How credit card interest is worked out
Each month the card adds interest to what you owe, and your payment then comes off the total. Interest = balance × monthly rate, so a payment that only just covers the interest leaves the balance almost untouched. The calculator repeats this month by month until the balance reaches zero.
A UK card statement quotes an APR, which is a yearly rate that already includes monthly compounding. By default the calculator turns it into a monthly rate with (1 + APR)^(1/12) − 1, the effective-rate basis the FCA sets for APR calculations. At 24% that is a monthly rate of 1.8088%. Some lenders divide the yearly rate by 12 instead, which is slightly dearer, so the APR convention field lets you choose between Effective APR (FCA) and APR divided by 12.
Worked example: £5,000 at 24% with £300 a month
On a £5,000 balance at 24% APR, paying £300 a month clears the card in 21 months (1 year 9 months). The first month's interest is £90.44, the total interest is £1,004.11 and you repay £6,004.11 in all. Lloyds Bank's online card interest calculator, with the same starting figures, showed total interest of £1,004.11 over 1 year 9 months when we checked it on 26 October 2026.
Dividing the rate by 12 instead gives £1,143.34 of interest over 21 months, so the choice of convention matters at the penny level but not over the length of the repayment.
Clearing the balance in a set time
To clear the same £5,000 in 24 months, choose the set number of months. The payment is £258.66 a month and the total interest is £1,207.92. Over 12 months it is £467.26 a month and £607.15 of interest. Paying faster costs more each month but far less in interest.
What paying only the minimum costs
FCA rules (CONC 6.7.5R) require a card's minimum payment to cover at least the interest, fees and charges plus a share of the balance. Many UK cards use the interest plus 1% of the balance, with a floor of a few pounds; your statement states your card's rule, so enter it in the minimum fields. With 1% plus interest and a £5 floor, the first minimum payment on this balance is £140.44, and paying only the minimum would take 390 months (32 years 6 months) and cost £8,831.86 in interest.
Against that, the £300 a month plan saves £7,827.74 of interest and 369 months. The minimum is recalculated each month as the balance falls, so it shrinks toward the floor and the payoff drags on.
When the payment is too low
If your payment does not cover the month's interest, the balance grows instead of falling. At 24% on £5,000 the first month's interest is £90.44, so a payment of £90 or less is refused with a message that says how much interest you need to cover. The FCA's persistent debt rules (CONC 6.7.27R) are aimed at exactly this pattern: if over 18 months you have paid less in principal than in interest, fees and charges, the lender has to write to you.
What this calculator leaves out
It models one card with no new spending, a payment every month and a fixed rate. It does not include 0% offers, balance transfer fees, annual fees, late fees or cash advance charges, and it does not model the day-by-day timing of statement and payment dates. Treat it as a planning tool and check figures against your statement.
To model a percentage-of-balance payment on its own, use the credit card repayment calculator.
To see the effect of extra payments, or a loan to clear the card, use the credit card loan payoff calculator.
Use the right country page
This page uses pounds and the UK convention for turning an APR into a monthly rate. The country choices above the inputs open separate local pages. Choosing United States opens the US credit card interest calculator, which works in dollars and divides the APR by 12 or compounds the daily rate. Inputs reset and amounts are not converted.
Inputs for the credit card interest calculator
APR convention
Effective APR (FCA) is the default and follows the FCA APR rule. APR divided by 12 is how many UK sites work and gives a slightly higher monthly rate.
How you want to pay
Enter a fixed monthly payment, or the number of months you want to take to clear the balance.
Card balance
How much you owe on the card now.
Card rate (APR)
The yearly rate on your statement. Enter the APR (effective annual rate).
Monthly payment
Used for a fixed payment. The amount you pay each month.
Months to clear
Used when clearing in a set number of months. A whole number up to 600.
Minimum payment: share of the balance
Used for the minimum comparison. Check your statement for the lender's rule.
Minimum payment also covers the month's interest
Many UK cards ask for the interest plus a share of the balance. Check your statement.
Minimum payment: lowest amount
The lowest amount the card asks for in a month, from your statement.
CALCULATOR QUESTIONS
UK credit card interest calculator questions
How do you calculate credit card interest?
Multiply the balance by the monthly rate, add it to what you owe, then take off your payment, and repeat each month. On £5,000 at 24% APR with £300 a month the total interest is £1,004.11 over 21 months.
Should I enter the APR or the EAR?
Enter the APR from your statement. Lloyds and Halifax label their input an effective annual rate; for a card the figure you are given is the one to use, and the default setting treats it as an effective annual rate.
Why does dividing the rate by 12 cost more?
A rate of 24% divided by 12 is 2% a month, which is higher than the 1.8088% a month that 24% effective gives. On £5,000 with £300 a month the totals are £1,143.34 and £1,004.11.
What happens if I pay less than the interest?
The balance would grow, so the calculator shows a message instead of a result. At 24% the first month's interest on £5,000 is £90.44, so you need to pay more than that to start clearing it.
Does this include fees?
No. It covers interest only. Annual fees, late fees and cash advance charges would add to the cost.