What Is Compound Interest?
Compound interest is interest calculated on both your original principal and on the interest that has already accumulated. Unlike simple interest, which applies only to the starting balance, compounding earns returns on returns. Each period, the interest earned gets added to the balance, and the next period's interest calculation uses that larger number. The result is exponential rather than linear growth. Given enough time, the difference is enormous.
A concrete example: $10,000 invested at 7% annual interest for 30 years grows to $76,123 with compound interest, compared to $31,000 with simple interest. The extra $45,123 comes entirely from interest earning interest. That gap widens with every additional year.
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The compound interest formula: A = P(1 + r/n)^(nt) where A = final amount, P = principal, r = annual interest rate (decimal), n = compounding periods per year, t = time in years. For a balance with regular contributions, each contribution starts its own compounding journey from the date it's added.
How to Use This Compound Interest Calculator
1
Enter your initial investment. This is your starting principal, the amount you have available right now. Enter $0 if you're starting from scratch and plan to build the balance entirely through monthly contributions.
2
Set your monthly contribution. This is how much you add each month. Use a negative number to model withdrawals instead. Contributions stack with compounding: each deposit starts earning interest from the moment it lands in the account.
3
Choose your interest rate. For savings accounts and CDs, use the APY from your institution. For long-term investment projections, the S&P 500 has historically averaged roughly 9.5% nominal or 7% after inflation. Use the rate presets to load common 2026 benchmarks instantly.
4
Set your compound frequency. Most savings accounts and HYSAs compound daily. CDs often compound monthly or quarterly. The more frequently interest compounds, the faster your balance grows, though the difference diminishes at lower rates.
5
Review the results. The calculator shows your final balance, total contributions, interest earned, a year-by-year stacked bar chart, and a rate variance card showing three scenarios. Scroll the year table to see exactly how the balance builds each year.
The Compound Interest Formula Explained
Basic Formula (Lump Sum, No Contributions)
A = P × (1 + r/n)^(n × t)
Where P = principal, r = annual interest rate as a decimal, n = compounding periods per year, t = years. Example: $5,000 at 5% compounded monthly for 10 years: A = $5,000 × (1 + 0.05/12)^(12 × 10) = $5,000 × 1.6470 = $8,235.
Formula with Regular Contributions
When you add contributions each period, the math extends to sum the future value of each individual contribution. In practice, the compound interest calculator runs this as a period-by-period simulation, which handles any compounding frequency and contribution timing accurately.
Daily vs. Monthly vs. Annual Compounding
The compounding frequency affects how quickly interest accumulates. With the same 5% annual rate, a $10,000 investment over 10 years produces:
| Compound Frequency | Final Balance | Interest Earned |
|---|
| Annually | $16,289 | $6,289 |
| Quarterly | $16,436 | $6,436 |
| Monthly | $16,470 | $6,470 |
| Daily | $16,487 | $6,487 |
* $10,000 principal, 5% annual rate, 10 years, no additional contributions. Daily compounds $198 more than annual compounding over the same period.
The Rule of 72: Quick Doubling Estimate
The Rule of 72 gives you a fast mental estimate of how long it takes to double your money at any given interest rate. Divide 72 by the annual interest rate and the result is the approximate years to double.
- At 4% (current HYSA range): 72 / 4 = 18 years to double
- At 7% (S&P 500 real return estimate): 72 / 7 = approximately 10.3 years
- At 10% (S&P 500 nominal estimate): 72 / 10 = 7.2 years to double
- At 0.38% (FDIC national average): 72 / 0.38 = approximately 189 years to double
The Rule of 72 works best for rates between 6% and 10%. For rates below 6%, it slightly overestimates doubling time. For rates above 10%, it slightly underestimates it. It's a back-of-the-envelope tool and a complement to precise formulas, but it's remarkably useful for quick comparisons between rate offers.
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Time is the most powerful variable. On a 7% annual return, $10,000 invested for 40 years grows to $149,745. The same $10,000 invested for 30 years grows to $76,123. Those extra 10 years add $73,622 without a single additional dollar contributed. Starting early beats contributing more later, in almost every scenario you can model.
Compound Interest Rate Benchmarks for 2026
The interest rate you use in the calculator determines everything. Here are the key benchmarks as of June 2026:
| Account / Asset | Rate / APY | Best for |
|---|
| FDIC national avg (savings) | 0.38% APY | Traditional bank savings, almost always better elsewhere |
| High-yield savings (HYSA) | 4.00%–5.00% APY | Emergency fund, short-term savings, liquid cash |
| Top HYSA (Varo Money) | up to 5.00% APY | Best available savings rate as of June 2026 |
| 1-year CD | ~4.30% APY | Fixed rate for defined timeframe, no market risk |
| S&P 500 (10-yr nominal avg) | ~9.5% annualized | Long-term equity investing (no guaranteed return) |
| S&P 500 (inflation-adjusted) | ~7% annualized | Conservative long-term equity projection |
| Federal funds rate | 3.50–3.75% | Fed policy benchmark (held June 17, 2026) |
* HYSA and CD rates as of June 2026 (Fortune/Curinos). S&P 500 figures are historical averages and represent no guarantee of future performance. Fed funds rate from Federal Reserve June 17, 2026 announcement.
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HYSA vs. FDIC average: the real cost of a bad savings rate. $20,000 in a 0.38% account grows to $20,777 after 10 years. The same $20,000 in a 4.50% HYSA grows to $31,122. The gap is $10,345, earned with zero additional effort or risk beyond choosing a better account. Use the rate presets in the calculator to model both scenarios in seconds.
APY vs. APR: Which to Use in the Calculator
APY (Annual Percentage Yield) already accounts for compounding. Banks quote savings account rates in APY because it reflects the actual return you earn per year. When your account statement says "4.20% APY compounded daily," the 4.20% is the effective annual yield after compounding is applied. Enter this number directly into the calculator's interest rate field.
APR (Annual Percentage Rate) is the rate before compounding. The compound interest formula uses the pre-compounding rate (what the formula calls "r"), so if your institution quotes APR, use it in the formula directly. If they quote APY, you can use it as a close enough approximation for the calculator's annual rate field, especially for short time periods. For precise conversion: APR = n × ((1 + APY)^(1/n) - 1), where n is the compounding periods per year.
Compound Interest and Regular Contributions
Adding consistent monthly contributions dramatically accelerates compound growth. The mechanism is straightforward: each new contribution immediately starts earning interest on top of all previous interest, creating a compounding stack that grows more powerful over time.
| Scenario | 10 Years | 20 Years | 30 Years |
|---|
| $10,000 lump sum at 7% | $19,672 | $38,697 | $76,123 |
| $0 start + $200/mo at 7% | $34,607 | $104,921 | $243,994 |
| $10,000 + $200/mo at 7% | $54,279 | $143,618 | $320,117 |
| $10,000 + $500/mo at 7% | $96,679 | $264,021 | $594,614 |
* All figures use monthly compounding at 7% annual rate. Monthly contributions added at beginning of each month.
Frequently Asked Questions
What is compound interest? −
Compound interest is interest calculated on both your principal and the accumulated interest from previous periods. Each period, the interest earned is added to the balance, and the next interest calculation uses that larger number. This creates exponential growth over time. The more periods that pass and the more frequently interest compounds, the more pronounced the effect. It is the mechanism behind long-term wealth building and also the reason high-interest debt grows so rapidly when left unpaid.
How do I calculate compound interest? −
The formula is A = P(1 + r/n)^(nt), where A is the final amount, P is the principal, r is the annual interest rate as a decimal, n is the number of compounding periods per year, and t is the time in years. Example: $5,000 at 6% compounded monthly for 5 years: A = 5000 × (1 + 0.06/12)^(12×5) = 5000 × (1.005)^60 = 5000 × 1.3489 = $6,744.25. For accounts with regular contributions, use the calculator above. The formula becomes a multi-step summation that is much faster to run than to solve by hand.
What is the difference between daily and monthly compounding? −
Daily compounding adds interest to your balance every day. Monthly compounding adds it once per month. With the same annual rate, daily compounding produces a slightly higher balance because interest is added more frequently and begins earning returns sooner. At 5% APR over 10 years on $10,000, daily compounding earns $6,487 in interest versus $6,470 with monthly compounding, a $17 difference. At 1%, the difference over 10 years is about $0.50. The frequency matters most at higher rates and over longer time horizons. Most high-yield savings accounts compound daily, while CDs often compound monthly or quarterly.
What is a good interest rate for a savings account in 2026? −
Anything above 4.00% APY is competitive in 2026. The FDIC national average is just 0.38%, weighed down by the millions of accounts at traditional banks earning next to nothing. Top high-yield savings accounts from online banks offer up to 5.00% APY as of June 2026, including Varo Money (5.00%), Axos Bank (4.21%), and Newtek Bank (4.20%). The Federal Reserve held the federal funds rate at 3.50%–3.75% in June 2026, and further rate cuts could pressure HYSA rates downward. Locking into a CD at the current rate offers protection if rates fall.
What is the Rule of 72? −
The Rule of 72 estimates how many years it takes to double your money at a given compound interest rate. Divide 72 by the annual rate: at 6%, money doubles in about 12 years; at 9%, about 8 years; at 4%, about 18 years. The rule works because 72 is mathematically close to the natural log of 2 times 100, which governs exponential doubling. It is an approximation, most accurate between 6% and 10%, and a useful mental shortcut for comparing rate offers without a calculator.
How much does $1,000 earn with compound interest? −
The rate and time horizon determine everything. At 5% compounded monthly: after 1 year, $1,000 grows to $1,051; after 10 years, to $1,647; after 20 years, to $2,712; after 30 years, to $4,467. At 7% compounded monthly: after 10 years, $1,967; after 20 years, $3,870; after 30 years, $7,612. At the FDIC national average of 0.38%: after 10 years, $1,038. The rate and time horizon determine everything. A 1% difference in rate compounds into thousands of dollars over decades. Use the calculator above to model your exact starting amount and rate.
How do regular contributions affect compound interest? −
Regular contributions multiply the power of compounding. Each new deposit immediately starts earning interest, and that interest then compounds on itself alongside all prior deposits. Starting with $0 and contributing $200/month at 7% annual return grows to $34,607 after 10 years, with only $24,000 of actual contributions. After 30 years at the same rate, those $200/month contributions build to $243,994 from $72,000 contributed. The $171,994 difference is pure compound interest. Contributions made earlier in the timeline carry the most weight because they have the longest runway to compound.
What is the difference between compound interest and simple interest? −
Simple interest applies only to the original principal, every period. Compound interest applies to the principal plus all previously accumulated interest. On $10,000 at 7% over 20 years: simple interest earns $14,000 (7% × $10,000 × 20 years), for a total of $24,000. Compound interest produces $38,697. The gap of $14,697 is entirely from interest earning its own returns. In personal finance, nearly all savings accounts, investment returns, and loans use compound interest. Simple interest appears mainly in short-term personal loans and some auto loans.
