Compound Interest Calculator
Enter your starting balance, monthly contributions, and interest rate to see your investment grow over time.
Final balance
$144,573
2.5× your money
Total invested
$58,000
Interest earned
$86,573
60% of total
At 7% p.a., compounded monthly
The Power of Compounding
Interest makes up 60% of your final balance — that's compounding at work.
Your Investment Insights
You will invest a total of $58,000 over 20 years.
$10,000 upfront + $200/month
You will earn $86,573 in interest — 149% on top of your contributions.
Every $1 you invest grows to $2.49 — a 2.5× return.
Your initial $10,000 doubles by year 3.
Monthly contributions add $104,185 extra to your final balance vs a one-time lump sum.
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Estimates only — not professional advice
Results are projections based on a constant annual return compounded at your chosen frequency. They do not account for inflation (unless toggled), taxes on gains (unless toggled), fund fees, contribution timing, or market volatility. Past market performance does not guarantee future results. This tool is for illustrative purposes only and should not be relied upon as financial, investment, or tax advice. Consult a qualified financial adviser before making investment decisions.
$263k+
$200/month at 7% for 30 years — from just $2,400/year
10.5×
return multiplier — $10k grows to $105k at 7% over 30 years
~7%
S&P 500 inflation-adjusted average annual return (historical)
Next steps
Start Growing Your Money Faster
Your projection is only the beginning. Here's how to put that compounding power to work with the right platform.
- 📈Compare investment platforms with different rates side-by-side
- 💸Low-cost index funds track S&P 500 returns (~7–10% historically)
- 🛡️Tax-advantaged accounts (ISA, 401k, Roth IRA) protect your gains
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Investment results are not guaranteed. Past market performance does not guarantee future returns. This calculator provides estimates for educational purposes only and does not constitute financial advice.
How does compound interest work?
Compound interest means your interest earns interest. In the first period, you earn interest on your principal. In the second period, you earn interest on your principal plus the interest already accumulated. This self-reinforcing loop is why investment growth looks like a hockey stick curve rather than a straight line.
The standard compound interest formula for a lump sum is:
Where: A = final amount, P = principal, r = annual interest rate (as a decimal), n = compounding periods per year, t = time in years.
But most real-world investors also make regular contributions — not just a one-time lump sum. Adding monthly contributions transforms the formula into a future value of annuity calculation. Our calculator handles both simultaneously, giving you a complete picture of investment growth including every monthly deposit.
Why time is your most powerful investment asset
The single biggest driver of wealth through compounding is not the interest rate — it is time. Consider two investors, both investing $200/month at 7%:
| Investor | Starts at | Invests for | Total contributed | Final balance at 65 |
|---|---|---|---|---|
| Early starter | Age 25 | 40 years | $96,000 | $528,000 |
| Late starter | Age 35 | 30 years | $72,000 | $243,000 |
The early starter contributes just $24,000 more — but ends up with over $285,000 more at retirement. That gap is entirely due to compounding. Starting a decade earlier more than doubles the final balance, which is why financial advisers consistently emphasise beginning to invest as early as possible.
The impact of regular monthly contributions
A lump-sum investment grows through compounding, but adding regular monthly contributions amplifies the effect significantly. Each monthly deposit starts its own compounding clock. A $10,000 lump sum invested at 7% for 20 years grows to approximately $38,700. The same $10,000 plus $200 each month grows to roughly $114,000 — nearly three times as much.
Monthly contributions also smooth out market timing risk. Rather than putting everything in at one moment (which might coincide with a market peak), regular contributions automatically purchase more shares when prices are low and fewer when prices are high — a strategy known as pound-cost averaging in the UK and dollar-cost averaging in the US.
Even small increases in monthly contributions compound powerfully over decades. Increasing contributions by just $50/month — roughly the cost of two restaurant meals — adds approximately $60,000 to a 30-year investment projection at 7%.
How inflation affects your real investment returns
Inflation is the silent tax on investment returns. If your portfolio grows at 7% per year but inflation averages 3%, your real (purchasing power) return is only about 4%. The future dollar figure shown in most calculators is a nominal figure — what your account will say in twenty years. The inflation-adjusted figure tells you what that money can actually buy in today's terms.
For long-term planning, the inflation-adjusted return is almost always the more meaningful number. A balance of $500,000 in 30 years at 2.5% average inflation is worth only about $238,000 in today's money. This is why most financial planners target a real return of 4–5% rather than focusing on nominal rates.
Asset classes that historically outpace inflation include equities (stocks), real estate investment trusts (REITs), and Treasury Inflation-Protected Securities (TIPS). Cash and short-term bonds have historically failed to keep pace with inflation over long periods, eroding purchasing power gradually.
Tax-efficient investing: keeping more of your returns
Taxes can dramatically reduce your effective compounding rate. The impact depends on account type, jurisdiction, and when gains are realised:
Tax-advantaged accounts
Roth IRA, Traditional IRA, 401(k) in the US — or Stocks & Shares ISA, SIPP in the UK — shelter gains from annual tax, allowing full compound growth. Maximising these before investing in taxable accounts is almost always optimal.
Capital gains tax
In taxable accounts, you typically only pay tax when you sell. Long-term capital gains rates (assets held 1+ year) are 0–20% in the US vs ordinary income rates of up to 37%. Holding investments long-term defers and reduces the tax bill.
Dividend reinvestment
Reinvesting dividends automatically purchases more shares, maintaining the compounding effect. In tax-advantaged accounts this is fully shielded. In taxable accounts, dividends are typically taxed in the year received even if reinvested.
Tax-loss harvesting
Selling underperforming assets at a loss to offset capital gains elsewhere is a legal strategy that reduces your net tax liability. It is particularly effective in years with large realised gains elsewhere in your portfolio.
Compounding frequency: does it matter?
The more frequently interest compounds, the more you earn — but the difference between monthly and daily compounding is surprisingly small in practice. On a $10,000 investment at 7% for 20 years, monthly compounding produces $39,343 while daily compounding produces $39,525 — a difference of just $182. The frequency of your contributions matters far more than the compounding frequency.
Most investment accounts (brokerage, ISA, pension) compound monthly or daily. Savings accounts and money market funds typically compound daily. The calculator above lets you toggle between monthly, quarterly, and annual compounding so you can see the effect on your specific projection.
Frequently asked questions
What is compound interest?+
Compound interest is interest calculated on both the initial principal and the accumulated interest from previous periods. Unlike simple interest (which only applies to the original principal), compound interest causes your balance to grow exponentially over time. The more frequently interest compounds — daily, monthly, quarterly — the faster your money grows. Einstein is often (apocryphally) credited with calling it 'the eighth wonder of the world'.
How often should I contribute to an investment account?+
Monthly contributions are the most practical and impactful approach for most investors. Investing consistently every month — regardless of market conditions — is called dollar-cost averaging. It removes emotion from investing, smooths out market volatility, and maximises your time in the market. Even small consistent contributions dramatically outperform irregular lump sums over 20+ year horizons thanks to compounding.
What interest rate should I expect from investments?+
The S&P 500 has delivered an average annual return of approximately 7–10% historically (adjusted for inflation: ~7%). Bond-heavy or balanced portfolios typically return 4–6%. High-yield savings accounts and cash ISAs currently yield 4–5% with low risk. The 'right' rate depends entirely on your risk tolerance and time horizon — longer horizons can weather higher-volatility, higher-return assets like equities.
Does inflation reduce my investment returns?+
Yes — inflation erodes purchasing power. If your investment earns 7% per year and inflation runs at 2.5%, your real (inflation-adjusted) return is approximately 4.4% (nominal rate minus inflation rate). This is why long-term investors focus on real returns, not nominal ones. Use the inflation adjustment toggle in the calculator above to see what your future balance is worth in today's money.
Should I reinvest earnings from my investments?+
Almost always, yes. Reinvesting dividends and interest is what drives compound growth. If you withdraw your earnings, you lose the compounding effect — your balance grows linearly instead of exponentially. Most modern investment platforms (brokerages, ISA providers, 401k plans) automatically reinvest dividends by default. Over 30 years, reinvesting dividends can account for more than 40% of total returns.
How does tax affect my investment returns?+
Tax on investment gains varies by account type and jurisdiction. In a taxable account in the US, long-term capital gains are taxed at 0%, 15%, or 20% depending on income. In the UK, gains above the annual exempt amount are taxed at 10% (basic rate) or 20% (higher rate). However, tax-advantaged accounts — Roth IRA, Traditional IRA, 401(k) in the US; ISA in the UK — shelter your gains from tax entirely or defer them. Maximising contributions to tax-advantaged accounts before taxable accounts is almost always the optimal strategy.
What is the Rule of 72?+
The Rule of 72 is a quick mental shortcut to estimate how long it takes to double your money. Simply divide 72 by your annual interest rate: at 6%, your money doubles in approximately 12 years (72 ÷ 6 = 12). At 9%, it doubles in 8 years. The rule works reasonably well for rates between 2% and 20%. Our calculator shows you the exact doubling year based on your inputs.