If you've ever wondered what the strange-looking row of numbers along the bottom of every US check actually is, you're looking at a MICR line: Magnetic Ink Character Recognition. It's the highest-throughput payment-processing technology ever deployed at scale, and it's been in continuous use since 1958.
What the MICR line encodes
A MICR line on a US personal or business check carries three required fields and one optional field, separated by special control symbols.
Field 1: Routing transit number (9 digits)
The routing number identifies the paying bank. The first four digits encode the Federal Reserve district, processing center, and state; the next four digits identify the specific bank; the ninth digit is a check digit calculated from the other eight.
Field 2: Account number (variable, typically 8–12 digits)
The account number identifies the specific account at the paying bank. Length and format vary by bank: a community bank might use 8 digits, a national bank might use 10, and a brokerage cash-management account might use 12 digits including a sub-account suffix.
Field 3: Check serial number (4 digits)
The serial number matches the printed check number in the upper-right corner. It is what allows the paying bank to find this specific check in your account history.
Field 4 (optional): Auxiliary on-us
Business checks above a certain physical size often include a fourth field at the far left of the MICR line, called the auxiliary on-us field. This is used for additional internal account information at the paying bank.
The E-13B font
The MICR line is printed in a font called E-13B, designed to be readable both by humans and by magnetic sensors. Each character has a unique magnetic signature, so a sensor can identify the digit even when the printed shape is partly obscured. The font has 14 characters: the digits 0–9 and four control symbols (transit, on-us, amount, dash).
What "magnetic ink" actually is
MICR characters are printed using toner or ink containing iron oxide particles. When the check passes through a reader-sorter, a magnet briefly magnetizes the iron oxide, and a downstream sensor reads the magnetic flux as the character moves past. Standard laser-printer toner contains too little iron oxide to be reliably read, which is why home-printed checks need MICR-formulated toner cartridges.
Why magnetic instead of optical?
Optical character recognition (OCR) was not reliable enough in the 1950s for the volumes the Federal Reserve was processing. MICR was designed to work even on checks that had been folded, stained, stamped, or written-over — magnetic flux passes through normal pen ink, fingerprints, and most types of damage. The standard has aged remarkably well: it still works perfectly, more than six decades later.
What goes wrong with MICR lines
- Folding across the MICR strip. Folding cracks the magnetic toner and the sorter can't read the field.
- Wrong toner. Standard toner is not magnetic enough — sorters reject the check.
- Misaligned printing. If the MICR line is not within the 5/8" band from the bottom edge, sorters can't lock onto it.
- Smudges and stains. Mostly tolerated, but a heavy stain across multiple characters will trigger manual review.
If a check fails the MICR read step, it falls out of the high-speed sort and is processed by hand, which costs the bank somewhere between $0.50 and $2.00 per item. Some banks pass that fee on to the issuer of the bad check, especially if the issuer is a small business printing its own.