What is Modulus Checking and why should I do it?

Read time: 2 mins

Last updated: 12 May 2025

What Is Bank Account Modulus Checking?

When businesses collect customer bank details for payments like Direct Debits or Bacs Direct Credits in the UK, it's vital to catch errors early. Modulus checking is a quick, automated way to check whether a bank account number and sort code pairing is likely to be valid - helping prevent payment failures and delays.

Why Is Modulus Checking Important?

Mistakes in bank account details can lead to failed payments, unhappy customers, and extra admin. Modulus checking helps by:

Reducing failed payments by flagging invalid sort code and account number combinations before a payment is processed.
Saving time and money by catching errors early, reducing the need to investigate returned or failed payments.
Improving cash flow by helping payments go through smoothly and on time.
Enhancing customer experience by avoiding missed payments and the frustration they cause.

For paperless Direct Debit sign-ups, modulus checking is even a requirement under Bacs' AUDDIS scheme.

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How Does Modulus Checking Work?

Modulus checking applies a mathematical formula (an algorithm) to a bank account number and sort code. This algorithm checks whether the combination follows the expected structure for that financial institution. In the UK, there are dozens of different modulus checking rules, depending on the bank and account type. These rules are maintained and regularly updated by Bacs and other providers to keep up with changes in the banking system. When a modulus check is run, it can return a few possible results:

Valid: The details match the expected pattern for that bank.
Failed check: The numbers don't pass the check - likely indicating a typo or invalid details.
Not checked: The financial institution hasn't made a check rule available for those details.
Invalid length: The sort code or account number doesn't have the required number of digits (always 6 digits for a sort code, usually 8 digits for an account number).

What Modulus Checking Can (and Can't) Do

What it does:

Confirms whether a sort code and account number pairing follows a valid pattern.
Helps reduce errors before payments are sent.
Can be integrated into payment systems or used via online services.

What it doesn't do:

Confirm that the bank account actually exists.
Verify that the account belongs to a particular person.
Guarantee that the account supports Direct Debits or Direct Credits.

For full validation - like confirming account ownership or Direct Debit eligibility - additional services are required. Mintly's tools can help confirm the payment types accepted by a finance institution.

How to Set Up Modulus Checking

If you're handling Direct Debit or bank transfers, modulus checking can be added to your payment process in a few ways:

Via your Direct Debit provider: Many providers, like GoCardless, handle modulus checks automatically when customers submit their details.
Using a third-party modulus checking service like Mintly: Available as online tools, APIs, or software integrations. Options range from free trials for low volumes to paid services for larger businesses.
Building it into your own systems: Large businesses with in-house development teams can implement modulus checking rules themselves, though these need regular updates to stay accurate with the latest data and rule changes.

Final Thoughts

Modulus checking is a simple but valuable step in ensuring the accuracy of customer bank details before making payments. While it doesn't confirm account ownership or guarantee payment success, it dramatically cuts down on avoidable errors and the operational hassle that follows.

If you process customer payments in the UK - especially via Direct Debit - modulus checking isn't just a helpful tool, it's often a necessary part of a reliable, customer-friendly payment system.

If you'd like to learn more about how Mintly can help you with modulus checking, or if you have any questions about our services, please get in touch. We'd be happy to help!