G-Sheet based Flex Payment Calculator / Estimator

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.


This is great!! Very useful.

Any chance you could make a copy (another tab maybe) that works it out over 24 months for us trialing this feature?
I could probably do it myself, but my brain isn’t firing on all cylinders are the moment.

I’m glad you like it.

I’ve added two more rows, for 18m and 24m. These are even more approximate than the first 12, since the main of error compounds over time, but they still should give you a reasonably close answer.

I’m not on the 24m trial, though, so I can’t compare them to in-app numbers, so I’m guessing, there.

1 Like

Thanks! Just tested it with a couple of transactions I have over 12 and 24 months and compared the spreadsheet with in-app.
For me, the app shows slightly less interest to be paid than your spreadsheet but it’s pretty close. Close enough for me to actually use it for working out an approximation.

So thank you! <3

Yeah, that’s probably as goos as I can get it. I’ve tried to do it in a way that will tell you a bad-case estimate, rather than best case, so it should hopefully never be BAD news when you move into the app.

Thanks for confirming that it’s not WAY OFF on the longer term ones, though :+1:

1 Like

For me, it was about £3 higher in your spreadsheet than in the app.
That was on a £500 purchase over 24 months at 19%, so not bad at all. :slight_smile:

1 Like

Apologies to anyone who has been trying to use this for the last month or so. There was an issue with the sharing permissions, which is now hopefully fixed.