Somebody eariler was asking about a Flex calculator, so they could work out before a purchase what the options would be if they were to Flex it.
Since the Monzo devs are probably quite busy, I threw some random numbers at google sheets until it looked vaguely useful.
DISCLAIMER : This is not an official Monzo product, and is not endorsed by them in any way It is also APPROXIMATE. It should be pretty close to the correct amounts, But use it only as a general indicator and assistant to see what the likely payments on a Flex purchase. will be. Check the interest rate matches yours, and enter the purchase amount in the pink boxes below. You can then see the calculated payments in the table underneath. If you want to see the maths, scroll further down. Feel free to copy this sheet to your own drive to have a fully editable version to play with.
Making a truly accurate one would involve knowing the purchase date and your payment date, since interest is calculated daily, so that can be anything from 2 to 6 weeks to the first payment, and then each month’s interest will actually depend on the number of days in the month. So this one is approximate. It assumes all months are the same length, and the purchase is one month before the first payment. So it isn’t terrible, and should give you a reasonable indicator of the monthly payments and total interest you’ll end up with for any purchase across 1-12 months. It gets a bit squirrely on very small transactions, since the monthly payments are always rounded up to the whole £ (which it deals with), and that means that small amounts and long times don’t mix and are not actually allowed in Flex (which it doesn’t deal with).
So, since it might be useful to someone, here’s a link to the google sheet. Help yourselves to copying it and having a play.