What is EMI on a debit card?
Nothing about a debit card lets you pay later. What is happening is a loan, arranged in advance, that your card is merely the key to.
The shop was paid at once. You entered a loan.
What actually happens at the till
Your bank has already decided, before you walked into the shop, that it would lend you a certain amount. That pre-approved limit sits against your account, usually based on your salary history and balances, and you were probably never told the exact figure.
When you choose EMI at checkout, the merchant is paid the full amount immediately by the bank. You now owe the bank, repayable in instalments debited from your account each month. The purchase is complete and the credit relationship has begun.
The important consequence: this is a loan, so it is reported like one. It affects your credit file, missed instalments are missed loan payments rather than a card inconvenience, and the obligation continues if you return the goods unless the cancellation is handled properly.
Where the cost is
Interest, at a rate that varies by bank and tenure and is often comparable to a personal loan rather than to anything cheaper.
A processing fee, charged up front, which is the part most people overlook when comparing offers.
No-cost EMI is a different structure. There, the merchant absorbs the interest by discounting the price, so you pay the sticker price in instalments and the interest is inside the price rather than added to it. It is genuinely free to you in most cases, and it is worth checking whether the same item is cheaper when paid outright, because sometimes the discount was available anyway.
Foreclosure fees. Paying the loan off early frequently costs a fee, which surprises people who assume clearing a debt early is rewarded.
The comparison worth making
Debit card EMI is usually cheaper than a credit card revolving balance and usually more expensive than a personal loan for larger amounts. It sits between the two, and its advantage is convenience rather than price.
The question that resolves most cases is simply: what is the total amount repaid, including processing fee, against the cash price? One number against one number. Every other framing in the shop is designed to make the monthly figure look small, which is what monthly figures are for.
Being handed the keys to a flat and told the deposit can be paid over six months. You live there from day one. The deposit is owed from day one. And the person who handed you the keys is now also your lender, which is a second relationship you did not think you were entering.
Where you meet it
Every checkout offering instalments on a card that has no credit line. Every pre-approved offer sitting in your banking app. Every EMI that kept running after the item was returned.