Miller’s law (7 plus or minus 2)

In short

If you read a phone number on one screen and type it on another, you keep it in your head for a few seconds. Miller’s law describes how narrow this short-term memory is. It is also called the magical number seven or the 7 plus or minus 2 rule.

According to the Laws of UX definition, the average person can only keep 7 (plus or minus 2) items in working memory. The key word is “item”. An item can be a single digit or a meaningful group.

See this difference yourself in the test below. Memorise a number of the same type first grouped, then ungrouped.

Live demo: remember the number

Choose a phone number or a card number, then the format. Press Show and memorise the number. When you are ready, press Hide and type, enter what you remember and compare.

Memory test

This test works with JavaScript. If it’s off, look at the two ways of writing the same number and think about which one you remember more easily.

Ungrouped

5551234567

Grouped

555 123 4567

The numbers are generated at random every round and belong to no one. They aren’t real phone or card numbers. The score checks whether each digit is in the right place. This isn’t a controlled experiment: do a few rounds in each format and compare the means.

Then try the card number. It has 16 digits, which are usually shown in four groups of four.

Don’t be surprised if you can’t keep even the grouped card number in full. 16 digits make 4 parts, and each part has 4 digits of its own to remember.

Origin: George A. Miller, 1956

The source of the law is psychologist George A. Miller’s paper “The magical number seven, plus or minus two: Some limits on our capacity for processing information”. It appeared in 1956 in volume 63 of Psychological Review, pages 81–97.

The paper opens with a famous sentence: “My problem is that I have been persecuted by an integer.” That integer is seven.

Miller sets two kinds of experiments side by side. The first is absolute judgement: the person hears tones one at a time and labels each with a number.

The second is the immediate memory span. According to the Laws of UX summary, both are limited to about 7 units of information.

On memory span, Miller writes that “for a lot of different kinds of test materials this span is about seven items in length”.

To me, the most useful idea in the paper is the distinction between bits and chunks. Miller calls turning small units into larger chunks recoding. According to him, it is an extremely powerful weapon for increasing the amount of information we can handle.

Miller doesn’t present seven as a fixed rule either. At the end of the paper, he writes that he is withholding judgement for now. He suspects that all these sevens may be “a pernicious, Pythagorean coincidence”.

A note: the phrase “plus or minus two” isn’t in the body of the paper, only in the title.

Common misuse: no more than 7 menu items

The sentence I hear most often about this law: “A menu can have no more than 7 items.” That is a myth. Both sources warn against it.

Kate Moran of Nielsen Norman Group writes that the magical number seven is often misunderstood. The wrong reading is that people can only process seven chunks at any time. Moran gives the example of designers who allow no more than seven options in the main navigation, and considers that an unnecessary restriction.

Her reasoning is simple. According to Moran, a menu relies on recognition: the items are on the screen, and users don’t have to keep them in their heads.

That is the same idea as the sixth of Jakob Nielsen’s 10 usability heuristics: recognition rather than recall. Nielsen recommends reducing users’ memory load by keeping elements, actions and options visible.

Laws of UX warns the same way: don’t use the magical number seven to justify unnecessary design limits. Short-term memory capacity varies from person to person and depends on prior knowledge and context.

The seven itself is also disputed. Moran notes that other researchers put the right number closer to three to six. So the limit may be narrower than seven. It still doesn’t apply to what is on the screen.

A long menu is really about choice time. That is described by Hick’s law.

Applying it the right way with chunking

The right counterpart of the law in the interface is chunking, that is, forming groups. In Moran’s definition, chunking means splitting content into small, distinct units. These units must make sense as a whole.

Figure: grouping cuts the number of items you have to keep in your head from 10 to 3. The number is an example.

Group numbers

In Moran’s example, a grouped phone number is easier to remember and scan than a long string of digits. Credit card numbers, too, are usually shown in four groups of four.

Show phone numbers like 555 123 4567 and card numbers in groups of four. If the person has to read the number somewhere and type it elsewhere, the displayed format makes that job easier.

Accept spaces in the input field

Support grouping not only when displaying, but also when typing. Moran also recommends that input fields chunk the input automatically. If a card number field rejects input with spaces, a number copied in groups leads to an error.

In these fields, I recommend silently removing spaces and showing the input grouped. Details are in the form design pattern.

Split long content

Chunking applies to text too. Among Moran’s examples for text are short paragraphs separated by white space and subheadings that clearly stand out from the text. Laws of UX recommends the same: organise content into smaller parts.

My note

I don’t use Miller’s law as a numeric rule. When I review a screen, I ask: does the person have to carry something in their head here?

The account number carried from screen to screen during a transfer is one example. So is the confirmation code from a text message at checkout. Group such details, or make them easy to copy and paste.

Best of all is when the person doesn’t have to remember anything.

I flag memory loads like these screen by screen in a heuristic evaluation. If you want this check done for your own product, take a look at the UX Audit.

Related pages

Miller describes what the person keeps in their head. How long they take to choose among many options is described by Hick’s law, and how long they take to reach the target by Fitts’s law.

The other laws are in the list of laws of UX.