An account number on its own is almost uncheckable. The arithmetic that makes UK account numbers testable depends on knowing which institution the account sits in, and that comes from the sort code. So the first thing to say about checking an account number is that you cannot usefully do it without the six digits that go with it, which is why every honest tool asks for both.
Why the sort code has to be in the check
UK modulus rules are not one rule, they are a table of rules indexed by sort code range, because institutions adopted different schemes as the system grew. Feed a checker an account number with no sort code and it has no way to know which weighting to apply, so it can only tell you whether the number is eight digits long. That is not a check. A tool that accepts an account number alone and returns a verdict is reporting on the length and nothing more.
What a real check catches
With both fields the arithmetic catches transposed digits, a mistyped character and a number copied from the wrong line of a statement. Those are the failures that come from reading details off paper, and they are common enough that payment software runs the check as you type. The check is fast, offline and free, and where it fails you almost certainly have the wrong number rather than an unusual account.
What no check reaches
Nothing about the account holder, nothing about whether the account is open, and nothing about whether the account has been used for fraud. The arithmetic is performed on the digits in front of you and does not contact the receiving bank at all. The step that does contact it is the name check your own bank runs when you add a payee, and that is the only routine confirmation available to an ordinary payer in the UK.
The order that actually protects a payment
Read the details from a source the payee controls. Run the arithmetic check to catch typing errors. Let the name check answer when you add the payee, and treat a close match as a reason to stop rather than a formality. Confirm any change to details you already hold by telephoning a number you had before the change was proposed. The arithmetic is the cheapest of those steps and the least informative, which is exactly the wrong way round from how it is usually presented.
Questions people ask about account number check
Can I check an account number without the sort code?
Not meaningfully. The UK modulus rule to apply depends on the sort code range, so without it a checker can only confirm the number is eight digits.
Does a passed check mean the account is real?
No. It means the digits are arithmetically consistent. Most consistent combinations belong to no open account.
Why did a number from my bank's app fail a check?
Some sort code ranges use exception rules that simpler checkers do not implement. Details read from your own bank's app are the authority, not the checker.
Is there a check for US account numbers?
No universal one. US account numbers have no fixed length and no shared check digit, which is why the nine digit routing number beside them carries its own checksum.