Compound Interest Calculator

Compound interest means you earn interest on both your original principal and the interest you've already accumulated. The longer you save, the more powerful this effect becomes — often called 'interest on interest'.

Compound interest is interest calculated on the initial principal and also on the accumulated interest from previous periods. Unlike simple interest — which pays a fixed percentage of your original deposit each year — compound interest lets every interest payment start earning its own interest. The result is exponential growth: the longer your money stays invested, the steeper the curve. Albert Einstein reportedly called compound interest 'the eighth wonder of the world,' and although historians dispute the quote, the math behind it is undeniable. In practice, compound interest is the mechanism behind every savings account, certificate of deposit, retirement plan, and mortgage. Understanding how it works is one of the highest-return investments you can make in your own financial education, because it changes how you think about every dollar you earn, save, or borrow for the rest of your life. Whether you are 18 and opening your first high-yield savings account, or 58 and trying to maximize your final decade of retirement contributions, the same underlying engine — interest earning interest — drives every outcome you will see on this page. The key intuition is that compound interest rewards time, not effort. Two savers who contribute exactly the same dollar amount over their lifetimes can end up with wildly different balances purely because one started a decade earlier. That time advantage is the only 'unfair' edge in personal finance, and it is available to everyone who begins now. Use the calculator above to model your own scenario — adjust the principal, monthly contribution, rate, and years to see how each variable interacts.

The standard compound interest formula is: A = P(1 + r/n)^(nt). Here, A is the final amount, P is the principal, r is the annual interest rate (as a decimal), n is the number of times interest compounds per year, and t is the number of years. For example, $10,000 invested at 5% compounded monthly for 10 years grows to about $16,470. The same principal at simple interest would yield only $15,000. If you add monthly contributions of $200 to the same scenario, the formula expands to account for an annuity of deposits, and the final balance rises to roughly $58,000 — a useful reminder that recurring contributions almost always matter more than the interest rate itself once you cross the ten-year mark. The expanded formula for regular contributions is: A = P(1 + r/n)^(nt) + PMT × [((1 + r/n)^(nt) − 1) / (r/n)]. The first term captures growth on your initial deposit; the second captures growth on every recurring deposit. Both terms are powered by the same compounding exponent, which is why a longer time horizon lifts both halves of the equation in unison rather than additively.

A = P(1 + r/n)^(nt)

Compounding frequency determines how often earned interest is added back to your balance. Daily compounding adds interest 365 times per year, monthly adds it 12 times, and annually just once. The more frequent the compounding, the higher your effective annual yield — but the difference between daily and monthly is usually under 0.1%. Most high-yield savings accounts in 2026 compound daily, while mortgages and student loans typically compound monthly. Certificates of deposit (CDs) often compound daily but pay interest monthly. The table below shows how $10,000 at 5% APY grows over 10 years at each frequency. As you can see, the difference between daily and annually is real but modest — about $300 over a decade — which is why frequency matters far less than the interest rate itself, your time horizon, and whether you keep adding money. The exception is when interest rates are high: at 15% APY, the gap between daily and annual compounding widens to about 1.2 percentage points, which can mean thousands of dollars over a 30-year horizon. That is why credit cards, which can charge 24%+ APR and compound daily, are so dangerous — the compounding frequency amplifies an already punishing rate.

Simple interest is calculated only on the original principal: A = P(1 + rt). Compound interest, by contrast, calculates interest on both principal and accumulated interest. Consider $10,000 at 6% over 30 years. With simple interest, you end up with $28,000 — your $10,000 plus $18,000 in interest. With monthly compounding, you end up with about $60,226 — more than double the simple-interest outcome. That $32,226 difference is the pure power of compounding, and it grows dramatically with longer time horizons. The same comparison at 5 years yields a difference of only about $830; at 50 years it balloons to over $90,000. This non-linear scaling is why financial planners hammer on the message 'start early' — the gap between starting at 25 and starting at 35 is not 10 years of growth, it is a multiple of your final balance. A worker who saves $5,000 per year from age 25 to 35 and then stops will often retire with more money than a peer who waits until 35 and saves the same $5,000 per year until 65 — purely because the early saver's money has more time to compound.

Compound interest appears across nearly every financial product, working for you in some and against you in others. In a high-yield savings account at 4% APY (a typical 2026 US rate), $25,000 grows to about $36,500 in 10 years with no additional contributions — your money doubled once over the decade. A 401(k) averaging 7% annual returns turns $500 monthly contributions into roughly $1.2 million over 40 years, with about 70% of that final balance coming from investment growth rather than your own contributions. A student loan balance of $30,000 at 5% interest will balloon to $36,000 in just 24 months if you make only minimum payments, demonstrating how unpaid interest gets capitalized into the principal. A 30-year fixed mortgage at 6.5% on a $400,000 home accrues more than $500,000 in total interest over the life of the loan — meaning you ultimately pay roughly 1.5x the original purchase price. And a credit card balance of $5,000 at 24% APR, with no payments, doubles to $10,000 in roughly 36 months — a textbook example of compounding working against you. Each of these scenarios illustrates the same math, with the sign flipped based on whether you are the lender or the borrower. Notice that none of these examples assumes an exotic investment product. The savings account uses a federally insured online bank, the 401(k) uses a broad index fund, and the credit card is a standard variable-APR card — yet the compounding effect produces wildly different outcomes purely because of time, rate, and contribution consistency.

The Rule of 72 is a quick mental-math shortcut: divide 72 by your annual interest rate to estimate how many years it takes for your money to double. At 7% returns, 72 / 7 ≈ 10.3 years. At 4% (a savings account), 72 / 4 = 18 years. At 10% (a long-term stock portfolio), 72 / 10 ≈ 7.2 years. The rule works best for rates between 5% and 10%, and it is one of the fastest ways to understand why starting early matters. You can also flip the formula: to estimate the rate required to double your money in a given number of years, divide 72 by that number. Want to double your money in 6 years? You need roughly 72 / 6 = 12% annual returns — a benchmark most active investors never reach consistently, which is why low-cost index funds remain the default recommendation for most long-term savers. The Rule of 72 also has a darker side: it applies to debt as easily as to savings. At 18% credit card APR, your balance doubles in just 4 years, which is how a single emergency can spiral into a multi-year repayment plan if you only make minimum payments. Use the Rule of 72 in both directions — to set realistic return expectations on the upside, and to size up the damage of any high-interest debt on the downside.

Years to double ≈ 72 / annual rate (%)

The most common compounding mistakes are starting too late, withdrawing early, and ignoring fees. Every decade you delay roughly halves the final balance, because each doubling cycle takes a decade at typical returns. Early withdrawals from retirement accounts trigger taxes plus a 10% penalty before age 59½, instantly erasing years of growth. And a 1% annual fee — common in actively managed funds — can reduce your final wealth by 20% or more over 30 years. Other pitfalls include: chasing last year's top-performing fund (which usually mean-reverts), panic-selling during market downturns (locking in losses that compounding could have recovered), and keeping too much cash in a 0.5% checking account when an emergency fund of 3–6 months is already set aside. The fix for all of these is structural: automate contributions on payday, choose low-cost index funds with expense ratios under 0.20%, increase your contribution by 1% of salary every raise, and never touch the principal if you can avoid it. Investors who review their statements once a year and rebalance their asset allocation back to target almost always outperform those who trade actively — because the biggest enemy of compounding is interruption, and the biggest friend is consistency.