How amex cc interest is calculated?

NachoGR

From their website I can read this :

To calculate the interest on your account each month, American Express multiplies the Daily Periodic Rate by the number of days in the billing cycle, and then multiplies that total by the average of your daily balances.

So, I have a question if I want to avoid paying a single penny in interest - what should I do?
When I buy something does it count on the same billing cycle? Meaning that if the balance stays more than 0 even for a single day, I will still have to pay some interest?

Example:

day 10 : buy sth that costs 20£
day 11: it appears on my balance as confirmed transaction
day 13: i repay

so the amount stayed for 2 days - does that count towards interest?

ViolaLass

Pay your bill in full by the date stated on your bill. That way you'll pay no interest.

ic

+1 - just pay the full statement balance by the statement date - that way you get the whole lot interest free - so depending on when you bought whatever you bought, you'll get up to eight weeks interest free. Best option is to set up a direct debit to make this payment for you.

If you underpay by even just a penny, then they will charge you the full interest as shown on your bill for that month. A part payment will only have an effect in reducing the interest charged the following month. Once you are charged some interest, it can take a couple of months to clear it, even if you start paying the bill in full.