A checker that takes both fields together can do something a sort code lookup cannot: it can test whether the account number is arithmetically consistent with the sort code it was given with. That is a real check and it catches real errors. It is also frequently mistaken for a check that the account exists, which it is not, and the gap between the two is where money goes missing.
The modulus check, and what it actually tests
Most UK account numbers carry a modulus relationship: the digits are weighted, summed and tested against a rule that depends on which sort code range the account sits in. A checker implementing that rule can tell you that a number is not a possible account number for that institution, which catches transposed digits and mistyped characters. What it cannot tell you is that a number which passes is a number in use. The vast majority of arithmetically valid combinations belong to no account at all.
Why passing is weaker information than failing
A failure is nearly conclusive: a number that does not satisfy the rule for its sort code range is almost certainly wrong. A pass is very weak: it narrows nothing meaningful. This asymmetry is worth internalising because interfaces usually present the result as a green tick, which reads as approval rather than as the absence of an arithmetic objection. Treat a pass as a spelling check rather than as a confirmation that the payee is real.
The check that reaches the receiving bank
The only routine check that asks the receiving institution anything is the name check that runs when you set up a payee in your own bank. It compares the name you typed against the name on the account behind the sort code and account number, and answers match, close match or no match. Pay.UK, which operates the service, treats it as an overlay on existing payments. It is the check to insist on, and it is free at the point of use because your bank runs it.
Using a checker sensibly
Run the pair through a checker when you have typed details by hand from a printed invoice, because that is where transposition errors come from and where the arithmetic earns its keep. Do not run one in order to decide whether a stranger's details are trustworthy, because it cannot answer that question and a green result will make you more confident rather than better informed. For that decision the answer is a telephone call to a number you already held.
Questions people ask about account number and sort code checker
Does a checker prove the account exists?
No. It tests whether the account number is arithmetically consistent with its sort code range. Most valid looking combinations belong to no open account.
Why did my details fail a checker but work at the bank?
Some institutions do not use the standard modulus rules, and some account numbers need padding to eight digits. A failure is worth re-reading the source for before assuming the checker is right.
Is there a checker that tells me the account holder's name?
Only the name check your own bank runs when you add a payee. No public lookup will name an account holder.
Does the checker work for business accounts?
Yes, the arithmetic is the same. Business payments fail the name check more often, usually because the invoice shows a trading name and the account is held in a registered name.