The "B" is a check digit that validates your routing number
The ninth digit of a routing number — the one marked "B" on a check — is a check digit. It is a single number (0 through 9) that the banking system uses to verify that the first eight digits are correct. When you wire money or set up a direct deposit, the receiving bank runs a mathematical formula on all nine digits. If the math works out, the routing number is valid. If it does not, the transaction gets rejected before it ever leaves your bank.
You do not calculate or choose this digit yourself. Your bank assigns it when they issue your account. The check digit exists to catch typos — if you accidentally transpose two numbers or misread a digit, the formula will fail and flag the error immediately. This is why wiring money to the wrong routing number usually stops at the sending bank rather than reaching the wrong account.
The formula used is called the mod-10 algorithm. It multiplies each of the first eight digits by a specific weight (7, 3, or 1, repeating), adds them up, and then checks whether the sum plus the check digit is evenly divisible by 10. If it is, the routing number passes. This happens automatically in the background — you will never see the calculation, but it runs every time you enter a routing number into a wire transfer form or direct deposit setup.
Key Takeaways
- The "B" digit is a check digit that validates the first eight digits of your routing number using a mathematical formula.
- If you mistype any of the first eight digits, the check digit will not match and the transaction will be rejected by the sending bank.
- You do not need to understand or calculate the check digit yourself — your bank assigns it and the system verifies it automatically.
- The check digit protects you from accidentally sending money to the wrong bank, because an invalid routing number will fail validation before the wire leaves.
Where to find the "B" on your check
On a physical check, the routing number appears in the bottom left corner. It is nine digits long, printed in a special magnetic ink that machines can read. The digits are usually labeled 1 through 9 from left to right, though not all checks show the labels. The first four digits identify the Federal Reserve district and the bank's location. Digits 5 through 8 identify your specific bank. Digit 9 — the "B" — is the check digit.
If your check does not have visible labels, you can count from the left: the ninth and final digit in that bottom-left number is the check digit. You can also find your routing number on your bank's website, in your online banking portal, or by calling customer service. When you look it up online, the check digit is included in the nine-digit number — it is not listed separately.
Why the check digit matters for wire transfers
When you initiate a wire transfer, you provide your bank with a routing number and an account number. Your bank uses the routing number to locate the receiving bank. Before sending the wire, your bank's system validates the routing number by checking the math on all nine digits. If the check digit does not match, the system rejects the routing number and asks you to re-enter it.
This validation step is one of the few safeguards that stops wires from going to the wrong place. Unlike ACH transfers (which can sometimes be reversed), wire transfers are usually final once they leave your bank. A bad routing number gets caught at the validation stage, which is why you rarely hear about wires going to completely wrong banks. The check digit is doing that work behind the scenes.
If you are wiring money to someone for the first time, double-check the routing number by asking them to read it aloud or by looking it up on their bank's website yourself. Transposing even one digit will cause the validation to fail, and you will know immediately that something is wrong before any money moves.
What happens if the check digit is wrong
If you enter a routing number with an incorrect check digit, your bank's system will reject it during the validation step. You will see an error message saying the routing number is invalid. This happens before the wire is sent, so no money leaves your account. You can then correct the number and try again.
The system cannot tell you which digit is wrong — it only tells you that the nine-digit number does not pass the check. If you are unsure which digit you mistyped, the safest approach is to get the routing number directly from the receiving bank or from the person sending you their banking details. Ask them to provide it in writing or read it to you digit by digit.
The check digit does not protect against fraud
It is important to understand what the check digit does not do. It validates that a routing number is mathematically correct, but it does not verify that the routing number belongs to the person or business you think it does. If someone gives you a valid routing number that belongs to a different bank entirely, the check digit will pass and your wire will go through — just to the wrong place.
The check digit protects you from typos, not from deception. If you are wiring money to someone you do not know well, verify the routing number through an independent source — call the business directly using a phone number from their official website, or ask your bank to confirm the receiving bank's name before you send the wire. The check digit will catch your mistakes, but only you can catch fraud.
Frequently Asked Questions
Can I use a routing number if the check digit is wrong?
No. Your bank will reject the routing number during validation if the check digit does not match the first eight digits. You will get an error message and the wire will not be sent. You must correct the routing number before proceeding.
Is the check digit the same for everyone at my bank?
No. The check digit is calculated based on the first eight digits of your routing number, which are specific to your bank and branch. Different banks have different routing numbers and therefore different check digits. Even two branches of the same bank may have different routing numbers and check digits.
What if I wire money to a valid routing number but the wrong account number?
The money will go to the receiving bank, but it may go into the wrong account if the account number does not exist or belongs to someone else. The routing number check digit only validates the bank, not the account. Always verify both the routing number and the account number before sending a wire.
Do I need to memorize the check digit?
No. Write down or save the entire nine-digit routing number, including the check digit. You do not need to understand how it works or calculate it yourself — just make sure you have all nine digits correct when you enter it into a wire transfer form.