Working out monthly interest charges

Jenny59_2

Hi everyone
If my credit card is 16.536%, do I just divide that into 12 to get the monthly % interest? I've just received my statement and for the first time, have incurred interest charges (although I paid the full balance, I was a day late). I am querying it because with a balance of £828, I was charged £24 interest. This is about 3%. The company have been more than fair, and waived the interest, plus the £12 late payment fee because it was a 'one-off' caused by a glitch in my online banking, however, I would still really like to know how to do the maths, so to speak.
When I asked them to explain it to me, all they would say was 'it is was it is'! Very helpful! The interest had been estimated at £17 - quite different to the £24 actual charge - bearing in mind the full balance was received the day after the due date. I don't get it.
Many thanks for your time.

CannyJock

If you don't pay the balance in full then you're normally charged interest from the date of the original transaction so can be more than a simple % of the closing balance on the card when it first occurs.

Interest is calculated daily and applied monthly. If you download the interest calculator spreadsheet here and enter in the details of the date and amount from your previous nil balance statement up to the date the interest appears, you'll see how it was worked out.

You'll need to enter the APR and it'll work out the rest.

EDIT: Ignore the bit about right-clicking on the link and saving target as. Just click on the Download option and save the file that way.