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What The ISA Calculator Works Out
- Projected ISA value: each monthly contribution compounds at your growth rate divided by 12 for the months it is invested. GBP 200 a month for 10 years at 5% grows to about GBP 31,000. Cash ISAs currently pay roughly 3% to 5%; stocks and shares ISAs have historically returned more over long periods, with short-term falls along the way.
- You paid in: monthly contribution multiplied by the number of months, plus any opening lump sum. The allowance is GBP 20,000 per tax year across all ISA types and is use-it-or-lose-it; from April 2027 the cash ISA portion is set to fall to GBP 12,000 for under-65s.
- Tax-free growth: the projected value minus what you paid in (and minus any government bonus). Inside an ISA this is permanently free of income tax and capital gains tax.
- Opening lump sum (advanced): an amount invested on day one, for example an existing ISA transferred in. Transfers between providers keep the tax wrapper but must go through the provider's transfer process rather than a withdrawal.
- Increase contribution each year (advanced): raises the monthly amount by a set percentage every 12 months, for example in line with pay rises.
- Annual charges (advanced): platform and fund fees taken from the balance each month. Stocks and shares ISAs typically cost 0.2% to 1% a year in total; over 10 years that can quietly remove several thousand pounds from the final pot, shown in the charges paid card.
- Lifetime ISA bonus (advanced): the government adds 25% to Lifetime ISA contributions of up to GBP 4,000 a year (a maximum GBP 1,000 bonus) for a first home or retirement after 60. You must be 18 to 39 to open one and other withdrawals carry a 25% penalty.
- Tax saved versus a taxable account (advanced): the same savings run with growth taxed each month at your marginal rate, compared with the ISA. It ignores the personal savings allowance (GBP 1,000 for basic-rate taxpayers, GBP 500 for higher-rate, nil for additional-rate), so the real saving is smaller for modest pots and the ISA's edge grows with the size of the pot.
- Value in today's money (advanced): the final pot deflated by your inflation assumption, so you can see what the money would actually buy.
- Year-by-year table (advanced): what went in and the pot's value at the end of each year, with the bonus shown separately for a Lifetime ISA.
Source and further reading: GOV.UK: Individual Savings Accounts.