CUSIP check digit calculator

Use compute mode when you hold only the eight-character body and need the trailing digit, or validate mode when you receive a full nine-character CUSIP and need a boolean plus the expected digit for error messages. Non-CUSIP characters and wrong lengths fail with a clear invalid_input error instead of silent corruption. Run it free in the browser or call the KIT API at the published price; both paths use the same pure arithmetic with no network lookup of security master data.

How the CUSIP check digit is computed

Each body character maps to a number: digits zero through nine stay as themselves, letters A through Z become ten through thirty-five, and the special symbols asterisk, at-sign, and hash become thirty-six, thirty-seven, and thirty-eight when they appear. This tool applies that standard walk exactly; it does not invent a house variant and it always names the algorithm as cusip in the JSON so downstream logs can record which scheme protected the field.

Compute mode versus validate mode

Send a cusip field that contains only allowed characters after optional spaces and hyphens are stripped. Mode validate expects exactly nine characters, with the ninth a digit zero through nine. Empty strings, missing cusip, wrong length for the chosen mode, non-digit ninth characters under validate, and invalid mode values fail before any weighted sum runs and are not billed. Letters are accepted case-insensitively and normalized to uppercase in the echoed input so batch jobs can feed mixed-case paste from spreadsheets. Mode compute is the default and requires exactly eight characters: the engine maps and weights the body, returns check_digit as a one-character string, and returns cusip as the full nine-character identifier so you can store or print a single string. The response reports valid true when the stored trailing digit matches the recomputed expected_check_digit, and always includes body without the last character so user interfaces can highlight what was typed versus what the algorithm expects. Because the path is pure parse with no clock, random source, or network, the same input always yields the same JSON on the free browser tool and on the API. At $0.002 per successful request you can protect bulk security-master imports, onboarding forms, and order-entry screens without negotiating a custom tier. Prefer compute when you mint or complete a body from a data vendor that omits the check digit; prefer validate when a user, a partner file, or a scanner submits a full CUSIP and you need a boolean gate before looking up the instrument.

Where CUSIP check digits fit real workflows

Portfolio and performance systems that accept manual security entry can show expected_check_digit next to a failed field so operations staff correct one mistype instead of opening a research ticket. Product teams building order tickets or watchlists can gate submission on validate mode before calling a market-data or trading API that would otherwise reject the symbol with a less clear error.