Which number on a check is the account number

Four numbers are printed on a US personal check, and three of them get handed over by mistake. Someone asks for your account number — a landlord setting up rent, a payroll department, a form wanting a voided check attached — and the check offers a routing number, an account number, a check number, and near the top right a small fraction that is a second way of writing part of the first one.

The layout is not arbitrary. It has been standardised since the late 1950s, and once you know what the symbols around each field mean you can pick the account number off any check without counting digits.

The line along the bottom, read left to right

That strip at the bottom edge is the MICR line — magnetic ink character recognition. On a standard personal check it carries three fields in this order:

1. The routing number. Nine digits at the far left, wrapped in a symbol on each side that looks like two vertical bars flanking a square. That is the transit symbol, and it appears twice because its whole job is to bracket the routing number so a machine knows where the field starts and stops.
2. The account number. Immediately to the right, closed by the on-us symbol — two small stacked squares. “On us” is bank shorthand for the part of the line the paying bank defines for itself.
3. The check serial number. The last group, matching the number printed in the top-right corner.

Routing, account, check number. Find the nine digits on the left, skip them, and the next run is the one you were asked for.

A fourth field at the far right is blank on an unused check. That is the amount field, encoded by the first bank to process it, so on a cancelled check those digits are the amount, not your account. The fourth symbol, the dash, separates elements inside a field rather than marking a boundary.

Those symbols exist because a read head cannot see

The typeface is E-13B. The American Bankers Association adopted it as the US standard for checks in 1958, ANSI followed, and it has barely changed since. The character set is fourteen characters: ten digits plus the four symbols above. No letters, which is the first hint this was never meant for people.

E-13B is read magnetically. A head passes over the ink and senses changes in the magnetic field, the way a tape head reads a cassette, and each character produces a waveform the reader matches against a known profile. The glyphs look strange — the fat foot on the 8, the blunt slabs on the 2 — because the designers were shaping magnetic signatures, not letterforms, tuning strokes until no two characters produced similar peaks. The name encodes the engineering: the characters sit on a grid of 0.013-inch squares.

The governing standards come from the Accredited Standards Committee X9 through ANSI: X9.100-160-1 for MICR placement and formatting, X9.100-20 for print and test specifications.

None of which helps a phone camera. A magnetic reader gets a signal an optical reader has to infer from pixels, and your phone does that harder job with no magnetic head at all.

The number in the top-right corner is not your account number

It is the check number, and it should match the last group of digits in the MICR line. The two are printed in different places for different readers: the corner for you, the bottom line for the sorter.

Many checks also carry a small fraction near the top right, something like 11-3160/1210. That is the fractional form of the routing number, and Regulation CC (12 CFR Part 229) recognises both forms explicitly. The denominator is the Federal Reserve routing symbol — the first four digits of the nine-digit number, with leading zeros restored where the fraction drops them. It predates the MICR line, survives because it is human-readable, and tells you nothing about your account.

Business checks move the check number to the front

If the check is longer than six inches — most business checks are — the check serial number moves out of the on-us field into an auxiliary on-us field at the far left, ahead of the routing number, bracketed by its own pair of on-us symbols.

So the order becomes check number, routing number, account number. Apply the personal-check rule here and you will read a check serial as a routing number. Look for the transit symbols instead: whatever they bracket is the routing number, wherever it falls.

The routing number can prove itself

The ABA created the routing system in 1910 to sort paper checks, still owns the scheme, and designates a registrar to assign the numbers — a role held today by LexisNexis Risk Solutions, which allocates them under the ABA Routing Number Policy and is Nacha’s exclusive preferred partner as official registrar.

The structure makes errors detectable. The first four digits are the Federal Reserve routing symbol, and Regulation CC spells out what the first two mean: they identify the Federal Reserve District, 01 for Boston through 12 for San Francisco, and adding 2 to the first digit marks a thrift institution, so 21 is a thrift in the First District and 32 a thrift in the Twelfth. The IRS states it as a flat constraint in its Form 8038-CP guidance — the routing number must be nine digits with the first two a 01 through 12 or 21 through 32.

The ninth digit is a check digit over the other eight, under a weighted modulus-10 scheme. Multiply the digits by 3, 7, 1, 3, 7, 1, 3, 7, 1 left to right, add the products, and a valid routing number divides by 10 with nothing left over — the formula Apache Commons Validator ships as ABANumberCheckDigit. It catches every single-digit substitution and almost every transposition of adjacent digits, so a routing number that survives it is very probably the one on the paper.

The account number cannot

There is no equivalent. No national standard governs the format of a US bank account number, no check digit is defined for it, and nothing about the string tells you whether it is a real account.

Length varies by bank. Chase’s own guidance puts the typical range at eight to twelve digits, noting some run to seventeen. Seventeen is not a coincidence — it is the width of the account number field in the ACH network, and the IRS repeats the ceiling in its filing guidance.

Watch how the industry handles the gap. Nacha’s WEB debit rule, effective 19 March 2021, requires an originator to validate an account before its first use and after any change to the number, and the methods it accepts are a prenotification entry, a micro-transaction, a commercial validation service or an API. Every one means asking a bank, because there is nothing to compute.

That asymmetry is why a check is risky to read by eye or by camera. A wrong digit in the routing number gets rejected on arithmetic. A wrong digit in the account number gets rejected days later by a human, or not at all — it lands in someone else’s account. Why OCR gets numbers wrong works through which digits get confused for which, and why a misread digit looks exactly as valid as the right one.

What a deposit slip shows instead

A pre-printed deposit slip carries your account number already, which makes it the easiest place to read one off — no transit symbols, no business-check variant. What it does not reliably carry is the routing number you want for an electronic payment. The IRS tells filers to get the correct ACH routing and account numbers from their financial institution rather than infer them, because the number on a paper form is not always the one the ACH network expects.

Credit unions add their own trap: your member number is not your account number. VyStar puts it plainly — “Don’t use your VyStar member number for direct deposit. It is different than your account number and the VyStar routing number.” The account number is on your statement, in the app under account details, or at the bottom of a check.

If you are going to photograph it

Assume the MICR line will be the hardest part of the image. It was designed for a magnetic head and you are pointing an optical sensor at it.

• Get close and flat. The characters are eight to the inch. Shot across a desk at an angle, that is not many pixels per stroke, and the symbols suffer worst.
• Keep the whole line in frame. Crop into the digits and you lose the transit and on-us symbols, which are how you tell the fields apart.
• Read the account number back against the paper. There is no checksum to catch you.

Your phone already does part of this for free. Live Text on iOS and Google Lens on Android lift digits off a photo with nothing installed, and for a one-off they are a reasonable first try. What they will not do is tell you which run is which field, or warn you that a routing number failed its check.

SwipeScan is our app, and its value here is narrow enough to state precisely: it filters the image down to digit runs in an editable field, so you compare against the picture before pasting, and where a number carries a checksum it runs the check — what the IBAN reader with its mod-97 validation does, and the same logic a routing number allows. For an account number it cannot, and does not pretend to.

Where the digits land matters too. A spreadsheet will quietly destroy a seventeen-digit account number dropped into a General cell, covered in the traps in Data from Picture, and a number that survives the paste can still sit there as text that vanishes from your totals.

The short version

• On a personal check the bottom line reads routing, account, check number, left to right. The account number is the middle group.
• Transit symbols bracket the routing number, the on-us symbol closes the account number. Find the symbols, not the digit count.
• The number in the top-right corner is the check number; the small fraction beside it is an older form of the routing number.
• Business checks longer than six inches put the check serial at the far left, ahead of the routing number.
• A routing number is nine digits with a 3, 7, 1 weighted mod-10 check digit, so a misread one is detectable. An account number has no check digit and no standard length, so it is not.
• Read an account number back against the paper every time, and confirm it with your bank before it carries a payment.