Why print maths catches people out
Screens are forgiving. Print is not. A layout that looks crisp on a monitor can arrive from the printer soft and pixelated, a paper described as "80 lb" can be either flimsy or stiff as card depending on a word nobody mentioned, and a book cover can come back with the title wrapped around onto the front because the spine was calculated a millimetre and a half too narrow.
All three of those failures come from the same root cause: print uses physical units and digital design does not. A pixel has no size until you decide how many of them go into an inch. A sheet of paper has no weight until you say which basis size you are weighing. A spine has no width until you know how thick one sheet of that specific stock is. Every conversion on this site sits on one of those bridges between the digital file and the physical object.
The tools here exist because the answers are genuinely simple once someone shows you the formula — and almost every calculator online hides it. We would rather you finished understanding the maths than kept coming back.
The three numbers that decide print quality
1. Resolution — pixels per inch
DPI (dots per inch) and PPI (pixels per inch) are used interchangeably in everyday conversation, though strictly PPI describes an image and DPI describes what a printer physically lays down. The practical relationship is one division:
Print size (inches) = pixel dimension ÷ DPI
A 3000 × 2000 pixel photo at 300 DPI prints at 10 × 6.67 inches. The same file at 150 DPI prints at 20 × 13.3 inches — twice the size, half the quality.
Nothing about the file changes when you change the DPI setting. You are not adding detail, you are only deciding how densely to pack the pixels you already have. This is the single most misunderstood idea in print preparation: "set it to 300 DPI" does nothing useful unless the pixel count supports the size you want.
Three hundred DPI is the conventional target for anything held in the hand — books, brochures, business cards, photo prints. It is not a magic threshold, it is roughly where the human eye stops resolving individual dots at normal reading distance. Step back and the requirement collapses: a poster read from two metres away is fine at 150 DPI, and a billboard read from thirty metres can be 15 DPI without anyone noticing. The DPI calculator works this out in either direction and tells you whether the result will actually hold up.
2. Paper weight — and the basis size trap
Ask for "80 lb paper" and you have not specified anything. Eighty pound text is a fairly ordinary brochure sheet at about 118 gsm. Eighty pound cover is stiff card at about 216 gsm. Same number, nearly double the substance, because the two are weighed on completely different basis sizes.
US basis weight is the weight in pounds of 500 sheets of a paper's basic size — and the basic size differs by paper grade. Bond is measured at 17 × 22 inches, text at 25 × 38, cover at 20 × 26. Nobody sells paper at those dimensions; they are historical reference sizes from the mills. The number on the label is the weight of five hundred sheets you will never see.
The metric system sidesteps all of this. Gsm is grams per square metre, full stop. One square metre of any paper, any grade, any size. A 120 gsm sheet is a 120 gsm sheet. This is why print shops outside the US almost never argue about paper weight and US print shops constantly do. Our paper weight converter handles all five common basis sizes and shows the working.
3. Caliper — thickness, which is not weight
Two papers of identical gsm can be noticeably different thicknesses. Bulk — how much air the fibres trap — varies with the pulp, the coating and the calendering. A coated 100 gsm sheet is compressed and thin; an uncoated bulky novel paper at the same weight is visibly thicker. Publishers exploit this deliberately, choosing high-bulk stock so a short novel still has a respectable spine.
This matters the moment you print anything bound, because spine width is driven by caliper and not by weight. That is why the spine width calculator asks for a paper stock rather than a gsm figure — it needs pages-per-inch, which only the mill or the printer can tell you.
A quick reference for the numbers people look up most
| Job | Target resolution | Typical viewing distance |
|---|---|---|
| Photo print, book, business card | 300 DPI | 25–40 cm |
| Brochure, magazine page | 300 DPI | 30–50 cm |
| A3 poster | 200–300 DPI | 0.5–1 m |
| Large poster, roll-up banner | 120–150 DPI | 1.5–3 m |
| Exhibition backdrop | 72–100 DPI | 3–6 m |
| Billboard | 10–20 DPI | 20 m+ |
| Common paper | Metric | US equivalent |
|---|---|---|
| Office copier paper | 75–80 gsm | 20 lb bond |
| Good letterhead | 90–120 gsm | 24–32 lb bond |
| Brochure / flyer | 130–170 gsm | 90–115 lb text |
| Postcard | 250–300 gsm | 92–111 lb cover |
| Business card | 300–400 gsm | 111–148 lb cover |
How we build these tools
Four rules, applied to every page on this site:
- Everything runs in your browser. The calculations are plain JavaScript on the page you are already looking at. Nothing you enter is transmitted, logged or stored.
- The formula is always shown. Every tool page sets out the arithmetic, a worked example with real numbers, and the assumptions it relies on.
- Limits are stated honestly. Where a result is an estimate — and in printing, most are — we say so and explain what would make it wrong.
- Fast on any connection. Each page is a single self-contained file with no external fonts, frameworks or libraries.