What this calculator works out
This calculator projects a starting balance and a fixed monthly contribution forward at a chosen annual rate. It returns the final balance, the total you will have contributed, and the growth on top.
Separating contributions from growth is the point. Over shorter periods most of the balance is money you put in; over longer periods growth takes over. Seeing which is which tells you whether the outcome depends mainly on your saving or mainly on the return.
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Before you rely on the result
- Use a monthly figure you can sustain through an ordinary year, including the months containing Christmas, a holiday or an insurance renewal.
- Choose a rate appropriate to where the money will actually sit. Cash and investments behave very differently, and the calculator cannot tell which you mean.
- Deduct charges from the rate before entering it. Platform fees and fund charges are taken every year.
- Treat the projection as one scenario among several rather than a forecast. Run it two or three times across a range of rates.
In the first few years, almost all of the balance is money you paid in — growth on a small balance is small in absolute terms. The crossover, where cumulative growth begins to exceed cumulative contributions, typically takes well over a decade at ordinary rates. This is why starting early matters more than optimising the rate.
How this calculator works
The calculator grows the opening balance and the stream of monthly payments separately, then adds them:
i = annual rate ÷ 12 ÷ 100, n = years × 12
Future value = start × (1 + i)n + monthly × [ ((1 + i)n − 1) ÷ i ]
Contributions = start + (monthly × n)The second term is the standard future value of an ordinary annuity: it assumes each contribution is made at the end of the month, so the final payment earns no interest. If the rate is zero, the calculator simply adds the contributions rather than dividing by zero.
Growth is the future value minus total contributions, which is the figure worth watching over time.
Worked example: £200 a month for five years
Using the default figures — £1,000 opening balance, £200 a month, 4% a year, over five years:
- Monthly rate: 4% ÷ 12 = 0.3333%, over 60 months
- Opening balance grows to: £1,000 × 1.00333360 = £1,220.99
- Contributions grow to: £200 × 66.298 = £13,259.80
- Final balance: £14,480.79
- Total contributed: £1,000 + £12,000 = £13,000
- Growth: £1,480.79
Growth is about 10% of the final balance. Over five years, the outcome is overwhelmingly determined by how much you saved, not by the rate you got. Extend the same contributions to fifteen years and the balance reaches roughly £51,038 against £37,000 contributed — growth becomes 27% of the total. That shift is the whole argument for time in the market.
Common mistakes
- Choosing an unsustainable monthly figure. A projection built on contributions you stop making after eight months describes nothing.
- Applying investment returns to a cash account. The two differ by several percentage points a year, which compounds into a very large gap.
- Ignoring charges. Enter the return net of fees, not the headline figure.
- Treating a smooth projection as a likely path. Real investment returns arrive unevenly, and the sequence matters as much as the average.
- Forgetting inflation. A balance of £51,000 in fifteen years will not buy what £51,000 buys today.
Frequently asked questions
Should I save monthly or invest a lump sum?
If you have the lump sum available and a long horizon, investing it immediately has historically produced a better average outcome, simply because the money spends more time invested. Regular saving is what most people are actually choosing between and doing, and it has the advantage of averaging your entry price and being far easier to sustain.
Does it matter whether I pay in at the start or end of the month?
Slightly. This calculator assumes end of month, which is the conservative assumption. Paying at the start of each month earns one extra month of interest on every contribution, which over five years at 4% adds roughly £45 to the example above.
What is a realistic rate for a cash savings account?
It broadly tracks Bank Rate, though providers vary considerably and easy-access rates are usually lower than fixed-term ones. Check current rates rather than relying on a figure quoted in an article, because they move whenever monetary policy does.
Are the returns taxable?
They may be. Interest above your Personal Savings Allowance is taxable, and gains and dividends outside a tax wrapper may be too. An ISA shelters both from UK income and capital gains tax within the annual allowance, which is why it is usually the first place to look.
Is what I enter stored?
No. Contributions, balances and rates are processed entirely in your browser and are never transmitted or retained.
Related tools
References
- Bank of England — Bank Rate, which underpins the rates available on cash savings
- MoneyHelper — guidance on regular saving, ISAs and choosing between cash and investments
- GOV.UK — current ISA allowances and the Personal Savings Allowance
Sources are checked at publication and can change — how I choose and check references.