What Is a Merchant ID (MID)?
A merchant ID (MID) is the unique number your acquirer assigns to your merchant account. It travels with every transaction you process, telling the card schemes and banks which business each payment, refund and chargeback belongs to.
You will find it on your merchant services statement, on the merchant copy of terminal receipts and in your provider's dashboard. You need it when cancelling, switching provider, disputing a chargeback or completing PCI DSS paperwork.
Rates checked: August 2026
What the MID actually does
Think of the MID as the account number for your card takings. When a customer pays, the transaction leaves your terminal stamped with your MID. The acquirer uses it to route the settlement into the right business bank account, and the card schemes use it to track your business's behaviour: volumes, refund rates, chargeback ratios. If a customer disputes a payment months later, the MID is how the dispute finds you.
One business can hold several MIDs. A shop with an in-store terminal, an online checkout and a phone-payment line will often have a separate MID per channel, because card-present and card-not-present transactions are priced and risk-scored differently. Multi-site businesses usually get one per location too, which is what makes per-branch reporting possible.
The MID is not secret in the way a password is, but treat it with care: combined with other details it can be used in social-engineering attempts on your provider. Quote it on cancellation letters and to your own provider; do not publish it.
MID vs TID vs merchant account: which number is which
| Term | What it identifies | Where you see it |
|---|---|---|
| Merchant ID (MID) | Your merchant account with the acquirer | Merchant statement, terminal receipt, dashboard |
| Terminal ID (TID) | One specific card machine; a MID can have several TIDs under it | Printed on terminal receipts, terminal settings menu |
| Merchant account | The holding account where card takings settle before payout | The thing the MID is the number for |
| Merchant category code (MCC) | What type of business you are, assigned alongside the MID | Set by the acquirer at onboarding; see our MCC code answer |
When you actually need your MID
- Cancelling or switching provider: quote it in your cancellation letter so the request cannot be misfiled
- Chargeback disputes: evidence you submit is matched to your account by MID
- PCI DSS compliance: self-assessment questionnaires ask for it
- Reconciling settlements: multi-MID businesses match payouts to channels or branches by MID
Note the flip side: if you use SumUp, Square or Zettle, you do not have a dedicated MID at all, because you sit under the provider's master account. Whether that matters for your business is really the question of whether you need your own merchant account, which we answer in do I need a merchant account in the UK.
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