Plan Your Wealth Growth with Compound Interest
See how your money grows year by year. Adjust your investment, contributions, rate, and risk profile — the chart updates instantly.
Your investment inputs
Final Value
$300,851
Interest Earned
$170,851
Total Contributed
$130,000
For educational purposes only. Assumes constant annual return compounded monthly. Actual returns vary and are not guaranteed.
What is compound interest — and why does it matter?
Compound interest is interest calculated on both your original principal and the accumulated interest from previous periods. Unlike simple interest (which only earns on the principal), compound interest earns on a growing base — producing exponential rather than linear growth over time.
Albert Einstein reportedly called compound interest the "eighth wonder of the world." Whether the attribution is accurate or not, the mathematics is undeniable: a $10,000 investment at 7% annually grows to $19,672 in 10 years, $38,697 in 20 years, and $76,123 in 30 years — without adding a single additional dollar. The growth is not linear; it accelerates because each year's interest becomes part of next year's principal.
Adding monthly contributions amplifies this dramatically. $500/month added to that same $10,000 at 7% for 30 years produces approximately $613,000 — over 10× the total cash invested ($190,000).
Compound interest formula explained
A = P × (1 + r/n)^(n×t) + PMT × [((1 + r/n)^(n×t) − 1) / (r/n)]
Risk profiles — what do Low, Medium, and High mean?
The risk profile selector applies a historically grounded expected annual return and a volatility range — the spread between bad years and good years. Higher expected returns come with higher year-to-year variance.
| Profile | Expected return | Typical assets | Best for |
|---|---|---|---|
| Low | 3–5% | Bonds, CDs, money market funds | Retirees, 1–5 year horizon, capital preservation |
| Medium | 6–8% | 60/40 stock-bond portfolio, balanced funds | 10–20 year horizon, moderate growth |
| High | 9–12% | Equity index funds, growth stocks | 20+ year horizon, maximum long-term growth |
Returns shown are historical long-run averages. Actual returns vary year to year and are not guaranteed. Past performance does not predict future results.
Why time is the most powerful variable
Compound interest growth is non-linear — the majority of wealth accumulates in the final years of a long investment horizon, not the early ones. This is called the "hockey stick" effect: the curve is nearly flat for the first decade then bends sharply upward.
| Years invested | $10,000 at 7% | +$500/mo at 7% | Growth multiple |
|---|---|---|---|
| 5 years | $14,026 | $45,410 | 4.5× |
| 10 years | $19,672 | $97,892 | 9.8× |
| 20 years | $38,697 | $268,738 | 26.9× |
| 30 years | $76,123 | $613,245 | 61.3× |
| 40 years | $149,745 | $1,321,590 | 132× |
Based on $10,000 initial investment + $500/month additional contributions at 7% annual return, compounded monthly. For illustration only.
The Rule of 72 — how to estimate doubling time instantly
The Rule of 72 is a mental shortcut: divide 72 by your annual interest rate to get the approximate number of years it takes for an investment to double.
18 years
to double at 4%
Conservative bond portfolio
~10 years
to double at 7%
Historical S&P 500 average
7.2 years
to double at 10%
Aggressive equity growth
At 7%, your money doubles approximately every 10 years. In 30 years it doubles three times — multiplying by 8×. This is why financial advisors emphasise starting early: every 10-year delay roughly halves the terminal value of a long-term investment.
How to use this calculator effectively
- 1
Start with your current savings as the initial investment.
This grounds the projection in reality — not a hypothetical starting point.
- 2
Enter a realistic monthly contribution — not an aspirational one.
Use what you are actually investing today. You can model increases separately.
- 3
Use 6–7% as a baseline rate for a diversified equity portfolio.
The S&P 500 has returned approximately 10% nominal / 7% real (inflation-adjusted) over 30-year rolling periods.
- 4
Try the risk profiles to see the range of possible outcomes.
Low: ~4% (bond-heavy portfolio). Medium: ~7% (balanced). High: ~10% (equity-heavy). The difference in terminal value over 30 years is dramatic.
- 5
Look at the year-by-year breakdown, not just the final number.
The breakdown shows when growth accelerates — typically the final 10 years contain more than half the terminal value.
What affects compound interest growth most?
Time horizon
The single most powerful variable. Starting 10 years earlier roughly doubles terminal value at 7%. Compound interest rewards patience above everything else.
Interest rate
A 2% rate difference (5% vs 7%) produces a 70% difference in terminal value over 30 years. Expense ratios on funds directly subtract from this rate.
Monthly contributions
Regular contributions benefit from dollar cost averaging and compound growth on each deposit. Even $100/month compounded over 30 years at 7% grows to $121,997.
Compounding frequency
Monthly compounding produces slightly more than annual compounding. At 7% over 30 years: annual = $76,123 vs monthly = $81,110 on $10,000 — a $4,987 difference.
Inflation impact
Real return = nominal return − inflation rate. If your investment earns 7% and inflation runs at 3%, your real purchasing power grows at approximately 4% per year.
Tax drag
Returns in taxable accounts are reduced by capital gains tax on dividends and realised gains. Tax-advantaged accounts (401k, IRA, ISA) let compound interest work on the full pre-tax amount.
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Employer-matched retirement savings growth
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Monthly payment and total interest on a home loan
Official resources and further reading
SEC Investor.gov — Compound Interest Calculator and Guide
The U.S. Securities and Exchange Commission's official compound interest calculator and investor education guide — includes explanation of compounding frequency and the difference between simple and compound interest.
Federal Reserve — Historical Return Data for US Equities
Federal Reserve research paper on long-run US stock market returns — the authoritative source for the historical 7% real return assumption used in retirement and compound interest projections.
IRS — 401(k) and IRA Contribution Limits 2026
The IRS official page for current retirement account contribution limits — 401(k): $23,500 under 50 / $31,000 age 50+. Roth IRA: $7,000 under 50 / $8,000 age 50+. Maximising tax-advantaged contributions is the most efficient way to apply compound interest growth.
Vanguard — The Power of Long-Term Investing
Vanguard's How America Invests study — data on real household investment behaviour, asset allocation by age group, and long-term return expectations across risk profiles.
Frequently asked questions
How often does compound interest compound?
It depends on the investment or account. Savings accounts and CDs typically compound daily or monthly. Most investment calculators (including this one) use monthly compounding as the default, which closely matches index funds and brokerage accounts. Annual compounding is simpler to calculate but slightly underestimates real returns for monthly-compounded accounts.
What is a realistic interest rate to use for long-term wealth projections?
For a diversified equity index fund portfolio (e.g. an S&P 500 or total market index fund), 7% is the most widely used real (inflation-adjusted) annual return figure, based on historical US market data since 1926. For nominal returns (before inflation adjustment), 9–10% is more accurate. For conservative projections, financial planners often use 6% real. Bond-heavy or money market portfolios should use 3–5%.
What is the difference between compound and simple interest?
Simple interest calculates interest only on the original principal: $10,000 at 7% for 10 years = $17,000 (principal + $7,000 interest). Compound interest calculates interest on the growing total (principal + accumulated interest): $10,000 at 7% compounded monthly for 10 years = $20,097 — $3,097 more, with no additional investment. The gap widens dramatically over longer time periods.
Does inflation affect compound interest calculations?
Yes significantly. If your investment returns 7% per year but inflation runs at 3%, your real purchasing power grows at approximately 4% per year. This calculator shows nominal returns (not inflation-adjusted). To see real purchasing power growth, subtract your expected inflation rate from the interest rate field — entering 4% instead of 7% shows what your money will actually buy in today's dollars.
How does tax affect compound interest growth?
In taxable brokerage accounts, dividends and realised gains are taxed annually, reducing the amount that compounds. In tax-advantaged accounts (401k, traditional IRA, Roth IRA, ISA in the UK), growth compounds on the full pre-tax amount. Over 30 years, the difference between taxable and tax-advantaged compounding can exceed 20–30% of terminal value. Maximising contributions to tax-advantaged accounts is one of the highest-leverage financial decisions available to most investors.