Miller's Law

People can hold only a small number of meaningful chunks in short-term memory at once, about seven in Miller's 1956 estimate and closer to four in later work.

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The law

People can hold only a small number of meaningful chunks in short-term memory at once.

The unit is the chunk, not the item. A chunk is anything a person already treats as one thing: a familiar word, an area code, a known date.

Where it comes from

George A. Miller described the limit in The magical number seven, plus or minus two, published in Psychological Review in 1956. The paper reviewed two lines of research. One measured how many categories people can reliably tell apart on a single dimension, such as pitch or loudness; the answer was usually around seven. The other measured immediate memory span, how many items people can repeat back right after hearing them; that was also around seven.

Miller's main point was that memory span is limited in chunks, not in raw information, and that recoding information into bigger chunks lets people remember far more. He treated the recurring number seven partly as a coincidence between the two lines of research.

Later work revised the number. Nelson Cowan's 2001 review, The magical number 4 in short-term memory, argued that when people cannot rehearse or chunk, the capacity is closer to three to five chunks.

What it means for an interface

  • Do not make people carry information between screens. If a person has to remember a code, a price or a choice from one step to use it on the next, show it again where it is needed.
  • Chunk what people must read or type. Phone numbers, card numbers and confirmation codes are easier to check and enter in groups.
  • Group long content. Splitting a long list or form into labeled sections lets people deal with one section at a time.
  • Prefer recognition to recall. A visible list of options asks people to recognize, not remember. That is the main reason menus work even when they are long.

Examples

One-time codes. A six-digit code shown as two groups of three is easier to hold while switching from a text message to the app than six digits in a row. Better still is a field that fills itself in from the message.

Checkout review. A final review step that repeats the shipping address, the delivery date and the total lets people confirm the order without remembering what they chose three screens earlier.

Comparison tables. When people compare plans, a table puts every value in view at once. Making them switch between tabs to compare forces them to remember one plan while reading the next.

Where it is misapplied

  • "Menus should have at most seven items." Miller's paper is about holding items in memory, not about choosing from a visible list. A person reading a menu does not need to memorize it. The right length depends on how well the items are grouped and labeled.
  • Treating seven as precise. Seven was an approximate figure from specific tasks, and later work puts the limit lower. Neither number is a design constant.
  • Ignoring chunking. Miller's own emphasis was that chunking expands what people can hold. The useful question is how to make content chunkable, not how to cut it to seven pieces.

Checklist

  • No step requires remembering information shown only on an earlier step.
  • Codes, numbers and identifiers are displayed in short groups.
  • Long forms and lists are split into labeled sections.
  • Comparisons happen side by side, not across separate views.
  • Menu length is decided by grouping and labeling, not by a fixed limit of seven.

Sources

  1. Miller, G. A. (1956). The magical number seven, plus or minus two: Some limits on our capacity for processing information. Psychological Review, 63(2), 81–97.
  2. Cowan, N. (2001). The magical number 4 in short-term memory: A reconsideration of mental storage capacity. Behavioral and Brain Sciences, 24(1), 87–114.