IMb decoder
Decode an Intelligent Mail barcode — including the 11-bit FCS round-trip that proves whether the encoder that printed it was telling the truth.
FAQ
What input does the decoder take?
Either the 65 bar states of an IMb as letters (A=ascender, D=descender, T=tracker, F=full), or the numeric IMb data string (20-digit tracking code plus a 0/5/9/11-digit routing code).
Is anything uploaded?
No. The decode runs entirely in your browser — the same pure USPS-B-3200 math Preflight uses server-side, compiled into this page. Nothing leaves your machine.
What does an FCS mismatch mean?
The bars are structurally valid but the CRC recomputed from the recovered payload disagrees with the frame check sequence embedded in the bars — the barcode was produced by a broken encoder and will misbehave in USPS processing even though it scans.
Can it decode mailer ID and routing straight from the bars?
The bar-to-fields decode on production documents runs in Preflight’s USPS module. This free tool proves encode integrity from bars, and parses fields from the digit string.
How the FCS round-trip works →Validate IMbs inside a document →What is an IMb? →