file stored 354.33 bare units, which 0.91 wrote at 90 per inch and 1.x reads at 96 per inch.","correct":true},{"text":"Inkscape 1.x defaults to a 93.75% document scale, which can be reset in Document Properties.","correct":false,"tempting":"Invents a default. Document scale is 1 unless someone changes it; the discrepancy comes from the units-per-inch constant, not a scale factor."},{"text":"SVG cannot store millimetres, so any physical dimension is approximated and drifts slightly on every save.","correct":false,"tempting":"SVG stores real unit suffixes perfectly well. The problem is files that omit them, leaving the renderer to apply its own constant."}]},"prev":{"slug":"reading-path-data","title":"What the `d` attribute is saying","url":"https://addaly.com/learn/vector-and-print/reading-path-data"},"next":{"slug":"a-path-has-no-thickness","title":"A path has no thickness","url":"https://addaly.com/learn/vector-and-print/a-path-has-no-thickness"},"authorship":"Founder and series editor: Dr Mustafa Mun. Written by AI models to a written standard. Exam keys checked blind by a second model: 3,665 of 3,666 agreed. Academic review invited.","review":{"status":"Draft, open for academic review"},"cite":"Addaly (2026) 'Points, millimetres, and two different inches', in Vectors, Logos and Things That Get Printed. Edited by M. Mun. Addaly. Available at: https://addaly.com/learn/vector-and-print/units-and-the-dpi-that-is-not-there (Accessed: 13 October 2026).","licence":{"name":"CC BY-NC-SA 4.0","url":"https://creativecommons.org/licenses/by-nc-sa/4.0/"},"reader":{"schema":1,"title":"Points, millimetres, and two different inches","courseTitle":"Vectors, Logos and Things That Get Printed","module":{"index":1,"slug":"what-a-vector-is","title":"The thing that is actually in the file"},"answer":null,"keyIdea":"A vector file stores unitless numbers plus a rule for how many of them make an inch, and because PostScript says 72 and the web says 96, a file that does not state its units can open at the wrong size in perfectly working software.","hero":{"kind":"figure","figure":{"t":"figure","label":"Figure","figure":{"type":"flow","steps":[{"label":"Draw 100 mm","detail":"Inkscape 0.91, at 90 units to the inch"},{"label":"Stores 354.33","detail":"100 divided by 25.4, times 90"},{"label":"Open in 1.x","detail":"Which reads units at 96 to the inch"},{"label":"Reads 93.75 mm","detail":"354.33 divided by 96, times 25.4"}],"title":"A 100 mm square that opens at 93.75 mm","caption":"Nothing is corrupt at any stage. The file stored a bare number and never said what a unit was, so two programs answered the question differently. Measure one known edge after every import."},"alt":"Figure: A 100 mm square that opens at 93.75 mm. Steps: 1. Draw 100 mm (Inkscape 0.91, at 90 units to the inch); 2. Stores 354.33 (100 divided by 25.4, times 90); 3. Open in 1.x (Which reads units at 96 to the inch); 4. Reads 93.75 mm (354.33 divided by 96, times 25.4). Nothing is corrupt at any stage. The file stored a bare number and never said what a unit was, so two programs answered the question differently. Measure one known edge after every import."}},"blocks":[{"t":"h2","id":"the-two-inches","runs":[{"text":"The two inches"}]},{"t":"p","runs":[{"text":"There are two competing definitions of a “unit” in graphics, and both are correct in their own world."}]},{"t":"ul","items":[[{"text":"The PostScript point.","b":true},{"text":" 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."}],[{"text":"The CSS pixel.","b":true},{"text":" 96 to the inch. This is the unit of the browser and therefore of SVG when no units are given."}]]},{"t":"p","runs":[{"text":"Neither has anything to do with the dots on a printer. "},{"text":"A vector file has no dpi at all.","b":true},{"text":" 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."}]},{"t":"h2","id":"the-arithmetic-that-catches-everyone","runs":[{"text":"The arithmetic that catches everyone"}]},{"t":"p","runs":[{"text":"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."}]},{"t":"p","runs":[{"text":"Draw a 100 mm square in Inkscape 0.91. The file stores:"}]},{"t":"code","lang":"","label":"Text","text":"100 mm ÷ 25.4 mm/inch × 90 units/inch = 354.33 units"},{"t":"p","runs":[{"text":"Open it in Inkscape 1.x, which reads those same 354.33 units at 96 to the inch:"}]},{"t":"code","lang":"","label":"Text","text":"354.33 ÷ 96 × 25.4 = 93.75 mm"},{"t":"p","runs":[{"text":"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."}]},{"t":"p","runs":[{"text":"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. "},{"text":"Always measure one known edge after importing.","b":true}]},{"t":"h2","id":"set-the-document-up-properly-once","runs":[{"text":"Set the document up properly, once"}]},{"t":"p","runs":[{"text":"Decide the unit from the output, not the software default."}]},{"t":"ul","items":[[{"text":"Print of any kind","b":true},{"text":": 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."}],[{"text":"Screen and icons","b":true},{"text":": pixels, and a viewBox that is a round number."}],[{"text":"Signage and cutting","b":true},{"text":": 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: "},{"text":"hoarding-1-to-10.svg","code":true},{"text":"."}]]},{"t":"p","runs":[{"text":"Where to set it:"}]},{"t":"ul","items":[[{"text":"Inkscape","b":true},{"text":": File ▸ Document Properties. Set "},{"text":"Display units","i":true},{"text":" and the page size, and check "},{"text":"Scale","i":true},{"text":" is 1."}],[{"text":"Illustrator","b":true},{"text":": File ▸ Document Setup ▸ Units, and Edit ▸ Preferences ▸ Units for the rulers."}],[{"text":"CorelDRAW","b":true},{"text":": the units dropdown on the property bar with nothing selected."}],[{"text":"Scribus","b":true},{"text":": File ▸ Document Setup ▸ Document ▸ Units."}]]},{"t":"h2","id":"stroke-weights-live-in-the-same-trap","runs":[{"text":"Stroke weights live in the same trap"}]},{"t":"p","runs":[{"text":"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."}]},{"t":"p","runs":[{"text":"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?”"}]},{"t":"h2","id":"the-one-number-worth-memorising","runs":[{"text":"The one number worth memorising"}]},{"t":"p","runs":[{"text":"25.4.","b":true},{"text":" Millimetres to the inch. Every other conversion in this subject is that number with a 72 or a 96 beside it:"}]},{"t":"ul","items":[[{"text":"mm → pt: "},{"text":"× 72 ÷ 25.4","code":true},{"text":" (so 1 mm = 2.835 pt)"}],[{"text":"mm → px at 96: "},{"text":"× 96 ÷ 25.4","code":true},{"text":" (so 1 mm = 3.780 px)"}],[{"text":"pt → px: "},{"text":"× 96 ÷ 72","code":true},{"text":" (so 1 pt = 1.333 px)"}]]},{"t":"p","runs":[{"text":"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."}]},{"t":"p","runs":[{"text":"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."}]}],"check":{"question":[{"text":"A 100 mm square drawn in Inkscape 0.91 is opened in Inkscape 1.x and now measures 93.75 mm. The file is not damaged and both programs are behaving correctly. What happened?"}],"options":[{"text":[{"text":"The older version saved in points and the newer one reads millimetres, so the square was converted using 72 rather than 25.4."}],"correct":false,"tempting":[{"text":"Mixes up the two conversions. Points and millimetres would give a far larger discrepancy than 6.25%, and the file’s units were never points."}]},{"text":[{"text":"The file stored 354.33 bare units, which 0.91 wrote at 90 per inch and 1.x reads at 96 per inch."}],"correct":true,"tempting":null},{"text":[{"text":"Inkscape 1.x defaults to a 93.75% document scale, which can be reset in Document Properties."}],"correct":false,"tempting":[{"text":"Invents a default. Document scale is 1 unless someone changes it; the discrepancy comes from the units-per-inch constant, not a scale factor."}]},{"text":[{"text":"SVG cannot store millimetres, so any physical dimension is approximated and drifts slightly on every save."}],"correct":false,"tempting":[{"text":"SVG stores real unit suffixes perfectly well. The problem is files that omit them, leaving the renderer to apply its own constant."}]}],"order":[2,3,0,1],"why":null},"prev":{"slug":"reading-path-data","title":"What the `d` attribute is saying","minutes":10},"next":{"slug":"a-path-has-no-thickness","title":"A path has no thickness","minutes":9}}}