The MICR Line on a Check and How It Maps to Positive Pay
The MICR line is the row of stylized numbers and symbols printed in magnetic ink along the bottom of a US check. MICR stands for Magnetic Ink Character Recognition. It lets bank sorting equipment read the check's routing number, account number, and serial number at high speed. The same three values are what a positive pay file reports to your bank, so understanding the MICR line makes it much easier to build a check issue file that matches.
This page explains each field on the MICR line, the special symbols that separate them, and how the printed values line up with the fields in a positive pay check-issue file.
What the MICR line contains
A US check uses the E-13B character set, which has the ten digits 0 through 9 plus four special symbols. Reading the line from right to left, it is divided into these fields:
- Amount field (rightmost): printed blank on a new check. The bank or first processor encodes the dollar amount here after the check is deposited.
- On-us field: contains your account number, and on many check stocks the serial (check) number follows it. The bank defines exactly what goes here.
- Transit field (also called the routing field): the nine-digit ABA routing number that identifies your bank. This field is fixed and is bracketed by transit symbols.
- Auxiliary on-us field (leftmost): present only on business-size checks longer than about six inches. It usually holds the check serial number.
So depending on the check size, the serial number lives either in the auxiliary on-us field on the left or inside the on-us field next to the account number. Both are common.
The four MICR symbols
The four E-13B symbols act as field separators so the reader knows where each number starts and stops:
⑆Transit symbol: brackets the routing number on both sides.⑈On-us symbol: marks the account number area.⑇Amount symbol: brackets the amount field.⑉Dash symbol: separates parts of a number, such as a prefix inside the account number.
The routing number in detail
The ABA routing transit number is nine digits in the form XXXXYYYYC. The first four digits are the Federal Reserve routing symbol, the next four identify your specific institution, and the last digit, C, is a check digit used for error detection. The check digit is computed with a weighted formula using the weights 3, 7, and 1 repeating across the first eight digits, so a single mistyped or transposed digit can be caught automatically. You do not normally send the routing number in a positive pay file, because the file already belongs to one account at one bank, but it is worth knowing it is the one MICR field your printer must never alter.
Mapping MICR fields to positive pay fields
A positive pay check-issue file is a list of the checks you wrote, so your bank can match each presented check against your own record. The core fields almost always are:
- Account number, which comes from the on-us field of the MICR line.
- Serial number (check number), which comes from the auxiliary on-us field or the on-us field, depending on your check stock.
- Amount, which is the value you wrote on the check. Note this is not read from the MICR amount field, because that field is blank until the bank encodes it. You supply the amount from your accounting system.
Many files also include the issue date, a void or issued status code, and sometimes the payee name for payee positive pay. The exact column order, field widths, and whether the file is fixed width or CSV are set by each bank, so there is no single universal layout. You can build any of these with the free positive pay file generator, or define a bank-specific layout with the custom format builder.
Why the serial number must match
Positive pay works by matching. When a check clears, the bank reads the serial number and account number from the MICR line and looks for the same serial number in the issue file you sent. If it finds a match and the amount agrees, the check pays. If the serial number on the printed check does not match any serial number you reported, the check becomes an exception and you have to approve or return it by the bank's cutoff.
This is why the number you print in the MICR line and the number you report in the file have to be identical. Common ways they drift apart:
- Leading zeros. If your check prints
001045but your file reports1045, some bank matching rules treat them as different. - Reprinted or manually written checks that use a serial number never sent in any issue file.
- Accounting software that exports a different serial number than the one your check printer used.
The safest practice is to pull the serial number, account number, and amount from the same source that printed the check, then report those exact values. For a deeper walkthrough of how matching stops fraud, see how positive pay stops check fraud, and for the field-level file details see the positive pay file format reference. The field layout described above follows the common E-13B placement covered in vendor documentation such as the Morovia MICR placement guide; your bank's own check stock may vary slightly.