Free Compound Interest Calculator
See how your savings or investments grow over time with the power of compound interest. Enter your principal, annual rate, compounding frequency, and time period. Free, private — all calculations run in your browser.
📊 Scenario Comparison
| Scenario | Future Value | Total Interest | Total Deposited | Interest % |
|---|---|---|---|---|
| ★ Base (7.0%, 20yr) | $144,573 | $86,573 | $58,000 | 60% |
| Rate +2% (9.0%) | $193,669 | $135,669 | $58,000 | 70% |
| Rate −2% (5.0%) | $109,333 | $51,333 | $58,000 | 47% |
| No Monthly Contributions | $40,387 | $30,387 | $10,000 | 75% |
| 2× Contributions ($400/mo) | $248,758 | $142,758 | $106,000 | 57% |
💡 What Your Numbers Mean
- At 7.0% compounded monthly, $10,000 grows to $144,573 in 20 years. 60% of your final balance ($86,573) comes from compound interest — money earning money.
- At 7.0%, money doubles every 9.9 years (exact Rule of 72: 72 ÷ 7 = 10.3). Starting earlier has exponential impact.
- Inflation at 3.0%/year reduces your future $144,573 to $80,046 in today's purchasing power — a $64,526 reduction. Consider inflation-adjusted targets.
- Your $200/month contributions add $48,000 in deposits but generate $56,185 of additional compounded growth — 117% return on every dollar contributed.
About This Compound Interest Calculator
The Compound Interest Calculator shows you the future value of a lump sum investment or savings balance, including the effect of regular contributions, over any time period and compounding frequency. It is one of the most important financial tools you can use, because compound interest — interest earned on interest — is the fundamental engine behind long-term wealth building. Albert Einstein reportedly called it the "eighth wonder of the world," and while that attribution is disputed, the math is undeniably powerful.
The Formula — How It Works
The core formula is the compound interest equation:
Where: A = final amount (principal + interest), P = principal (starting amount), r = annual interest rate as a decimal (e.g. 0.07 for 7%), n = compounding frequency per year (12 for monthly, 365 for daily), t = time in years.
When regular contributions are added, the calculator also applies the future value of an annuity formula to calculate the accumulated value of all periodic deposits, then sums the two results to give the total balance.
Assumptions and Limitations
This calculator makes the following simplifying assumptions:
- •The interest rate is constant throughout the entire period — real investment returns fluctuate annually
- •Contributions are made at the start or end of each period consistently — no missed periods
- •Taxes on interest or capital gains are not deducted — results shown are pre-tax
- •Inflation is not factored in — the final amount shown is in nominal (today's dollar) terms
- •Investment fees (expense ratios, advisory fees) are not subtracted — these reduce real returns
Who Should Use This Calculator
This tool is designed for anyone who wants to understand or demonstrate the power of compounding. Students learning personal finance can use it to see why starting early matters so much. First-time savers can compare a high-yield savings account to a standard account. Investors can model different return scenarios to understand outcome ranges. Parents planning education funds can estimate how monthly 529 contributions grow over 18 years. Anyone comparing two savings or investment products — where the only difference is the rate or compounding frequency — can use this tool to instantly quantify the dollar difference.
When to Consult a Professional
This calculator is an educational and planning tool. Before making real investment decisions — choosing between a Roth IRA and taxable brokerage, selecting funds, or allocating a large lump sum — consult a qualified financial planner (CFP) or fee-only fiduciary advisor. They can account for your tax bracket, risk tolerance, time horizon, and complete financial situation. Visit napfa.org to find a fee-only advisor.
Privacy Notice
All calculations in this compound interest calculator run entirely in your browser. No data you enter — including principal amounts, interest rates, or time periods — is transmitted to any server or stored anywhere. Your financial information stays completely private on your device. See our Privacy Policy for full details.
When to Use This Calculator
See exactly how much a savings account or CD balance will grow over 1, 5, or 20 years at a given interest rate and compounding frequency.
Compare a high-yield savings account (4.5% APY daily compounding) to a standard savings account (0.5% APY) over 10 years to quantify the difference in dollars.
Project how monthly contributions to a 529 plan will grow over 18 years to estimate whether your savings rate will cover future college costs.
Model different expected return scenarios (conservative 5%, moderate 7%, aggressive 10%) to understand the range of possible outcomes for a lump-sum investment.
Estimate the future value of a single lump sum (like an inheritance or bonus) invested today, as a complement to your regular retirement contributions.
💡 Pro Tips
A 1% difference in annual rate sounds tiny, but it is enormous over time. $10,000 at 6% for 30 years grows to $57,435. At 7%, it grows to $76,123 — a $18,688 difference from a single percentage point. Always pursue higher returns through low-cost index funds, which historically outperform actively managed funds after fees.
Compounding frequency matters much less than rate or time. The difference between monthly and daily compounding on $50,000 over 20 years at 7% is only about $200. Focus your energy on maximising your rate and contribution amount rather than obsessing over compounding intervals.
Use the Rule of 72 to make compound interest intuitive: divide 72 by your annual rate to find the doubling time. At 8%, money doubles in 9 years. That means $25,000 today becomes $50,000 in 9 years, $100,000 in 18 years, and $200,000 in 27 years — without any additional contributions.
Starting 10 years earlier can have more impact than doubling your monthly contribution. $300/month from age 25 to 65 at 7% grows to ~$798,000. Starting at 35 with $600/month — double the contribution — only reaches ~$726,000. Time in the market is your greatest asset.
Frequently Asked Questions
Financial Disclaimer
Compound Interest Calculator — Results are projections based on a constant interest rate and the inputs you provide. They do not constitute financial or investment advice. Real investment returns vary by year and are not guaranteed. Taxes, inflation, and fees are not included in results. Always consult a qualified financial planner before making investment decisions.
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