Finding your own account number takes seconds once you know which screen to open. Finding somebody else's is not a thing that can be done, and any service suggesting otherwise is either selling a directory that does not contain account numbers or is something worse. This page covers the first question properly and answers the second plainly.
Where yours appears
The account details screen in your banking app is the best source because it reads the live record and normally lets you copy the number rather than transcribing it. A statement header carries it beside the sort code and the account name. A cheque carries it in the machine readable band along the bottom, and a paying in slip carries it on the face. A card sometimes carries it and increasingly does not, because several UK banks now issue cards with no printed account details.
When the number looks too short
UK payment systems expect eight digits, and some banks hold internal account numbers that are shorter. Where that happens the number is padded with leading zeros to make eight, and the padded form is what a payer needs. If you are reading a number from an old passbook or a legacy product and it has fewer than eight digits, do not guess at the padding: ask the bank for the form to give a payer, because the wrong padding produces a rejected payment rather than a helpful error.
Why you cannot find someone else's
There is no register of account numbers, public or licensed, and there could not usefully be one: the number identifies an account inside an institution and is meaningful only with the sort code. If you need to pay someone, ask them for the pair in writing and let your bank's name check confirm that the name you typed matches the account. If somebody has offered to look up a stranger's account number for you, that offer is the problem, not the solution.
Giving the number out safely
The sort code and account number pair is designed to be shared: it appears on invoices, cheques and paying in slips because that is how people get paid. What deserves care is the context. Never supply it in response to an unexpected message, never alongside a full set of identity details on a form you did not go looking for, and if you are a business, print the exact account name beside the pair so that payers do not trip the name check and stall.
Questions people ask about find bank account number
Where is my account number in my banking app?
On the account details or share details screen, next to the sort code. Most apps will copy both fields for you.
My account number is only six digits. Is that wrong?
No, but a payer needs the eight digit form. Ask your bank how the number is padded rather than adding zeros yourself.
Can I look up someone else's account number?
No. There is no register of account numbers, and no legitimate service offers one.
Is it risky to put my account number on an invoice?
It is how invoices have always worked. UK pull payments need a mandate, so the pair on its own does not let anyone take money from you.