An account number identifies one account inside one institution. That is all it does, and the confusion around it comes almost entirely from the fact that different countries chose different lengths and different rules, so a number that is obviously valid in one place looks malformed in another. This page sets out what the number is, what shape it takes on each side of the Atlantic, and what it does not carry.
What it identifies, and what it does not
The account number names an account, not a person. One person can hold several, a joint account has one number for two holders, and a business can run dozens. Because it identifies an account rather than an owner, the number carries no name, no address and no balance, and nothing about it can be decoded to reveal any of those. It is only meaningful alongside the code that says which institution to look in, which is the sort code in the UK and the routing number in the US.
UK: eight digits, usually with a check
A UK account number is normally eight digits. Some banks internally use shorter or longer numbers and pad or map them to eight for payment purposes, which is why an account number from an old passbook sometimes needs a leading zero. Most UK account numbers carry a modulus check, an arithmetic relationship between the digits that lets a payment system spot many single character errors before the payment leaves. That is why a mistyped account number is often rejected immediately rather than sent somewhere wrong.
US: variable length, no universal check
A US account number has no fixed national length. Depending on the institution it can run from a handful of digits to seventeen, and there is no universal check digit scheme across them, which is one reason the nine digit routing number beside it carries its own checksum. Large US banks publish guidance on where to read the number from a cheque, because the position on a cheque is standardised even though the length is not. U.S. Bank and PNC both publish such a page.
Why the number alone is not sensitive, and why it still matters
An account number on its own does not let anyone take money from you: UK and US pull based payments both require a mandate, and a mandate can be challenged. That is why the pair appears on invoices, on paying in slips and on the back of many cheques without alarm. What the pair does enable is money being sent to the wrong place, and a convincing request to update saved details. Treat a change of bank details in an email as suspicious by default, and confirm it by telephone on a number you already had.
Questions people ask about what is a bank account number
How many digits is a UK bank account number?
Eight for payment purposes. Where a bank's internal number is shorter, it is normally padded with leading zeros to make eight.
Is a US account number always the same length?
No. There is no fixed national length, and it varies by institution. The routing number beside it is always nine digits.
Can someone take money using just my account number?
Not by itself. A direct debit or ACH debit requires a mandate, and both systems provide a route to challenge one that was not authorised.
Why does my account number need a leading zero?
Because the bank's internal number is shorter than eight digits and the payment system expects eight. Your bank will confirm the padded form to give a payer.