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Routing Number

General

ABA Routing Transit Number

A 9-digit code that identifies a US bank or credit union for electronic transfers, wire transfers, and check processing, validated by a weighted checksum formula.

Definition

A routing number, formally called an ABA Routing Transit Number, is a 9-digit code that identifies a specific US bank or credit union for the purpose of directing checks, ACH transfers, and wire transfers to the correct financial institution. It was originally developed by the American Bankers Association (ABA) in 1910 to speed up check clearing, long before electronic banking existed, and remains the backbone of the US payment-routing system today.

A routing number always accompanies an account number to complete a transaction — the routing number identifies where the money should go (the institution), while the account number identifies whose account within that institution.

Formula

The 9th digit of a routing number is a check digit derived from the first 8 digits (d1–d8) using a weighted-sum formula defined by the ABA:

3×(d1 + d4 + d7) + 7×(d2 + d5 + d8) + 1×(d3 + d6 + d9) ≡ 0 (mod 10)

In other words, applying weights of 3, 7, and 1 cyclically across all 9 digits (including the check digit itself) must produce a sum that is an exact multiple of 10. This weighted-modulo design catches the vast majority of single-digit and adjacent-transposition errors, making it possible for banking systems to validate a routing number's structure instantly without a network lookup.

Worked Example

A structurally valid example following the checksum formula: 021000021, which is the routing number publicly associated with JPMorgan Chase's New York operations.

Key Things to Know

  • 9 digits, no separators: unlike an IBAN, a routing number is a plain 9-digit string with no letters or hyphens.
  • Weighted checksum: the 3-7-1 weighted-sum formula lets the Routing Number Validator confirm structural validity offline, though only the ABA registry confirms an actual bank match.
  • Domestic only: routing numbers work for US transfers; international wires need a SWIFT/BIC code alongside the routing number and account number.
  • One bank can have several: large banks often maintain different routing numbers per region or per transaction type (ACH vs. wire).
  • Always paired with an account number: a routing number alone cannot identify or move funds — it only routes a transaction to the right institution.
  • Origin predates computers: the ABA created the numbering scheme in 1910 for manual check sorting, decades before electronic verification existed.

Related Terms

IBANInternational Bank Account NumberSWIFT / BICSociety for Worldwide Interbank Financial Telecommunication / Bank Identifier Code

Frequently Asked Questions

A routing number's 9th digit is a check digit computed from the first 8 using a weighted-sum formula: (3×(d1+d4+d7) + 7×(d2+d5+d8) + 1×(d3+d6+d9)) mod 10 must equal 0. This ABA formula lets banking systems detect most typos and transposition errors instantly, without a database lookup. You can test a number against this formula with the [Routing Number Validator](/validators/routing-number-validator/).
A routing number is printed on the bottom-left of a personal check, just before the account number, and is also listed in online banking under account or wire transfer details. Different routing numbers are sometimes used for domestic ACH transfers versus international wire transfers at the same bank.
No. A routing number identifies a US bank for domestic transfers (ACH, checks, wires within the US), while a [SWIFT/BIC code](/glossary/swift-bic/) identifies a bank internationally for cross-border wire transfers. International payments to a US account typically require both the routing number and a SWIFT code.
No, each routing number is unique to a specific financial institution, and large banks may hold multiple routing numbers for different regions, subsidiaries, or transaction types (such as one for checks and another for wires). The American Bankers Association assigns and maintains the full registry of active routing numbers.
A routing number identifies only the bank, and must be paired with a separate account number to complete a transaction, whereas an [IBAN](/glossary/iban/) — used in Europe and elsewhere — encodes the bank, branch, and account number together in a single string. The US has not adopted IBAN, so domestic transfers rely on the routing number plus account number pair instead.