Compound Interest Calculator

Compound interest is the mechanism by which you earn returns not only on your original investment but also on the returns it has already generated — growth on top of growth. This tool projects that exponential curve so you can see, in pounds and on a chart, how a starting balance and regular contributions turn into a much larger sum over time.

It separates your total principal (the money you actually paid in) from the interest earned (what the markets or your savings account generated), making the time value of money visible rather than abstract.

• Long-term investors projecting the future value of a Stocks & Shares ISA or brokerage account over decades.
• Savers comparing how different interest rates on Cash ISAs or regular savers affect the final balance.
• Students and educators visualising the fundamental concept of exponential growth.
• Anyone weighing "start now vs start later" — the calculator makes the cost of delay obvious.

• Your starting balance — what's already invested or saved today.
• The regular contribution you can commit to and how often you'll make it (monthly is most common).
• A realistic expected annual return matched to the asset: 3–5% for cash savings, 6–8% for a diversified global equity fund over long periods. Future returns are not guaranteed.
• Your time horizon in years — the single biggest driver of the result.

• Starting balance — the lump sum already in the account on day one.
• Regular contribution — the fixed amount added each period. Consistency is what powers the snowball.
• Contribution frequency — how often you add money (monthly, quarterly, annually). More frequent contributions compound slightly faster.
• Annual return (%) — the yearly growth rate assumed for the whole term. Enter a net figure after fund charges for a realistic projection.
• Compounding frequency — how often interest is added back to the principal. Daily compounding yields slightly more than monthly, which yields more than annual, over long horizons.
• Time horizon (years) — how long the money stays invested. Because growth is exponential, the final years contribute the largest gains.

The three headline figures tell the story: Final Balance is the projected pot at the end; Total Principal is what you paid in; Interest Earned is the difference — the market's contribution. Over long horizons, interest usually exceeds principal, which is the whole point of compounding.

The growth chart curves upward, and the steepness in the later years is the compounding effect accelerating. The year-by-year table lets you check any point in the journey — useful for matching a projection to a specific life event, like the year a child starts university. Remember the figure is nominal; for purchasing power, subtract inflation from your return rate and re-run.

Compound interest uses the formula A = P(1 + r/n)^(nt), where A is the final amount, P the principal, r the annual rate, n the compounding periods per year, and t the years. Regular contributions add a future-value-of-a-series calculation on top. The frequency of compounding matters because interest added to the principal sooner starts earning its own interest sooner.

An investor using their annual ISA allowance:

• Initial Investment: £10,000
• Monthly Contribution: £500
• Annual Return: 6% (a conservative estimate for a diversified global index fund)
• Time Horizon: 20 years

After 20 years, total out-of-pocket contributions are £130,000, but the projected final balance is over £264,000. Compound interest alone generated more than £134,000 — slightly more than the investor actually put in. Delaying the start by five years would cut roughly a third off that final figure.

Assumptions

• A fixed annual interest rate throughout — real markets are volatile, with up and down years.
• Contributions made at the end of each compounding period, consistently and without withdrawal.
• Interest compounded at the selected frequency (daily / monthly / annually).
• Gross returns shown before tax and fees; an ISA or SIPP wrapper can eliminate tax on gains.

What is not included

• Investment fees or platform charges (typically 0.15%–0.75% annual fund expense ratios) — enter a net return to account for these.
• Inflation adjustment — for real purchasing power, use the real return (nominal rate minus inflation).
• Tax on interest or gains outside an ISA or SIPP.
• Sequence-of-returns risk for market-based investments — a bad early decade is far more damaging than the smooth average suggests.

• Underestimating the time horizon. The most explosive growth happens in the later years. Delaying investing by five years can cut your final portfolio value dramatically.
• Ignoring inflation. A 6% return with 3% inflation is only a 3% real return in purchasing power.
• Not accounting for fees. A 1.5% annual fee turns a 6% return into 4.5%, which over decades drastically reduces the compounding effect.
• Assuming smooth returns. The calculator shows a tidy curve; real markets lurch. Stay invested through the dips to capture the average.

• Run the same numbers twice — once with your realistic return, once with a pessimistic one — to see the range of outcomes.
• If you're investing, use a tax-advantaged wrapper (ISA, SIPP) so gains aren't eroded by tax. See our Tax-Free Savings guide.
• Automate your monthly contribution via standing order so compounding continues without you having to remember.
• Pair this with the Savings Goal Calculator to work backwards from a target amount.

Does compounding frequency really matter?

Yes, though the impact depends on the timeline and balance. Daily compounding generates slightly higher returns than monthly, which generates more than annual. Over decades, frequent compounding adds a noticeable boost.

What is a realistic interest rate to use?

Historically, the global stock market has returned about 7–9% annually on average over long periods, before inflation. High-yield cash savings accounts typically offer 3–5% during high-rate environments. Choose a rate that aligns with the specific asset class you are investing in.

• Savings Goal Calculator: Work backward to find the monthly deposit needed to hit a target.
• Pensions Calculator: Apply compounding specifically to your retirement planning.
• Compound Interest Monthly Deposits Guide: How regular deposits create exponential wealth.
• Tax-Free Savings (ISAs) Guide: Shielding your compounded gains from tax.

SmartMoneyTools checks statutory rates and thresholds against official government publications. Results are estimates for educational purposes and may not reflect every individual circumstance. Last reviewed: 11 September 2026.

The compounding formulas used are standard financial mathematics. Context and figures informed by:

• MoneyHelper – Savings and ISA accounts
• Financial Conduct Authority (FCA) – Investments
• Bank of England – Inflation data