Compound Interest Calculator
A compound interest calculator that separates what you put in from what the interest earned — with regular contributions, any compounding frequency, a year-by-year table and an inflation-adjusted “real” result.
How to use Compound Interest Calculator
- Enter your starting amount, the annual interest rate and how many years you will stay invested.
- Choose how often interest compounds — daily, monthly, quarterly or annually.
- Add a regular contribution and say whether you pay it at the start or the end of each period.
- Read the split between total contributed and total interest, and scan the year-by-year table and chart.
- Switch on the inflation adjustment to see what the balance is worth in today’s money.
About Compound Interest Calculator
Compound interest is the mechanism by which small, boring, regular amounts turn into large ones, and the reason it feels counter-intuitive is that the growth is exponential while our intuition is linear. This calculator makes the effect visible by splitting the final balance into two parts — the money you contributed and the interest that money earned — and charting both year by year. The moment where the interest bar overtakes the contribution bar is the single most instructive thing a savings calculator can show you, and most of them do not show it at all.
The model handles the combinations that matter in practice. Compounding can be daily, monthly, quarterly or annual; contributions can be monthly or annual, paid at the start or the end of each period. Internally the nominal rate is converted to an effective annual rate and then applied per contribution period, which keeps daily compounding with monthly deposits exactly consistent rather than approximated. The result is that the number here matches what a bank or fund platform would compute for the same inputs.
The inflation toggle exists because nominal projections quietly mislead. At 3% inflation, money halves in purchasing power roughly every 23 years, so a £500,000 pot in 30 years buys what about £206,000 buys today. Showing the real figure alongside the nominal one is not pessimism — it is the only way to judge whether a savings plan actually reaches a goal that is itself denominated in future prices.
Frequently asked questions
What is the compound interest formula?
For a lump sum, A = P(1 + r/n)<sup>nt</sup>, where P is the principal, r the annual rate, n the compounds per year and t the years. With regular contributions a future-value-of-an-annuity term is added on top. Both formulas are printed with your figures substituted.
How much difference does compounding frequency make?
Less than most people assume. At 7% a year, moving from annual to monthly compounding raises the effective annual rate from 7.00% to about 7.23%; moving on to daily adds roughly another 0.02 points. Contribution size and time invested dominate the outcome far more than frequency does.
Should contributions be at the start or the end of the period?
Start-of-period contributions (an “annuity due”) earn one extra period of interest each, so they always finish ahead. Over long horizons the gap is meaningful — which is a real argument for paying into a pension or ISA at the beginning of the month rather than whatever is left at the end.
What does the inflation-adjusted result mean?
It converts the final balance into today’s purchasing power by dividing by (1 + inflation)<sup>years</sup>. A balance that looks large in 30 years’ nominal money can be far less impressive in real terms, and this is the number that reflects what it will actually buy.
Is this a SIP calculator?
Yes — a systematic investment plan is exactly a fixed regular contribution compounding at an assumed rate, so set a monthly contribution and your expected annual return. Note that real investment returns vary year to year, while this model assumes a constant rate.
Are my figures private?
Completely. Every calculation runs in your browser; nothing is uploaded, saved to a server, or shared.
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