How Credit Card Interest Is Actually Calculated

The annual rate is not what's applied

Your statement quotes an annual rate. Your issuer does not charge you once a year. It converts that annual figure into a daily periodic rate — the annual rate divided by the number of days in the year — and applies it every day to whatever you owed that day.

An annual rate of about 22% becomes a daily rate of roughly 0.06%. That sounds trivially small, which is exactly why it's easy to underestimate. Applied to a balance every day for a year, and compounding, it produces slightly more than the headline figure.

Two things follow immediately, and they're the practical core of this whole subject:

• Interest accrues by the day, not by the month. Paying a week early genuinely costs less than paying a week late, even within the same billing cycle.
• The balance the rate is applied to changes daily. There is no single "the balance". There's a different one every day, and the calculation cares about all of them.

Average daily balance, step by step

The most common method works like this. For each day in the billing cycle, the issuer records what you owed at the end of that day. At the end of the cycle it averages those daily figures, then applies the daily rate to that average, multiplied by the number of days.

A worked illustration, using round numbers for clarity. A 30-day cycle. You start owing £1,000. On day 10 you spend £500. On day 20 you pay £300.

• Days 1–9: £1,000 → 9 × 1,000 = 9,000
• Days 10–19: £1,500 → 10 × 1,500 = 15,000
• Days 20–30: £1,200 → 11 × 1,200 = 13,200

Total 37,200, divided by 30 days = £1,240 average daily balance. At a daily rate of 0.06%, that's about £0.744 a day, or roughly £22 for the cycle. Notice that the average, £1,240, matches neither your opening balance nor your closing one.

The grace period, and how it's lost

Most cards give you an interest-free window on purchases: spend during the cycle, and if you pay the full statement balance by the due date, you're charged no interest on those purchases at all. This is why a card can be genuinely free to use for someone who clears it every month.

The part that catches people is what happens when you don't clear it. On many agreements, once you carry a balance, the grace period stops applying — and it stops applying not just to the old balance but to new purchases too. New spending starts accruing interest from the day of the transaction rather than from the end of the cycle.

Getting the grace period back typically requires clearing the balance in full and often keeping it clear for a further cycle. This is the mechanism behind a common and painful experience: someone pays off almost everything, leaves £30 outstanding, and is surprised to be charged interest on a full month of new spending.

Two details worth knowing:

• "Full balance" means the statement balance, not the current balance. Purchases made after the statement was issued belong to the next cycle. Paying the statement figure is what preserves the grace period.
• Cash advances usually have no grace period at all, regardless of how you pay. More on that below.

Which balance gets paid first

A single card frequently carries several balances at different rates: purchases, balance transfers, cash advances, promotional 0% spending. When you make a payment, the order in which it's applied changes what you end up paying.

In a number of jurisdictions, regulation now requires that anything above the minimum payment goes to the highest-rate balance first, which is the arrangement that costs you least. Where that rule doesn't apply, agreements have historically allocated payments to the cheapest balance first — which keeps the expensive one alive longer.

Whichever regime you're under, the minimum payment itself is usually allocated by the issuer's own rules. That's part of why paying only the minimum while carrying a mixed balance is so slow: the expensive portion can barely move.

Why cash advances are different

Withdrawing cash on a credit card is treated as a different product, and it typically differs on three axes at once:

• A separate, higher rate than purchases.
• No grace period. Interest starts on day one, even if you pay in full at the end of the cycle.
• A fee on top, often a percentage with a minimum.

The category is broader than an ATM withdrawal. Depending on the issuer it can include buying foreign currency, gambling transactions, some money transfers, and buying certain financial instruments. These are classified by the merchant category code the transaction carries, not by what you thought you were doing — which is how people occasionally find a purchase treated as a cash advance without warning.

Why minimum payments last so long

A minimum payment is typically a small percentage of the balance, subject to a floor amount, plus interest and fees. The structure has a specific consequence: because the percentage is applied to a shrinking balance, the payment shrinks too, and the proportion of it going to interest stays stubbornly high for a long time.

This is why many jurisdictions now require card statements to print how long the balance would take to clear at the minimum, and what it would cost in total. That box is often the most informative thing on the statement, and it's there precisely because the arithmetic is counter-intuitive.

The mechanical point, stated neutrally: every unit paid above the minimum goes straight at the balance, and because interest is charged daily on that balance, the effect compounds in your favour for the remaining life of the debt.

What happens when a 0% deal ends

Promotional rates are a genuine zero — no interest accrues during the promotional window on the promoted balance. What matters is the structure around them.

• The end date is fixed, not floating. It runs from when the card was opened or the transfer was made, not from when you last used it.
• Whatever remains reverts to the standard rate on that date. There's no partial credit for having nearly cleared it.
• Missing a minimum payment can end the promotion early on many agreements. This is in the terms and it is enforced.
• Balance transfers usually carry an up-front fee, expressed as a percentage of the amount moved. A 0% deal is not the same as a free deal.
• New spending on a balance-transfer card may not be at 0%, and because of the grace-period rule above, it can start accruing immediately.

Where people lose money without noticing

• Paying the current balance instead of the statement balance — or the reverse. Only one of them preserves the grace period, and it's the statement balance.
• Leaving a small amount outstanding. A few pounds carried over can switch new purchases into immediate-interest mode for the whole next cycle.
• Paying on the due date via a method that takes days to arrive. The date that counts is when it reaches the issuer.
• Treating a cash-like transaction as a purchase. Currency, gambling and some transfers can be classified as cash advances with no grace period and a fee.
• Assuming the promotional rate covers everything on the card. It usually covers one specific balance.
• Not reading the statement's own repayment box. It's the one number on the page calculated specifically for your situation.
• Ignoring a card that failed to charge. A missed minimum can cost a fee, a rate increase and a promotional deal at once — see why payments get declined.