Is there an online calculator for how much return you would get per month for a sum of money invested at a certain interest rate?

[ViciousSquirrelSpotter]

My mother is selling her house and will have half a million pounds to invest. She will want to live off the interest of this while she is renting, while she looks for a new house.

Am trying to work out what her budget will be for renting, council tax etc., but need to work out what her income would be.

X * Rate / 100
So if you have 500K and get 4%, it's:

500,000 * 4 /100 = 20K per year.

I am not a qualified IFA (but a CQF), so cannot give detailed advice, but the best option is probably to put most of it in a "savings" account which has restrictions on withdrawal, but leave (say) 20K in quick access account.

Of course she should shop around, since this varies a lot, and almost certainly the worst option is her current bank.

Thank you DC

I thought it was that but it seems too easy. Thought there must be more complications to it - for example, that sum must be before tax, no?

Yep, before tax, but typically you'd expect this to be deducted at source.
The more complex calc is if you leave it in the bank, and let the interest accrue.