Compound Interest Calculator
Calculate how your savings grow with compound interest. See the impact of regular monthly contributions and compounding frequency on your long-term wealth.
Key Features
- •9 compounding frequencies: Annually, Semiannually, Quarterly, Monthly, Semimonthly, Biweekly, Weekly, Daily, and Continuously
- •Initial lump sum investment + recurring monthly contributions
- •Year-by-year growth schedule with balance, contributions, and interest breakdown
- •Visual bar chart of balance progression over time
- •Final balance summary with total contributed vs total interest earned
- •Supports both APR (monthly) and APY (annual) compounding modes
About Compound Interest Calculator
Compound interest is the force that turns modest savers into millionaires. Our Compound Interest Calculator models the exponential growth of your savings — whether you're investing in a high-yield savings account, a retirement account, or a brokerage portfolio. Unlike simple interest calculators, this tool visualizes the "snowball effect" where your interest earns interest, accelerating growth over time.
The Secret to Wealth: Time + Consistency
Let's run a motivating scenario. You are 25 years old. You invest $200 per month into an S&P 500 index fund earning an average of 8% compounded monthly. By age 65 (40 years), you will have contributed only $96,000 of your own money. But your ending balance will be over $620,000 — meaning $524,000 is pure interest. If you wait until 35 to start, that same $200/month yields only $274,000. The cost of waiting 10 years is $346,000. Use our calculator to run your own scenarios.
Compounding Frequency: Daily vs. Monthly vs. Annually
Not all compound interest is created equal. The more frequently interest is compounded, the higher the effective yield. A 5% annual rate compounded:
- Annually: Effective APY = 5.00%
- Monthly: APY = 5.12%
- Daily: APY = 5.13%
- Continuously: APY = 5.127% (theoretical maximum)
On a $100,000 balance over 20 years, daily compounding yields ~$271,000 vs. ~$265,000 for annual — a $6,000 difference just from checking the right box.
Understanding the Year-by-Year Breakdown
The growth schedule shows your projected balance at the end of each year alongside total contributions and total interest earned. This makes it easy to see when interest begins to outpace contributions — the hallmark of compounding kicking in.
Contributions: The Most Controllable Variable
While the interest rate is largely outside your control, how much you contribute is entirely up to you. Even small increases in monthly contributions have an outsized effect over decades. Try adjusting the monthly contribution field to see how an extra $100/month changes your final balance.
FAQ: Compound Interest Calculator
What is the best compounding frequency for savings accounts?
Daily compounding is best for savers. Some online banks (Ally, Marcus, SoFi) offer daily compounding. Traditional banks often compound monthly. Check your account terms.
What is the difference between APR and APY?
APR (Annual Percentage Rate) is the stated rate. APY (Annual Percentage Yield) includes the effect of compounding. Use this calculator to see the difference — select "Monthly (APR)" vs "Annually (APY)" with the same rate to compare outcomes.
What does continuous compounding mean?
Continuous compounding is the mathematical limit of compounding infinitely often. It uses the formula A = Pe^(rt) and represents the theoretical maximum growth for a given rate. In practice, daily compounding is very close to continuous.
Compound Interest Calculator is optimized for fast browser-based use, so you can test multiple scenarios in seconds.
Formula & Logic
- 01Discrete compounding: balance = balance × (1 + r/n) + (PMT × 12/n) applied n times per year.
- 02Continuous compounding: balance = balance × e^(r/12) + PMT applied 12 times per year.
- 03Total interest earned = final balance − total contributions (including initial principal).
- 04contributionPerPeriod = (monthlyPMT × 12) / n, ensuring annual contribution is consistent across all frequencies.
Practical Examples
- 01Baseline check: Use realistic inputs in Compound Interest Calculator to generate a first-pass estimate.
- 02Sensitivity check: Change one key input at a time to compare how the output shifts.
- 03Decision check: Save two or more scenarios and use the differences to choose the better option.
Important Limitations
- •Results depend on the accuracy of your inputs.
- •Displayed values may be rounded for readability.
- •Financial outputs are planning estimates and do not include institution-specific fees unless provided.
Frequently Asked Questions
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