Modulus checking: what is it anyway?

Modulus checking is a procedure for validating sort code and account number combinations.


In the UK, modulus checking is a procedure for validating a sort code and account number combination. It's useful for Bacs Direct Debit and Bacs Direct Credit as it confirms that the sort code and account number could be a valid combination.

Modulus checking can reduce the number of failed payments, by automatically highlighting invalid bank account details. However, it doesn't show whether the account actually belongs to that customer in question. It also doesn't show whether the account is set up to support Direct Debit.

Why should you use modulus checking?

  • You may have no choice; paperless DD users have to use modulus checking. Modulus checking is mandatory at the point of sale for AUDDIS users (if you’re using paperless Direct Debit) but as we mentioned in our article on AUDDIS messages it’s a great way for anyone collecting Direct Debit to reduce payment failures.

  • Modulus checking helps you reduce Direct Debit payment failures. It means you can make sure customers’ account details are taken accurately at the point of sale and fix them there and then rather than dealing with returned payments and trying to fix errors later.

  • Modulus checking helps you keep your customers happier. If a Direct Debit payment isn’t collected when your customer expects it to be they’ll probably blame you - even if the hold up is because they provided incorrect details! By helping you to reduce errors (and therefore customer inconvenience), modulus checking improves the customer experience and takes some of the hassle out of keeping your customers happy.

What do I need to do to set up modulus checking?

There are a few providers you can use for modulus checking (some of which offer free trials). Which works best for you will depend on factors including the number and frequency of checks required.

If you have a lot of details to check, using a modulus checker service can be pretty costly and you may want to consider whether a Direct Debit provider could help. However, if you’re collecting payments from a handful of people a free trial may be enough.

At GoCardless we carry out modulus checking for you as soon as your customer submits their details. This means we can help you to resolve any errors before we submit a mandate avoiding future errors and unhappy customers.

Got questions about modulus checking?

GoCardless are Direct Debit experts, that offer modulus checking as part of the service. If you have any questions about how to perform a modulus check, call our team on 020 8338 9537. You can also leave your details below to receive a free callback from a member of our team.

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