Sending inside the US
A nine-digit routing number plus the account number. The US does not use IBANs, and an IBAN field will not accept US details.
Reference
IBAN, SWIFT/BIC, sort code, routing number, BSB, IFSC, NUBAN — what each one identifies, which countries use it, and a free checker for every format.
Every checker here validates structure. None of them can confirm that an account exists or belongs to the person you intend to pay.
A bank code identifies an institution or a branch. It is not an account number, and on its own it never proves an account is real.
A nine-digit routing number plus the account number. The US does not use IBANs, and an IBAN field will not accept US details.
An IBAN. Most SEPA transfers need nothing else; some banks also ask for the BIC, which is often derivable from the IBAN itself.
A six-digit sort code and an eight-digit account number domestically, or the GB IBAN for international transfers — both describe the same account.
Hold multiple currencies, send and receive internationally, and pay from one account.