Is there any rules-of-thumb or actual research on how to make long numbers more legible?

We are dealing with 14 digit numbers:


This can be hard to read back over the phone as it's hard to keep track of where you're at.

I want to divide this up. The most natural break, to me, would be 4 characters. This is what credit cards do (and portions of phone numbers).

1234 5678 9123 45

2 related questions:

  1. Is there any reason to consider other breaks than every 4?
  2. Is there a usability problem here in that I might be implying that these breaks mean something? (They don't...it's just one large number)
  • What are these numbers for, if you can answer that? Account numbers? Reference Numbers? I assume they will always be numeric and not alpha-numeric too, yes?
    – JonW
    Commented Mar 13, 2015 at 16:03
  • @JonW it's a record. You could call it the GUID but one that the user sees directly. If they call in to ask a question, they'd need to reference this number to look up the item. A user would likely see many of these (list of records). It's purely numeric (no alpha).
    – DA01
    Commented Mar 13, 2015 at 16:14
  • 3
    Not really an answer to your question, but if you are going to the trouble of segmenting your numbers for easier data entry, you might also want to add a checksum digit to the end (like the credit cards do) so that your data-entry field can check its content's validity (using javascript). You might also consider displaying it to them (and allowing them to enter it) in hexadecimal as that cuts the length down to a more manageable 8 characters. Commented Mar 13, 2015 at 16:16
  • 1
    A good question: and probably one that's quite easy to test on your colleagues - What you are doing is called 'chunking' by psychology people, as people can remember the chunks better than the raw data. I suspect 3 or 4 digits are what people are familiar with from phone numbers.
    – PhillipW
    Commented Mar 13, 2015 at 16:31
  • You could take advantage of letters if you convert the numbers to base 36 (26 letters + 10 digits). This shortens the GUID to 8-9 characters. For example: 12345678912345 => 4DJIYGV2X. Might want to exclude certain visually similar characters though (1I and 0O), giving you a base 32.
    – Supr
    Commented Mar 13, 2015 at 20:49

7 Answers 7


4 digits is time-tested chunking for large numbers

  • 3 to 4 digit chunks are easy to read accurately. Perceptually, the eye tends to read words and not letters across a page, and a 3-4 letter word allows the eye to read the end points and the middle letters of the word accurately without disorientation. Once the word gets too long, the letters in the middle add cognitive load.

  • The word superiority effect shows that chunking letters into words aids recall and recognizability. This study is pretty interesting.

  • 4 digits words are easy to read over the phone. In most languages, digits are monosyllabic words ('one' 'two' 'eins' 'zwei' 'ee' 'ar', etc). A chunk of 4 letters provides a reasonable phrasing (or foot) for reading numbers.

  • There are a lot of industrial examples of chunking to 3 or 4 letters. Credit cards, phone numbers, license codes, etc. For long numbers, 4 is a lot more common. A canonical example is the old logarithmic tables, used to display long logarithmic results (see below):

log tables

One more thing

This may or may not work for you, but a popular approach to these long serial numbers is to convert them to alphabetical GUID's to shorten the digits.

Assuming you have no ID's starting with '0', the number of ID's for a 14 digit number is: 1e14 - 1e13 So by converting to 26-letter alphabetical, the ID can be shortened to:

log(1e14 - 1e13) / log(26) = 9.86 or 10 letters instead of 14.

  • 2
    Agree with everything except your "canonical answer". The log table has four digits because that is all the accuracy that is supported for the granularity of the inputs - not because four digits in a row is particularly legible. A fifth digit would add no value; in the days of tables, people still knew this. Five figure tables were also published - these used additional interpolation values and were generally very cumbersome. See for example archive.org/stream/fivefigurelogari00mcauuoft/… and be grateful for scientific calculators...
    – Floris
    Commented Mar 13, 2015 at 20:25
  • 1
    @Floris actually you're right. I had heard log tables brought up before as an example of 4-digit chunking but hadn't gone back to check why. I tried to find my green CRC Numerical Tables handbook but realized I threw it away years ago :-(
    – tohster
    Commented Mar 13, 2015 at 20:29
  • Seriously doubt the Word Superiority Effect applies here - the process involving word recognition shares little with that of numbers. You could tell what the missing letters in le_t and b_rd are, but you wouldn't be able to tell the missing digit in 38_1.
    – Izhaki
    Commented Mar 24, 2015 at 21:55
  • le_t... S? N? F? b_rd.. A? I? Y?
    – AlexC
    Commented Mar 26, 2015 at 14:10

Digit Span Tests

6 is the average

The adult average for the famous (auditory) digit span test is just above 6. The average for visual digit span test is roughly the same.

Around 80% of adult population will score between 5-8 in such tests, and people scoring 4 or below will be suspected of some cognitive impairment.

4 is the boundary

This means that most adults should be able to recall a chunk of 4 digits (which one party will have to do over the phone).

Accessibility matters

But do bear in mind that both children and the elderly score lower in these tests. So do people with cognitive impairments, like dyslexia.

Just because you can doesn't mean it's easy

Most of us can calculate 7*9/2, but would find it easier to calculate 14/2.

Similarly, it is easier to recall 3 digits over 4, as both these graphs demonstrate (source):

A graph showing lower correct proportion for higher digits

A pupil dilation plot showing increase for higher digits

Group count/items tradeoff

So with 4 being safe and 3 being easier, it's really a question of striking a balance between the amount of digits per cluster and the amount of clusters. It is worth remembering that as people read these numbers out loud, they have to keep track of which group is being read - a seemingly simple task, but even simple tasks can be challenging given a particular context.

3 per cluster

For 14 digits, you'll end up with:


5 groups.

4 per cluster

Will give you:


4 groups.

5-3 vs 4-4

Although you can't regard in the same way the amount of digits to the amount of clusters, a 4-4 system seems more balanced than a 5-3 one.

5 chunks would be perceived as more effortful compared to 4, particularly as most people will have no problem with a group made of 4 digits.

What's more, when one person says it and another has to write it, the back and forth toggling which happens between each group takes time. By way of analogy, if you have to move 12 chairs between two rooms, would you prefer to move 3 at a time (4 rounds), or 4 at a time (3 rounds)?

The only reason to prefer a 5-3 system is accessibility (which would include children and the elderly). But in this specific case such claim is based on reasoning - I wouldn't pick the 5-3 system without further research into this specific issue with the cognitively impaired.

  • +1 For the charts, quite interesting (and I'm not a chart guy). I think 3 digit chunks are only preferred if the overall length of the number is shorter, e.g. 9 digits.
    – Vince C
    Commented Mar 25, 2015 at 8:05

Telephone numbers are an interesting example. Different countries chunk differently. The US uses 3-3-4 (a hangover from the days of area code (3), exchange (3), subscriber (4)) - but in France they use five 2-digit pairs, and pronounce them as the combined number:

"zero-six, quatre-vingt-treize, soixante-onze, douze, dix-huit" for

06 93 71 12 18

(and this really makes life awkward for foreigners since 93 is spoken as "four twenties thirteen" and 71 is pronounced "sixty-eleven". Maybe we shouldn't take our usability cues from the French...)

  • Except that 93 is not conceived of 4 twenties and 13 anymore than 93 is thought of as ninety and 3 in english. It's absorbed as one number. From a French perspective you're remembering 5 numbers as opposed to 3 sets of numbers in America: 718-123-4567
    – Mayo
    Commented Mar 25, 2015 at 2:36
  • @Mayo - which is why I said "awkward for foreigners". I am perfectly aware that French speakers (native, or fluent) don't think in these terms. I was just illustrating the point about chunking in pairs and thought to add a little twist because of the difficulty this causes when a French person reads a phone number to a non-fluent speaker.
    – Floris
    Commented Mar 25, 2015 at 3:00
  • I see. You did say "awkward for foreigners." Sometimes when you skim material you do miss important points. :-)
    – Mayo
    Commented Mar 25, 2015 at 3:02

Three or four digit chunks make the string more readable and repeatable. The advantage of three digits is that there will be either a remainder of one or two, so the last item in the list would be 4 digits or the last two would be 4 digits.

Using four digits is easiest when copying information - say a license key from the cd case (remember those?). I would have said five, but now that I have kids, I'm lucky to get 5 uninterrupted seconds. I say that tongue in cheek, in reality, everyone has to deal with distractions.

Using three digits is easier when communicating verbally over the phone or over a radio (or reading the model number with your head under the dishwasher to your spouse standing nearby) and the receiver repeats back the digits. As the caller and receiver fall into a rhythm a four digit chunk is a good balance.

I recommend chunking into four and three digit chunks, maximizing the number of four digit chunks, and leading with the three digit chunks. For your example of fourteen digits, this would be:

14: 3-3-4-4


I don't know about the thumb rule. But if it is about making the user easily readable then may be its fine with progressive number pattern For 14 digit

12 123 1234 12345

Like if i am telling you the number my mind will find the first two digits a simple then my mind will be ready for a little bit crazy length of the number which may go up to five digits. If five is bit lengthy for human readable then may be push the 5th letter in fifth chunk as

12 1235 1234 1234

All i want to say is that start with easy pattern followed by complex.


Thumb Rule: 3-4 chunks with 3-4 digits each.
real life examples :

While using credit card there are 16 digits they are chunked to 4 chunks of 4 digits.

Mobile numbers (10 digits) are also chunked 3-3-4.

This article cites many psychology papers an research studies. Read the part "optimal size of chunking"

Also I learned "this rule of thumb" from "Human Factors International" .

  • 1
    Where is this thumb rule from? Because in UX you will rarely find identical, or even similar problems - context changes a lot in what already involves many many variables. Michael Jackson (not the late singer) said that rules of thumbs are for people who apply a solution without really thinking about the problem.
    – Izhaki
    Commented Mar 25, 2015 at 1:36
  • Hi! Could you edit your answer and add a reference to your "Rule of Thumb"? Thank you Commented Mar 25, 2015 at 9:58
  • Edited. And added one link that could support my answer. There can always be different opinions about UX based on various factors, user study and observations although human cognitive heuristics stay same.
    – Vatsal
    Commented Mar 25, 2015 at 17:28

Is there a usability problem here in that I might be implying that these breaks mean something? (They don't...it's just one large number)

If you use chunks of 3, like in

123 456 789

it might be read as “123 million, 456 thousand and 789”, which might not be desired.

  • Have you seen evidence of people doing that?
    – Mayo
    Commented Mar 26, 2015 at 12:58
  • Actually, this would at least come to my mind. And I think I do have seen (heard) people doing this.
    – Scz
    Commented Mar 26, 2015 at 16:26

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