Research poster font size for A0

Every poster in a conference hall is competing with the aisle. People walk, they slow down, they read the title from three or four metres, and only if that works do they come close enough to read anything else. Type size is the mechanism by which that sequence either happens or does not, and it is one of the few design decisions on a poster that has an arithmetic answer.

The viewing-distance rule

Start with what the eye can do. A person with 20/20 vision resolves detail down to about one minute of arc, and a letter at that threshold subtends five arcminutes. Threshold is not comfort: reading continuously is comfortable at roughly three to five times the threshold size.

Rather than derive a comfort factor from first principles, it is easier to anchor on a published figure and scale it. SPIE’s poster guidelines state that at 24 point body text “most people can comfortably read the text from about 3 ft away”. Three feet is 914 mm, which gives a ratio of about 38 mm of viewing distance per point of type. Scale it and you get a ladder:

• 0.5 m away: 13 points. This is a reference list, read by someone standing at the board.
• 1.0 m: 26 points. Body copy, read by someone who has stopped.
• 1.5 m: 39 points. Section headings, read by someone deciding whether to stop.
• 2.0 m: 53 points. A key number, read across a gap.
• 3.0 m: 79 points. The title, read from the aisle.

Sanity check that against the acuity figures: 24 point type has a cap height of roughly 5.9 mm, which at 914 mm subtends about 22 arcminutes, or four and a half times the five arcminute threshold letter. That is the right order of magnitude for comfortable reading, so the ladder holds.

The rule is linear and it is unforgiving. If you want the title read from four metres, it needs to be about 105 points, and no amount of bold will substitute.

Recommended sizes by element

Published guidance is more consistent than poster advice generally is. SPIE gives 72 to 100 points for the title, 36 to 60 for author names and affiliations, 36 to 48 for section headings, 24 to 32 for body text, 24 to 30 for figure titles, 18 to 24 for axis labels, and 14 to 18 for footnotes and references. The NYU library guide takes a different route to a similar place: important information should be readable from about ten feet, with a total of 300 to 800 words.

Put those next to the distance ladder and they line up. Body copy at 24 to 32 points is comfortable at 0.9 to 1.2 m, which is where a reader stands. A title at 72 to 100 points is comfortable at 2.7 to 3.8 m, which is the aisle.

What the A0 layouts actually use, in points

The table on this page is the measured version. It comes from the same 40 A0 layouts counted in what 40 poster templates have in common, read block by block and converted from canvas pixels to print points.

The conversion is exact and worth understanding, because it makes every number on this page checkable. A0 portrait is 841 by 1189 mm, and the working canvas is 1656 by 2340 pixels. 841 mm is 33.11 inches, and 1656 divided by 33.11 is 50.0, so the canvas is exactly 50 pixels per inch. A point is one seventy-second of an inch, so one canvas pixel is 1.44 points. A 19 pixel body size is 27.4 points on the printed board.

Three things stand out from the measured ramp.

Body text is remarkably consistent. 173 of the 178 section blocks across all 40 layouts use exactly 19 pixels, which is 27.4 points. That sits in the middle of the 24 to 32 point recommendation and is comfortable at about 1.0 m. The five exceptions run at 14 or 15 pixels, which is 20 to 22 points, and those are in tight sidebar blocks.

Section headings are fixed at 28 pixels, exactly 40 points, on every one of the 178. That is at the lower end of SPIE’s 36 to 48 and it is comfortable at 1.5 m, which is the distance at which a passer-by decides whether to stop. Headings are doing navigation work, not reading work.

The gap between title and body is enormous, and deliberate. A portrait title at 89 points is more than three times the body size. That ratio is what lets the board work at two distances at once.

Line length and columns

Size is only half of legibility. The other half is how many characters sit on a line.

Purrington recommends text boxes of 45 to 65 characters. WCAG’s visual presentation criterion sets 80 as the upper bound for readable body text. Measuring the 178 section blocks here, assuming an average character width of half the font size, gives a median of 69 characters per line, with 93 of the 178 above 65 and 22 above 80.

That is the single most fixable weakness in a poster layout. A two-column portrait board gives each section roughly 900 pixels of inner width, and at 19 pixel body text that is around 95 characters. The fix is not a smaller font, which makes it worse; it is a narrower block, a third column, or a shorter paragraph.

Figures: the labels inside the chart

A figure’s own type is the size people forget, because charts are usually drawn at screen scale and pasted in. SPIE asks for 18 to 24 points on axis labels and 24 to 30 on figure titles.

The captions in these layouts are set at 16 pixels, which is 23 points, comfortable at about 0.9 m. Axis labels drawn inside a chart need to be at least that, and preferably larger, because they are what makes the picture mean something. A chart whose axis labels are half the size of the caption underneath it is a decorative shape.

References: the one place small is allowed

Reference lists in these layouts run at 12 to 15 pixels, which is 17 to 22 points, and the median is 20. That is comfortable at about 0.75 m, which is a person standing at the board with their nose close to it. It is also right at the bottom of SPIE’s 14 to 18 point footnote range, which means there is no room to go smaller.

Six of the 40 layouts drop references altogether. If yours keeps them, keep the list to the few that a reader might follow, and put the rest behind the QR code.

Check your own file

Two checks before you send anything to a print shop.

First, measure the type against the physical sheet, not against your screen. The A0 poster checker opens a print-ready PDF, JPEG or PNG, draws the trim, bleed and keep-clear margins over the artwork, and marks low-resolution images and small type, then produces a report you can paste into an email to the printer. It does not fix anything and it cannot proof colour, but it will tell you where the problems are.

Second, print one column at 100% on A4 and read it at arm’s length. If the body copy is uncomfortable at that distance, it will be worse at a metre and a half.

If you are still choosing a sheet size, the flyer sizes guide has A0 alongside every other canvas, and poster DPI covers why 100 dots per inch at A0 is enough for a board and not enough for a photograph.

The example

The public health poster is a portrait board built on a layout that opens with four key numbers. Its title is set at 62 canvas pixels, which is 89 points, readable from about 3.4 m. Its four key numbers are 58 pixels, or 84 points. Its section body is 19 pixels, or 27 points, comfortable at a metre. Its reference list is 14 pixels, or 20 points, the smallest text anywhere on the board and still above the 14 point floor. Five sizes, five distances, one board.