cognitive psychology
Miller's law
Working memory can hold roughly seven items, plus or minus two, at once. Exceed that limit and things start to fall out. The constraint isn't about intelligence; it's about the size of the mental workspace everyone is working with.
The demo
A sequence of digits will flash on screen for 1.5 seconds, then disappear. Type what you saw. Each round adds one more digit. See how far your working memory stretches.
Didn't catch it?
What this demo shows (text version)
A digit-span test: sequences of random digits flash on screen for 1.5 seconds, then disappear. The reader types what they saw. The sequence starts at 4 digits and grows by one each round, up to 10. Most people recall sequences up to 5-9 digits reliably before errors begin, with an average around 7.
The result is Miller's law: working memory holds roughly 7 (+/- 2) items. The test makes the abstract limit personal, letting you find your own span, not a statistic.
That's your working memory limit in action. The number wasn't made up: George Miller measured it in 1956 and found the same result across people, senses, and kinds of information.
Miller's original "magical number seven" has been refined since: modern estimates put the chunk limit closer to four, once rehearsal tricks are controlled for. But the design implication holds: working memory is small, and overloading it costs errors and effort. Seven navigation items, seven form fields on one screen, seven steps in an onboarding flow, each is quietly testing the limit.
The "+/- 2" caveat matters more than the headline number. Some people manage nine, some five. Designing for the mean fails the people at the edges. Chunking (grouping items into fewer, meaningful units) is the practical answer: it doesn't expand the workspace, but it makes better use of it.