Account Number

An account number is the unique identifier that a bank, credit union, payment institution or other financial institution assigns to a customer's account. It is used to post deposits and withdrawals, to route incoming and outgoing payments and to link statements, cards and services to the right ledger. On its own an account number is usually unique only within one institution, so payments also need an identifier for the institution itself.

That second identifier differs by country: a routing number in the United States, a sort code in the United Kingdom, an IFSC code in India and a bank code in many other systems. Combined, they form a domestic account identifier, often called the basic bank account number, which in turn sits inside the IBAN where IBANs are used. An account number is different from the 16-digit primary account number printed on a payment card.

An account number works like a flat number in a large apartment block. The street address gets a visitor to the right building, but there are hundreds of front doors inside. The flat number tells them which door is yours. In banking the bank or branch code is the street address and the account number is the door.

Like a flat number, an account number on its own is not secret, and people share it so others can pay them. Knowing it does not let a stranger walk in and take money out, because withdrawals need passwords, cards or signatures. It can, however, be misused, which is why banks add checks.

There is no global standard for the length or structure of account numbers. Common examples as of 2026:

• United Kingdom. Eight digits, paired with a six-digit sort code; some building societies also use a roll number.
• United States. Lengths vary by bank, and ACH files allow up to 17 characters; the number is paired with a nine-digit ABA routing number.
• Euro area. Domestic account numbers have largely been replaced by IBANs since SEPA migration, though the account number remains inside the IBAN.
• Pakistan. Domestic account numbers vary by bank, but all accounts carry a 24-character IBAN starting with PK.
• India. Account numbers vary in length by bank and are paired with an 11-character IFSC code identifying the branch.

Some institutions issue virtual account numbers that point to a master account, so that each customer or invoice has its own number for incoming payments; see also virtual IBANs.

Many account number schemes include check digits, which let software catch most typing mistakes before a payment is sent. A common approach multiplies each digit by a weight, adds the results and requires the total to divide evenly by a set number. In general form:

• $d_i$ is the $i$-th digit of the number.
• $w_i$ is the weight assigned to that position.
• $m$ is the modulus, often 10 or 11.

As an illustrative example with weights 4, 3, 2, 1 and modulus 11, the number 6 1 5 1 gives $4 \times 6 + 3 \times 1 + 2 \times 5 + 1 \times 1 = 24 + 3 + 10 + 1 = 38$. Since 38 does not divide evenly by 11, the number fails, so the sender is warned before the money leaves. Real rules vary by bank and country; the UK publishes its modulus checking rules, and IBANs use a single check based on modulus 97. Tools such as an IBAN checker apply these tests.

Because account numbers are shared openly, they can be used for unauthorised debits if a fraudster also knows the routing code; in the US this is a known risk with ACH debits and printed cheques. Defences include debit blocks, positive pay for businesses and bank account verification before a payee is added. A correct account number also does not prove who owns the account, which is why name checks such as verification of payee and account ownership verification exist. Account numbers that suddenly change on an invoice are a warning sign of payment diversion fraud. Sending money to a wrong but valid number usually results in a returned payment only if the receiving bank spots the mismatch.

Account numbers can also change when banks merge or systems are replaced, and in most countries a customer who moves bank receives a new number; the debate over keeping one number for life is covered under bank account portability.

• Vocalink: Modulus checking (UK sort code and account number validation)