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Interest on a Cash ISA balance is the balance multiplied by the rate, adjusted for how often the provider adds interest. This guide works through examples at assumed rates of 2%, 3% and 4%, which are not forecasts.
The usual ISA pitfalls are withdrawing instead of transferring, paying in over the allowance, Lifetime ISA age limits, and assuming protection that does not exist. Here is what the rules say about each.
gov.uk says you cannot hold an ISA with someone else. Reading the limit as applying to each adult, giving a couple two between them, is this site's own interpretation. Here is how that works, with a worked example.
The 25% Lifetime ISA charge applies to the total amount in the ISA, including the bonus, when the whole balance is withdrawn, so an early withdrawal can pay out less than was paid in. The examples below show the arithmetic.
How an ISA calculator turns balance, contributions, rate and time into a projected return, how to read nominal and real figures, and what it leaves out.