No. 04
Why Phone and Card Numbers Are Split into Groups
010-1234-5678, 1234 5678 9012 3456. The habit of splitting long numbers into groups of three or four is closely tied to the limits of working memory. Here is why, along with the actual numbering standards.
Long numbers are almost always written in groups. South Korean mobile numbers split their 11 digits 3-4-4, and payment cards print 16 digits as 4-4-4-4. Sometimes a standard came first, but the main reason is that this format is easier for people to read and copy.
From 11 digits to 3 units
Holding 11 separate digits exceeds most people's digit span. But seen as the familiar prefix 010 plus two groups of four, there are only three units to remember. That fits within the range of "about 4 chunks at once" proposed by Cowan (2001). When part of the number is a chunk you already know, like the prefix, the load is even lighter.
| Number | Total length | Common grouping | Units to remember |
|---|---|---|---|
| --- | --- | --- | --- |
| South Korean mobile | 11 | 3-4-4 (010-XXXX-XXXX) | 3 |
| Payment card (most) | 16 | 4-4-4-4 | 4 |
| American Express card | 15 | 4-6-5 | 3 |
| International phone number (E.164) | Up to 15 | Country code + national grouping | 3–5 |
| IBAN (international bank account number) | Up to 34 characters (varies by country, includes letters) | Groups of 4 | 5–9 |
| One-time passcode | 6 | 3-3 or all together | 1–2 |
The actual numbering standards
- International phone numbers: ITU-T Recommendation E.164 limits international phone numbers, including the country code, to a maximum of 15 digits. How they are grouped varies by country.
- Payment card numbers: ISO/IEC 7812 defines the leading digits of the card number (PAN) as the Issuer Identification Number (IIN) and makes the last digit a check digit computed with the Luhn algorithm. Numbers can be up to 19 digits long, and 16 is the most common.
- IBAN: ISO 13616 recommends no spaces in electronic transmission but a space every 4 characters in the printed form meant for people. It's an example of human-friendly grouping written into a standard.
Mistakes that grouping prevents
The most common mistakes when copying numbers are writing one digit wrong and swapping two neighboring digits. With groups, you can check your copy group by group, which makes errors easier to find. A card number's Luhn check digit catches every single-digit error and most swaps of adjacent digits (except 09↔90). Human grouping and machine checking work together to prevent errors.
When reading out or writing down a number
- The person reading should say it group by group, pausing briefly between groups.
- The person listening should repeat each group back or write it down right away. Don't try to hear all 11 digits before writing.
- At the end, read the whole number back from the start, group by group, to check it.
Try 11 or 16 digits in this site's all-at-once presentation and you'll notice your eyes looking for groups on their own.
Sources: ITU-T Recommendation E.164 · ISO/IEC 7812-1 · ISO 13616-1 · Cowan (2001) Behavioral and Brain Sciences 24(1):87–114