What does this ISA calculator estimate?

It gives you a quick projection of how an ISA could grow based on your starting balance, monthly contribution, time horizon, expected return, and ISA type.

Does this use guaranteed ISA rates?

No. This is a planning tool, so the projected return is only an assumption. Cash ISA rates can change, and investment ISA returns are not guaranteed.

Can I compare different ISA types here?

Yes. The calculator lets you switch between Cash, Stocks and Shares, Junior, and Lifetime ISA assumptions so you can see how the projection changes.

Do small changes in return or contribution really matter?

Yes. Even modest differences in contribution level, timeframe, or return can materially change the final value because compounding builds over time.

What if I want to model this in more detail?

Use the advanced ISA calculator for deeper modelling, richer charts, and inflation analysis. Sign in when you want extras like saved scenarios and account-linked features.

ISA Calculator UK: Estimate Interest and Growth

ISA Calculator FAQs

Short answers to the common questions people ask before using the tool.

What This Calculator Shows

This calculator is useful when you want to see how your ISA could grow from a starting balance, regular contributions, and the type of account you have in mind.

It gives you a clearer view of the projected end value, how much your monthly contributions are doing over time, and whether switching ISA type, timeframe, or return assumptions changes the result enough to matter.

Worked Example

One simple scenario to show how the inputs translate into an ISA projection.

Example scenario

ISA type

Stocks and Shares

Starting balance

£15,000

Monthly amount

£500

Term

15 years

Return

5.5%

Increase

2% yearly

In a case like this, small changes in return assumptions, contribution growth, or time horizon can make a noticeable difference to the projected outcome, so it is usually worth testing a few nearby versions.