See how your savings and investments grow over time with the power of compound interest. Enter your starting amount, expected return rate, and monthly contributions to visualize your wealth building — complete with interactive charts, milestones, and inflation-adjusted projections.
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The lump sum you're starting with today
S&P 500 avg: ~10% (7% after inflation)
Longer = more compounding power
Regular investing builds wealth
Historical US average: ~3%/year
Compound interest is often called the eighth wonder of the world, and for good reason. It works by reinvesting your earnings so that your interest earns interest — creating an exponential growth curve that accelerates over time.
The three key factors that drive compound growth are your initial investment (principal), the interest rate (annual return), and time. Of these three, time is the most powerful and the only one you can't get back. Starting early — even with small amounts — is one of the most impactful financial decisions you can make.
For perspective: someone who invests $200/month starting at age 25 will have significantly more at 65 than someone who invests $400/month starting at age 35 — despite contributing less total money. That's the power of compounding over time.
Enter your initial investment (the lump sum you're starting with), your expected annual return rate, and how long you plan to invest. Add any monthly contributions you plan to make regularly.
The calculator will instantly show your projected future value, a growth chart showing contributions vs. interest over time, a breakdown donut chart, and financial milestones you'll hit along the way. Toggle the inflation adjustment to see what your money will actually be worth in today's dollars, and expand the year-by-year table for a detailed breakdown.
Compound interest is interest calculated on both the initial principal and the accumulated interest from previous periods. Unlike simple interest (which only earns on the original amount), compound interest creates a snowball effect — your earnings generate their own earnings, leading to exponential growth over time. This is why Albert Einstein reportedly called it the eighth wonder of the world.
More frequent compounding means interest is calculated and added to your balance more often, resulting in slightly higher returns. For example, $10,000 at 7% for 20 years yields: $38,697 compounded annually vs. $39,927 compounded monthly vs. $40,552 compounded daily. The difference between monthly and daily compounding is minimal, but the jump from annual to monthly is meaningful. Most savings accounts compound daily, while many investments compound quarterly or annually.
The Rule of 72 is a mental math shortcut to estimate how long it takes to double your money. Simply divide 72 by your annual interest rate. At 6% interest, your money doubles in about 12 years (72 ÷ 6 = 12). At 10%, it doubles in about 7.2 years. This rule works best for rates between 4% and 12%. Our calculator shows this estimate automatically in your results.
Regular monthly contributions dramatically accelerate wealth building because each contribution starts compounding immediately. For example, investing just $200/month at 7% for 30 years turns $72,000 in contributions into over $243,000 — more than tripling your money. The key insight is that consistent investing matters more than timing the market. Even small amounts grow substantially over long periods.
The nominal return is the raw percentage your investment earns. The real return adjusts for inflation to show your actual purchasing power gain. If your investment earns 7% but inflation is 3%, your real return is roughly 4%. Our calculator's inflation toggle shows your future value in today's dollars, giving you a more realistic picture of what your money will actually be worth.
The S&P 500 has historically returned about 10% annually before inflation (~7% after inflation) over the past century. However, this varies significantly by time period. Conservative estimates use 6-7%, moderate estimates use 8-10%, and aggressive estimates use 10-12%. For retirement planning, many financial advisors recommend using 6-7% to build in a margin of safety. Bond-heavy portfolios might return 4-5%, while high-yield savings accounts currently offer 4-5%.
Simple interest is calculated only on the original principal. If you invest $10,000 at 7% simple interest for 20 years, you earn $1,400 per year — always the same amount — for a total of $28,000 in interest ($38,000 total). With compound interest, you'd earn $29,836 in interest ($39,836 total with monthly compounding) because each year's interest earns interest in subsequent years. The longer the time period, the bigger the gap between simple and compound returns.
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