Compound Interest Calculator — Monthly Contributions & Rule of 72
Calculate compound interest with monthly contributions free — see how your investment grows year by year. Includes the Rule of 72 (how long to double your money), growth chart and investment scenario presets.
Last updated: June 2026
Results
- Final balance
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- Total contributions
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- Total interest / returns
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📈 Rule of 72
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Growth over time
Year-by-year growth
| Year | Contributions | Interest earned | Total balance |
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What if you started earlier?
Same monthly amount and rate, invested until age 60.
| Start age | Years invested | Balance at 60 |
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A free compound interest calculator to see your money grow.
A compound interest calculator reveals one of the most important ideas in personal finance: the way money grows on itself over time. When you earn interest not just on your original deposit but on all the interest that came before it, your balance doesn't grow in a straight line — it accelerates. This tool lets you enter a starting amount, an optional monthly contribution, a rate of return, and a time horizon, then shows your final balance, your total contributions, and exactly how much of the result is pure growth.
To use it, start with your initial amount and add a monthly contribution if you plan to invest regularly. Set the annual rate — or tap one of the scenario presets, from a conservative 4% typical of savings and bonds, through a moderate 7% balanced fund and a 10% equity fund, up to an aggressive 15%. Choose how often interest compounds: daily, monthly, quarterly, or yearly. The results, the growth chart, and the year-by-year table all update instantly, so you can see the impact of every change as you make it.
The Rule of 72 and the cost of waiting
The built-in Rule of 72 gives you an instant feel for compounding: divide 72 by your rate of return and you get the approximate number of years for your money to double. At 9% it's about eight years; at 6%, twelve. The "start earlier" comparison drives the lesson home by showing the same monthly investment begun at ages 25, 30, 35, and 40 — and how much more you'd have at 60 simply by starting sooner. The extra years happen at the end, when your balance is largest, which is why a five-year head start can be worth far more than it first appears.
This is a global, educational calculator: it works in any of six currencies and isn't tied to a specific country's tax rules, so it's ideal for understanding the mechanics of investing, comparing scenarios, or teaching the concept of compounding. Remember that real-world returns vary year to year and aren't guaranteed — a fixed rate is a simplification. Everything runs privately in your browser, with nothing uploaded, so explore as many "what if" scenarios as you like.
Investment Disclaimer: Past performance does not guarantee future results. For educational purposes only and not financial advice. Real returns vary year to year; a fixed rate is a simplification. Consult a qualified professional before investing.
How it works
Three quick steps — no account, nothing uploaded to a server.
Enter your amounts
Set an initial amount and an optional monthly contribution.
Pick a rate and time
Choose a return rate (or a preset), tenure and compounding frequency.
See your growth
Read the final balance, Rule of 72, growth chart and start-early comparison.
Investing concepts on this page align with investor education from Investor.gov (US Securities and Exchange Commission). Last checked July 2026.
FAQ
Frequently asked questions
What is compound interest?
Compound interest is interest earned on both your original principal and on the interest that has already accumulated. In other words, you earn interest on your interest. This is what makes long-term investing so powerful: as your balance grows, the amount of interest it generates each period grows too, creating an accelerating snowball effect. The opposite is simple interest, which is calculated only on the original principal and grows in a straight line.
How is compound interest calculated?
The basic 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 times interest compounds per year, and t is the number of years. When you also add regular contributions — like a monthly deposit — each contribution starts compounding from the moment it's invested. This calculator handles both the initial amount and recurring monthly contributions, at daily, monthly, quarterly or yearly compounding.
What is the Rule of 72?
The Rule of 72 is a quick mental shortcut to estimate how long it takes an investment to double. You simply divide 72 by the annual rate of return. At 8% a year, your money doubles in about 72 ÷ 8 = 9 years; at 6%, in about 12 years. It's an approximation that works best for rates between roughly 5% and 12%, but it's a powerful way to grasp the impact of compounding and to compare investment options at a glance. The calculator updates this figure live as you change the rate.
How does compounding frequency affect returns?
The more often interest is compounded, the more you earn, because interest starts earning its own interest sooner. Daily compounding produces a slightly higher final balance than monthly, which beats quarterly, which beats annual — all at the same stated annual rate. The difference is modest at low rates and short horizons but grows with higher rates and longer periods. Use the compounding selector to compare daily, monthly, quarterly and yearly side by side.
Why does starting early matter so much?
Because compounding rewards time more than almost anything else. Money invested earlier has more years to grow, and those extra years happen at the end — when the balance is largest and each year's growth is biggest. That's why someone who starts investing a modest amount at 25 can finish well ahead of someone who invests more but starts at 35. The 'start earlier' comparison above shows exactly how many years of growth you'd gain, and what it's worth.
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