Six lines, six words. That is the entire 6x6 rule, and it sits at the top of most versions of a slide design checklist for a reason: no more than six lines of text on a slide, no more than six words per line. Nobody agrees on who invented it. It shows up in guidance from schools, military training programmes and corporate decks with no one person’s name reliably attached, and the reasoning behind it traces loosely to research on processing text and images at the same time rather than to one specific, citable study fixing the numbers at exactly six and six.
Why the 6x6 rule works anyway
The number six itself is a round approximation, not a measured threshold discovered in a lab. What holds up regardless is the underlying mechanism: a slide dense with text competes directly with whoever is presenting it. An audience reading a paragraph is not, at the same moment, listening closely to a sentence being spoken aloud, and a slide that demands both at once usually loses the reader’s attention to whichever channel is easier to process, which is normally the text rather than the voice.
That mechanism is also exactly why the 6x6 rule has real, reasonable critics. A slide meant to be read later, shared as a PDF rather than presented live in a room, is not competing with a speaker at all, so the six-and-six limit stops being useful and starts being an arbitrary constraint on a document that needed the detail. The rule is a fit for one specific situation, live presentation, not a universal law of slide design.
The 10-20-30 rule, with a name attached
Unlike the 6x6 rule, the 10-20-30 rule has a clear, well-documented origin. Guy Kawasaki developed it for investor pitch presentations, where losing an audience’s attention carries an immediate and often costly consequence. Ten slides as a hard ceiling, not a target, forces a pitch down to its essential structure and cuts the padding a nervous presenter is tempted to add for reassurance. Twenty minutes respects something real about attention in a room: it does not decline gradually as a talk runs long, it falls off steeply past a certain point, and twenty minutes is roughly where Kawasaki judged that drop-off begins for a pitch audience specifically.
The thirty-point minimum font size is the cleverest part of the rule, because it is really a proxy for content discipline rather than a readability rule on its own. Text that large physically cannot fit much onto a single slide, so the constraint pushes a presenter to speak the supporting detail out loud instead of printing it and reading from the screen, which is a stronger presentation in almost every case anyway.
Where the two rules overlap, and where they don’t
Both rules attack the same underlying failure from different angles: too much text on a slide, competing with a speaker who should be the primary channel of information in the room. The 6x6 rule caps it by counting lines and words directly. The 10-20-30 rule caps it indirectly, through font size and a hard slide ceiling, which has the side benefit of also disciplining how many separate ideas make it into the deck at all, not just how much text sits on each individual slide.
Where they diverge is scope. The 6x6 rule is a per-slide test that can be applied to any single slide in isolation. The 10-20-30 rule is a whole-deck constraint, and its ten-slide ceiling makes it a poor fit for anything longer than a short pitch, such as a technical training deck or a lengthy quarterly review, where ten slides would force out information the audience genuinely needs.
Examples of each rule catching a real problem
A slide titled “Q3 Sales Overview,” carrying a paragraph of context, a bulleted list of caveats and a small-print source line, is a textbook 6x6 failure: eleven or twelve lines of text competing with whoever is meant to be talking through it, none of them large enough to read from the back of a room. Rewriting that same slide down to a headline stating the actual finding and a single supporting chart usually gets it under six lines without losing anything the audience needed, since most of what got cut was throat-clearing rather than substance.
A pitch deck that runs to twenty-two slides and forty-five minutes is a 10-20-30 failure of a different kind, and it is rarely a text-density problem at all — it is a structure problem, too many separate ideas competing for one meeting’s worth of attention. Cutting a deck like that down to Kawasaki’s ten-slide ceiling usually means asking which three or four points the pitch actually depends on and building the other slides as backup material, held in reserve for questions rather than presented by default.
Font size failures are the easiest to spot and the easiest to excuse. A dense financial table shrunk to fourteen-point type to fit onto one slide might look complete on a laptop screen a few feet from the presenter, and unreadable from the fifth row of an actual room. The thirty-point rule exists precisely to force that table into a decision: either split it across two slides, or extract the one number the audience actually needs and speak the rest.
A practical slide design checklist
Pulling both rules together into something usable under real deadline pressure: count the lines of text on the slide (six or fewer is the target); count the words on the longest line (about six or fewer); check the smallest font size anywhere on the slide (thirty points or larger for anything meant to be read from more than a few feet away); and, for a full pitch deck specifically, count the total slides against a ten-slide ceiling and time a full run-through against twenty minutes.
None of these four checks require design training, and running all four on a finished deck takes a few minutes. The fix the slide game below applies exactly this checklist to three real slides, each hiding five separate faults, some of them 6x6 or 10-20-30 violations and some of them other design problems entirely — a legend sitting too far from its chart, a bar chart with a cut axis, false precision in a headline number. Spotting five faults per slide, deliberately, is a faster way to build the instinct than memorising the two rules and hoping to notice a violation cold, in the middle of building an actual deck against a deadline.
What these rules don’t catch
Neither rule says anything about whether the right chart type was chosen in the first place, or whether an axis has been cut to exaggerate a difference. A slide can pass both the 6x6 rule and the 10-20-30 rule cleanly, six words a line, thirty-point font, and still mislead its audience with a distorted chart underneath the tidy text. The which chart should I use guide and the misleading graph lesson cover exactly the layer of slide design these two text rules leave untouched, and a genuinely reliable slide design checklist needs all three working together, not any one of them alone.