Hey DiS. I think I'm useless at Googling, but basically, finally got round to being sensible enough to put my paltry savings into the best ISA I could find last year (interest barely above inflation, economics win!). Anyway, it was one of those with a special 12-month 'bonus' interest period, at which point the interest rate reverts to something stupidly small, so I'll move it somewhere else then.

BUT I'm trying to work out how interest works. Interest on the crap amount of money in there is 3.5%, easy. But a few months ago I had to take some money out of it to pay my rent. The interest, when the year is up (April 5th or whenever the end of the tax year is) will go into my current account in one lump sum. But how is the interest calculated then, if I've had a variable amount of money in there over the course of the year?

Sure there is a really simple answer but my C in GCSE economics isn't really helping me here, I'm a numpty.