What compound interest actually is
Compound interest is interest earned not just on the money you originally put in, but on all the interest that money has already earned. Each time interest is added to a balance, the next round of interest is calculated on the new, larger total — so growth accelerates the longer money is left in place. The calculator above models this month by month, taking a starting amount, a regular monthly contribution and an assumed annual rate, to project a future value alongside how much of that total came from your own contributions versus growth.
Why time matters more than almost anything else
Because each period's growth is calculated on an ever-larger base, compounding is heavily weighted towards time rather than the size of any single contribution. Money left invested for 25 years at a given rate will typically grow to far more than double what the same money would grow to over 15 years at the same rate, simply because there are more compounding cycles working in the background. This is the core reason financial guidance so consistently emphasises starting early: a smaller amount given more time to compound often ends up ahead of a larger amount given less time.
The difference a small rate change makes
Because growth compounds, small differences in assumed rate produce surprisingly large differences in outcome over long periods. Try running the calculator above at 3%, then 5%, then 7% for the same starting amount, contribution and term, and you'll see the projected value diverge considerably rather than scaling in a straight line. This is exactly why fees matter in investment platform charges — a seemingly small annual fee is itself compounding against you every year, quietly eating into the same growth mechanism working in your favour elsewhere.
Contributions versus starting capital
The calculator splits your result into total paid in and interest or growth earned, which is a useful way to see how much of the final figure is genuinely "free" growth rather than money you put in yourself. Early on, a large share of the total tends to come from contributions; over a long enough term, growth can eventually overtake contributions entirely, particularly at higher assumed rates. This is worth bearing in mind if you're deciding between a lump sum now and smaller regular contributions — our lump sum versus regular investment calculator compares that trade-off directly.
Where this applies in real life
This calculator is deliberately generic, because the maths of compounding is identical whether the money sits in a savings account, a stocks and shares ISA, a pension, or a general investment account — only the realistic rate of return differs. A cash savings account might realistically compound at an interest rate in the low single digits; a diversified investment portfolio has historically delivered higher average returns over the long run, but with the value fluctuating year to year rather than growing smoothly. Our savings calculator and investment growth calculator use the same underlying formula with framing suited to each.
A worked example
Take the calculator's own defaults: a £5,000 starting amount, £150 invested every month, at an assumed 5% annual rate, for 15 years. Over that period you'd pay in £5,000 up front plus £27,000 in monthly contributions (£150 × 180 months), for total contributions of £32,000. Because that money compounds monthly rather than sitting still, the projected result comes out at roughly £42,300 — meaning growth alone has added around £10,300 on top of what you actually paid in, without you doing anything beyond leaving it invested and continuing the monthly contribution.
Now compare that with stretching the same plan to 25 years instead of 15, keeping every other input the same: total contributions rise to £50,000 (a 56% increase), but the projected value climbs to roughly £96,700 — nearly two and a half times the 15-year figure, driven almost entirely by the extra decade of compounding rather than by paying in much more. That gap is the clearest illustration of why starting early matters more than almost any other single decision in long-term saving or investing.
Common mistakes to avoid
The most common mistake with a compounding projection like this is assuming a single rate will hold perfectly steady for the whole term and then treating the resulting figure as a promise rather than an illustration. A second common mistake is stopping contributions during a market dip or a tight month and not resuming them, which quietly costs far more over a long horizon than the missed contribution itself, simply because that money then misses out on every subsequent period of compounding too. Treat interruptions as something to restart from as soon as reasonably possible, not as a permanently lower baseline.
Frequently asked questions
Does this account for tax?
No. Interest and investment growth can be subject to tax depending on the account it's held in and your personal allowances. Money held inside an ISA or pension typically grows free of Income Tax and Capital Gains Tax, which is one reason those wrappers are worth understanding before you invest outside them.
What rate should I assume?
There's no single right answer — it depends entirely on where the money is held. A cash account's rate is usually published by the provider; an investment portfolio's future return can't be known in advance, so it's sensible to test a range of assumptions (a cautious, a middling and an optimistic rate) rather than relying on one figure.
Why does my contribution total look larger than I expected?
Because it's the sum of your starting amount plus every monthly contribution added up over the whole term, before any growth is applied — over 15 or 20 years, even a modest monthly amount adds up to a substantial total paid in.
Can I use this to plan for a specific goal, like a deposit?
Yes — enter your target amount as the future value informally by adjusting the years or monthly contribution until the projected figure matches roughly what you need, though remember the result is an estimate, not a guarantee of hitting that exact figure by that exact date.