Compound Interest & Wealth Growth Calculator
Map year-by-year balance growth from your initial investment and monthly contributions. See when compound interest overtakes your regular deposits.
Wealth Growth Projector
See exactly how your money grows over time
You have $10,000 in savings and you are thinking about putting $500 a month into an index fund. But will that actually be enough for retirement? This calculator shows you the year-by-year growth of your money β including how much comes from your contributions and how much comes from compound interest β so you can see the numbers before you commit.
Step 1: Enter your starting balance
Type the lump sum you already have invested or saved. If you are starting from zero, enter 0. This initial amount compounds alongside your monthly contributions. A $10,000 starting balance at 7% annual return grows to about $76,123 over 30 years even without any additional contributions.
Step 2: Set your monthly contribution
Enter the amount you plan to add each month. This is the variable you have the most control over. A common guideline is 15% to 20% of gross income for retirement savings. The calculator adds this amount to your balance every month after applying the monthly interest rate.
Step 3: Choose your annual return rate
The default is 7%, which reflects the historical average real return of the S&P 500 after adjusting for inflation over the past century. For a high-yield savings account, use 4% to 5%. For a bond portfolio, use 2% to 4%. Remember that higher returns come with higher risk β a 7% rate assumes a diversified stock portfolio held for decades.
Step 4: Set your time horizon
Enter the number of years you plan to keep the money invested. The calculator accepts 1 to 50 years. Time is the most powerful variable in compound growth. A 25-year-old investing $300 per month for 40 years at 7% ends up with about $720,000. A 35-year-old investing the same amount for 30 years ends up with about $340,000 β less than half, despite contributing for only 10 fewer years.
Step 5: Read the year-by-year table
The output shows three numbers: your final balance, total amount contributed, and total interest earned. Below that, a year-by-year table breaks down balance, contributions, and interest for each year. In the early years, your contributions make up most of the growth. In later years, interest on interest begins to dominate β this is the crossover point where compounding truly accelerates.
How compound interest actually works
Monthly compounding is more accurate than annual
The calculator applies interest monthly, which is how most savings accounts and investment funds actually compound. Each month, your current balance is multiplied by (1 + annual rate divided by 12), then your monthly contribution is added. This process repeats for every month across the investment period. Monthly compounding captures the effect of interest earning interest more frequently than annual compounding. On a $100,000 balance at 7% annual return, monthly compounding produces about $2,000 more per year than annual compounding because each month's interest starts earning its own interest immediately.
The crossover point where interest overtakes contributions
The year-by-year table reveals the fundamental principle of compound growth. In year 1, your balance grows almost entirely from your contributions. By year 15, interest on interest starts matching your annual contributions. By year 25, interest earned exceeds what you contributed that year. This crossover point is why time in the market matters more than timing the market β the longer your money compounds, the harder the interest works relative to your contributions.
A worked example: $10,000 starting balance, $500/month, 7% return
Starting with $10,000 and adding $500 per month at 7% annual return compounded monthly: after 10 years your balance is about $103,000 β you contributed $70,000 and earned $33,000 in interest. After 20 years, your balance is about $282,000 β you contributed $130,000 and earned $152,000 in interest. After 30 years, your balance is about $604,000 β you contributed $190,000 and earned $414,000 in interest. By year 30, you are earning more in interest each year than you contribute annually. This is the exponential effect of compounding made visible.
Why starting 10 years earlier almost doubles your outcome
A 25-year-old investing $300 per month for 40 years at 7% ends up with about $720,000. A 35-year-old investing the same amount for 30 years ends up with about $340,000. The difference is $380,000 β despite the 35-year-old contributing only $36,000 less over their shorter timeframe. The extra 10 years of compounding on the earlier contributions is what creates the massive gap. This is the strongest mathematical argument for starting to invest as early as possible.
What this calculator does not model
The calculator uses a fixed annual return rate and does not account for taxes, inflation, management fees, or year-to-year market volatility. Real investment returns fluctuate β the S&P 500 has returned more than 20% in some years and dropped more than 30% in others. The 7% rate is a long-term average, not a guarantee. For tax-advantaged accounts like superannuation or 401(k)s, the effective return may be higher due to deferred taxation. Always consult a qualified financial advisor for personalised advice.
Frequently asked questions
What is compound interest?
Compound interest is interest calculated on the initial principal plus all previously accumulated interest. Unlike simple interest, which only applies to the original principal, compounding means you earn interest on your interest, leading to exponential growth over time. This is the fundamental mechanism behind wealth accumulation through long-term investing.
What annual return rate should I use?
For long-term stock market investments, 7% is a commonly used real return (after inflation) based on historical S&P 500 averages spanning over a century. For high-yield savings accounts, use 4% to 5%. For bonds, 2% to 4%. These are estimates only β past performance does not guarantee future results, and actual returns will vary year to year.
How much should I invest per month?
A common guideline is to save 15% to 20% of your gross income for retirement. Even small amounts benefit enormously from time due to compounding. Starting 10 years earlier can roughly double the final balance even with the same total contributions, which is why starting early matters more than investing a large amount later.
Does this account for inflation?
No. The calculator shows nominal returns. To estimate inflation-adjusted (real) returns, subtract your expected inflation rate from your interest rate input. A 7% nominal return with 3% inflation equates to roughly 4% real growth, which still compounds significantly over long periods.
Why does the year-by-year table show interest growing faster in later years?
This is the core effect of compounding. In early years, your monthly contributions make up most of the balance growth. As interest accumulates, you begin earning interest on increasingly larger amounts, creating a snowball effect. In later years, interest earned can exceed your annual contributions β this is the crossover point where compound growth truly accelerates.
Should I prioritise increasing contributions or finding a higher return rate?
For most investors, increasing contributions has a larger impact than chasing higher returns. A 1% increase in annual return is meaningful, but doubling your monthly contribution has a far greater effect on the final balance. Additionally, higher returns typically come with higher risk, while increasing contributions is a guaranteed improvement to your outcome.
What happens if I miss a month of contributions?
Missing a single month has a small immediate effect but a larger long-term impact due to lost compounding. On a $500 monthly contribution at 7%, missing one month costs you about $3,500 in final balance after 30 years β the $500 you missed plus roughly $3,000 in interest it would have earned. Missing multiple months compounds this loss. If you cannot contribute one month, try to make it up the following month if possible.
Is compound interest the same as simple interest?
No. Simple interest is calculated only on the original principal. If you invest $10,000 at 5% simple interest, you earn $500 every year regardless of how much interest has accumulated. With compound interest, you earn interest on your interest as well. After 10 years at 5% compound interest, your $10,000 grows to about $16,289. With simple interest, it would only be $15,000. The gap widens dramatically over longer time periods.