Free Compound Interest Calculator
Project your savings growth with compounding and monthly contributions — no signup needed.
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Contributions grow by this percentage each year (e.g., 3% to model salary increases)
Capital gains / income tax applied to investment earnings
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What is a compound interest calculator?
A compound interest calculator is a free online tool that helps you compute investment growth with compounding, showing balance over time for any interest rate and compounding frequency. It runs entirely in your browser — no signup, no app, and no data is ever sent to a server.
I've run the numbers for dozens of friends who were debating whether to invest $200 a month or wait until they had a bigger lump sum, and the answer is always the same: start now, even if it's small. Compound interest is where the magic happens — Einstein allegedly called it the eighth wonder of the world, and once you see the numbers, you'll get why. This calculator starts with whatever you've saved, adds your monthly contributions, and projects it all forward over time. The chart alone is worth playing with: watch the curve bend upward as the interest starts earning its own interest.
A lot of compound interest calculators ignore two big things: inflation and taxes. If you're getting 7% but inflation is 3%, your real return is closer to 4%. And if you're paying capital gains tax, that eats into the total too. This one lets you factor both in, plus you can model annual contribution increases (like if you plan to increase your 401(k) contributions as your salary grows).
How the compound interest calculator works
The calculator applies the compound interest formula A = P(1 + r/n)^(nt) plus an additional term for regular monthly contributions. Each year, interest is calculated on the growing balance and added according to your chosen frequency — daily, monthly, quarterly, semi-annually, or annually. With monthly compounding, for example, one-twelfth of the annual rate is applied each month to the current balance including that month's contribution. Over time this compounding effect causes your money to grow at an accelerating pace, which is why long-term investors see such dramatic results.
For example: a $10,000 initial deposit with $200 monthly contributions at 7% annual interest compounded monthly grows to approximately $256,000 after 30 years. Of that, only $82,000 came from contributions — the remaining $174,000 is compound interest earned along the way. The year-by-year table in the calculator shows exactly when the curve bends upward, typically around year 10 to 15, when the accumulated interest begins to rival and then surpass your annual contributions.
The calculator uses the standard compound interest formula with monthly contribution support. It runs entirely in your browser, so no data is sent to any server. Your inputs are automatically saved using local storage, so you can close the page and return later to find your numbers exactly where you left them. The interactive growth chart updates in real time, making it easy to compare different scenarios side by side.
When to use a compound interest calculator
Play around with the numbers and you'll quickly see why starting early matters more than starting big. Drop $200 a month into a retirement account starting at 25 with a 7% return, and by 65 you've got roughly $480,000. Wait until 35 to start the same $200 a month, and you end up with about $225,000 — less than half, even though you only saved for ten fewer years. That ten-year delay costs you a quarter million dollars.
Use it to model your 401(k), an IRA, a kid's college fund, or just to see what happens if you up your monthly contribution by $50. The year-by-year table shows exactly when the compounding starts to snowball — typically somewhere around year 10 to 15, when the interest earned each year surpasses what you're contributing. That's when the curve on the chart really bends upward.
Compound frequency and its effect
The frequency of compounding matters because more frequent compounding means interest starts earning interest sooner. Daily compounding produces the highest ending balance, followed by monthly, quarterly, semi-annual, and annual compounding at the same stated annual rate. In practice the difference between monthly and daily compounding is small — typically less than 1% of the total after 30 years — so focus on your contribution rate and time horizon rather than chasing accounts with daily compounding. The most important factor by far is starting early and staying consistent.
Use the compound frequency dropdown in the calculator to see the exact difference for your specific numbers. On a $50,000 portfolio with $500 monthly contributions at 7% over 20 years, annual compounding yields about $297,000 while monthly compounding yields about $305,000 — a difference of roughly $8,000. The chart visualizes this gap clearly, helping you decide whether seeking a higher compounding frequency is worth switching banks or brokerages. In most cases, increasing your contribution rate by even a small amount will have a far larger impact than switching from quarterly to monthly compounding.
Common mistakes people make
The mistake I see most often is people underestimating how much fees eat into compound growth. A 1% annual fee on a $100,000 portfolio growing at 7% over 30 years costs you roughly $76,000 in lost returns — that's not what you pay in fees, that's what the compound growth would have been on that 1% each year. You can approximate fees by subtracting them from your expected return rate. I also find that people obsess over daily vs monthly compounding when what really matters is the contribution rate and time horizon — a $50/month increase in contributions has a far bigger impact than switching from annual to daily compounding. Another common error: forgetting to account for inflation. Getting 7% nominal returns with 3% inflation means your real return is only 4%, and the calculator's inflation toggle shows you what your balance is actually worth in today's dollars.
How to use this calculator
- Enter your savings details — Type your initial deposit, monthly contribution, expected annual interest rate, and the number of years you plan to save.
- Choose compound frequency — Select how often interest compounds — daily, monthly, quarterly, semi-annually, or annually — to match your savings account or investment.
- Review your projected growth — Instantly see your ending balance, total contributions, and total interest earned. The year-by-year table and growth chart show how your money grows over time.
Frequently asked questions
How does compound interest work with a simple example?
Compound interest earns interest on both your principal and previously earned interest. $1,000 at 10% simple interest earns $100/year = $1,500 after 5 years. At 10% compound interest: year 1 = $1,100, year 2 = $1,210, year 3 = $1,331, year 4 = $1,464, year 5 = $1,611. The difference ($111) grows dramatically over longer periods.
How much will $10,000 grow in 10 years at 7% compound interest?
$10,000 at 7% annual compound interest grows to $19,671 after 10 years — nearly doubling without any additional contributions. After 20 years: $38,697. After 30 years: $76,123. This demonstrates the Rule of 72: divide 72 by the interest rate to find doubling time. At 7%, money doubles roughly every 10.3 years.
What is the difference between daily, monthly, and annual compounding?
More frequent compounding produces slightly higher returns. $10,000 at 6% for 10 years: annual compounding = $17,908; monthly compounding = $18,194; daily compounding = $18,221. The difference between monthly and daily is small ($27 over 10 years) but the difference between annual and daily compounding adds up to $313. High-yield savings accounts and most bonds compound daily.
How do I calculate compound interest manually?
Formula: A = P(1 + r/n)^(nt), where A = final amount, P = principal, r = annual interest rate (decimal), n = compounding frequency per year, t = years. For $5,000 at 5% compounded monthly for 3 years: A = 5000 × (1 + 0.05/12)^(12×3) = 5000 × (1.004167)^36 = 5000 × 1.1614 = $5,807. The calculator handles this instantly.
What is the Rule of 72 for compound interest?
The Rule of 72 estimates how long it takes money to double: divide 72 by the annual interest rate. At 6%, money doubles in 72/6 = 12 years. At 9%, it doubles in 8 years. At 3%, it takes 24 years. It works in reverse too: if you need money to double in 10 years, you need a 7.2% return. The rule is an approximation; the calculator gives exact values.
How much should I invest monthly to reach $100,000?
At 7% annual return: investing $200/month reaches $100,000 in about 23 years; $400/month in 14 years; $700/month in 9 years. Starting earlier matters enormously — $200/month for 30 years grows to $227,000, while $400/month for 15 years grows to only $125,000. Enter your monthly contribution, rate, and target amount in the calculator to find your exact timeline.