How it works
The annual rate (minus fees) is turned into an equivalent monthly rate, (1 + annual rate)^(1/12) − 1, and applied every month to the balance, so returns are compounded. Extra monthly payments are added at the end of each month. First-year return = amount × annual rate; monthly figure = first-year return ÷ 12.
A worked example
£10,000 in savings at 4% earns £400 in the first year, about £33.33 a month, and grows to £14,802.44 after 10 years. In an S&P 500 fund returning its long-run average of 10% a year it would grow to £25,937.42, but that return is not guaranteed and can be negative in any year.
What to keep in mind
Savings rates are variable and investments can fall as well as rise; past returns, including the S&P 500's, do not guarantee future returns. Rates shown are assumptions, not offers: change them to match a real account. Excludes tax on interest or gains outside ISAs, inflation and currency movements. This is an estimate, not financial advice.
Savings or investing?
Money in a bank or building society savings account earns a set interest rate that can change, and it is protected by the FSCS up to £120,000 per person, per banking licence. Money invested in funds, shares or an index such as the S&P 500 can grow faster over the long term, but its value goes up and down and you can get back less than you put in. As a rule of thumb, keep money you may need within about five years in savings and only invest money you can leave alone.
Where the default rates come from
In October 2026 the Bank of England base rate is 3.75% and the best easy-access savings and cash ISAs pay roughly 4% to 5%, so the calculator assumes 4%. Many high-street current and savings accounts pay much less, so check your own rate. The S&P 500 has returned about 10% a year on average since 1957 including dividends, or about 6.5% to 7% after inflation, but with big swings, including falls of more than 30% in a single year. 5% for a global fund or managed fund is a cautious long-term assumption, not a promise. Pick 'My own rate' to enter any figure.
Worked examples
£50 in a savings account at 4% earns £2 in a year. £10,000 at 4% earns £400 in the first year, about £33.33 a month, and becomes £14,802.44 after 10 years. The same £10,000 growing at 10% a year in an S&P 500 fund would reach £25,937.42 after 10 years and £67,275 after 20. £1 million in savings at 4% would earn about £40,000 a year, roughly £3,333 a month, before tax.
ISAs, tax and protection
You can put up to £20,000 a year into ISAs in total, and interest and gains inside an ISA are tax-free. From 6 April 2027, people under 65 will be able to put at most £12,000 of that into cash ISAs. Outside an ISA, basic-rate taxpayers can earn £1,000 of savings interest a year tax-free and higher-rate taxpayers £500, and gains on investments may be subject to Capital Gains Tax. FSCS protection covers up to £120,000 per person, per banking licence, so large sums should be spread across separate banks.
Fees matter more than they look
A fund charging 1% a year turns a 5% return into 4%. On £10,000 over 10 years that is the difference between £16,288.95 and £14,802.44, almost £1,500. Index funds tracking the S&P 500 often charge 0.1% or less; managed funds and some platforms charge 0.5% to 1.5% in total. Enter the total yearly charge in the fees box.
Frequently asked questions
How much interest will I earn on £10,000?
At 4% a year, about £400 in the first year, or £33.33 a month, before any tax. At 5%, about £500 a year.
How much would £10,000 in the S&P 500 be worth in 10 years?
If it matched the long-run average of about 10% a year, about £25,937. Actual returns vary a lot from year to year, can be negative, and depend on the pound-dollar exchange rate.
How much interest does £1 million earn in the UK?
At 4%, about £40,000 a year, or about £3,333 a month, before tax. Only £120,000 per banking licence is protected by the FSCS, so spread large sums across banks.
Is the return guaranteed?
Savings interest is paid at the account's rate, which can change unless it is a fixed-rate account. Investment returns are never guaranteed and you can get back less than you put in.
Is this financial advice?
No. It is an estimate based on the rates you choose. For advice on your own situation, speak to a regulated financial adviser; MoneyHelper has free guidance.
Sources & rates
Rates checked on 7 October 2026.
For general information and planning, not personalised financial or tax advice. Verify important decisions with a qualified adviser.