People usually meet the sort code and the account number as one blob of digits on an invoice, and treat them as a single address. They are not. They are two fields doing two different jobs, and understanding which does what explains almost every odd thing that happens to a domestic payment: why a typo in one is caught and a typo in the other is not, why the code is sometimes printed in pairs, and why a code alone is never enough to pay anyone.
One half routes, the other half identifies
The sort code is the routing half. It points the payment at an institution, and in the older model at a clearing point inside that institution. The account number is the identifying half: it names one account inside whatever the sort code points at. Neither means anything without the other. A sort code with no account number is an address with no house, and an account number with no sort code is a house number with no street. This is why every UK payment form asks for both and why neither field is optional.
How the six digits are read
A UK sort code is six digits, conventionally printed as three pairs. The leading pair identifies the institution's allocated range, and the remaining digits historically identified a clearing point or branch within it. That branch meaning has faded: many banks now operate on a single code for the whole institution, and several of the newer banks never had branches to identify in the first place. So the three pair layout is now mostly a readability convention, and any payment form should accept the digits with or without the separators.
Why an account number typo is often caught and a sort code typo is not
Most UK account numbers carry a modulus check, an arithmetic relationship between the digits that makes many single character errors detectable before the payment leaves. Sort codes carry no such check digit: a mistyped sort code is simply a different sort code, and if it happens to be a live one the payment will be routed there. That asymmetry is the practical reason to read the sort code off something the payee controls and to let the name check answer before you send.
What the pair does not tell you
An account and sort code pair does not carry the account holder's name, and it does not prove that the account is open. It also says nothing about what kind of account sits behind it: a current account, a savings account and a client money account can all look identical from the outside. If you need to know the name on the account, the name check is the only routine way to find out, and your own bank runs it when you set the payee up.
Questions people ask about bank account sort code
Can I pay someone with only a sort code?
No. The sort code routes the payment to an institution and the account number identifies the account inside it, so a domestic payment needs both.
Is a sort code the same as a routing number?
They do the same job in different countries. A UK sort code is six digits, a US routing number is nine and carries a check digit. Neither is valid in the other country's system.
Why does my bank show one sort code for every customer?
Several UK banks, particularly the app based ones, operate on a single institution wide sort code because they have no branch network to distinguish. Your account number is what makes your account unique.
Do I need the sort code for an international payment into a UK account?
Usually not directly. A payment arriving from abroad is addressed with the IBAN, which already contains the sort code and account number inside it, together with the bank's BIC.