ABA routing check digit

US bank routing transit numbers (RTNs), also called ABA routing numbers, are nine digits long: eight payload digits that identify the Federal Reserve district, institution, and check-digit series, plus one trailing check digit that proves the whole string was typed or pasted without a single-digit slip. This capability implements the official three-seven-one weighting used by the American Bankers Association and Federal Reserve: positions cycle through weights three, seven, and one, and the full nine-digit sum must be divisible by ten. Non-numeric input and wrong lengths fail with invalid_input instead of silent stripping, so you never store a half-cleaned RTN. The same pure arithmetic runs free in the browser and at the published API price.

How the ABA routing check digit is calculated

To compute it, walk the eight payload digits from left to right and multiply by the repeating weight sequence three, seven, one, three, seven, one, three, seven. Compared with multi-format payment-number validators, the search intent here is narrow: people who need an ABA routing check digit calculator for eight-digit payloads or nine-digit RTNs, not a general IBAN or card-number checker.

Input contract, modes, and error behaviour

Send a number field that contains only characters zero through nine. Wrong length is an explicit error that names the mode and the length you sent—eight for compute, nine for validate—so client forms can show “need eight digits” without guessing. Empty strings, missing number, and invalid mode values fail before any weighted sum runs and are not billed. Prefer compute when you mint test RTNs or complete partial institutional identifiers; prefer validate when you gate ACH setup or import on a full nine-digit code from a user or partner file. Spaces, hyphens, letters, and decimal points are rejected as non-numeric input with invalid_input so a silently stripped value never reaches storage. Mode compute is the default: the engine requires exactly eight digits, returns check_digit as a one-character string, and returns with_check as the nine-digit routing number you can print or persist. Mode validate requires exactly nine digits; the response reports valid true when the trailing digit matches the recomputed expected_check_digit, and always includes payload (the first eight digits), check_digit, expected_check_digit, length nine, algorithm aba-3-7-1, and weights 3-7-1. Alias keys such as digits, routing, rtn, and code are accepted for the same payload field so integrations that already speak “routing” do not need a rename. Because the path is pure parse with no clock, random source, or network, the same JSON body always yields the same JSON result on the free browser tool and on the API at $0.002 per successful request.

Where ABA routing check digits fit real workflows

Payroll and benefits platforms receive bank routing numbers from employees and must reject mistyped ninth digits before the first direct-deposit file is submitted to the ODFI. Treasury and AP teams validate every routing column in a vendor CSV export, quarantine rows where valid is false, and avoid failed ACH batches that cost delay and rework fees. Sandbox and demo environments often need synthetic but checksum-valid routing numbers; compute mode turns any eight-digit body into a nine-digit string that passes local format gates without inventing a real bank relationship. Fintech onboarding forms recompute expected_check_digit and show a friendly correction when a customer pastes a nine-digit RTN with a single transposition. Do not treat a passing ABA check as proof of an open account, OFAC clearance, or current Federal Reserve assignment; it only proves the three-seven-one digit is consistent with the payload under the standard weights.