Free Online Compound Interest Calculator
Runs 100% in your browser — your numbers never leave your device.
Educational estimates only — not financial advice. Ignores taxes, fees and inflation.
Project your growth
About this compound interest calculator
Compound interest is interest earning interest. In year one, a 5% rate on $1,000 adds $50. By year ten, the same 5% applies to a much larger balance, so the dollar gains accelerate — the growth curve bends upward. The table above makes this visible: early rows are mostly your own contributions, later rows are mostly interest.
Two levers matter more than anything: time and contributions. Doubling the rate helps, but starting ten years earlier or adding a monthly contribution usually wins. Try the defaults ($1,000 at 5% for 10 years with $100/month) against the same setup with $0 monthly contributions to see the difference.
The formula, with a worked example
The calculator uses the standard compound-interest formula A = P(1 + r/n)nt, where P is the starting amount, r the annual rate as a decimal, n the compounding periods per year, and t the years. Try it by hand: $10,000 invested at 7% compounded monthly for 10 years gives A = 10000 × (1 + 0.07/12)120 = $20,096.61 — the money roughly doubles. Run the same inputs in the calculator above and you will get exactly that figure; the year-by-year table then shows how the balance climbs from $10,000 to $20,096.61.
Compounding frequency matters less than you think
That same $10,000 at 7% for 10 years grows to $19,671.51 with yearly compounding, $20,096.61 with monthly compounding, and $20,136.18 with daily compounding. Notice the pattern: moving from yearly to monthly is worth $425.10, but moving from monthly to daily adds only $39.57. Chasing the most frequent compounding is a distraction; the rate and the time horizon decide the outcome. Time is the heaviest lever: $5,000 at 6% compounded monthly becomes $16,551.02 after 20 years — the interest alone ($11,551.02) is more than double what you put in.
Contributions usually do the heavy lifting
Take $1,000 at 5% compounded monthly for 10 years with no contributions: it grows to $1,647.01, so interest earned is just $647.01. Now add $100 a month — the calculator reports a future value of $17,175.24, made of $13,000 in contributions and $4,175.24 in interest. Your own payments are three times the interest. That is the normal shape of saving for the first decade: contributions dominate, and compounding takes over only in the later years, when the balance is large enough for interest to compound on itself.
How to use
- Enter the starting amount, annual rate, time horizon and compounding frequency.
- Add an optional monthly contribution.
- Press Calculate growth for the future value, total contributions, interest earned and a year-by-year table.
Limitations
- Educational estimates only — not financial advice.
- Ignores taxes, fees, inflation and variable returns; real investments fluctuate.
- Monthly contributions are assumed at each compounding period, spread evenly.
- The rate you enter is the nominal annual rate (like a bank's quoted APR); the calculator does not convert it to an effective annual yield.
- Contributions are added at the end of each compounding period in the model, which matches most savings accounts but not every investment product.
Frequently asked questions
What is compound interest?
Compound interest is interest earned on both your original money and the interest already added. Over time the growth curve bends upward — the later years earn far more than the early ones, which is why starting early matters more than the exact rate.
How do monthly contributions change the result?
Dramatically. Regular contributions usually dominate the final balance for most savers — the growth table shows your contributions and interest separately so you can see which does the heavy lifting.
What does compounding frequency mean?
How often interest is calculated and added: yearly, half-yearly, quarterly, monthly or daily. More frequent compounding grows slightly faster, but the difference between monthly and daily is small compared to the rate and the time horizon.
Is this financial advice?
No. This is an educational estimate tool, not financial advice. It ignores taxes, fees and inflation, and past market returns do not predict future results. Talk to a qualified professional before investing.
Is my data uploaded anywhere?
No. The calculator runs 100% in your browser with JavaScript. Your numbers never leave your device, are never stored, and never sent to any server.
How do I calculate compound interest by hand?
Use A = P(1 + r/n)nt: multiply the principal P by (1 + annual rate ÷ periods per year) raised to the power of (periods per year × years). Example: $10,000 at 7% compounded monthly for 10 years is 10000 × (1 + 0.07/12)120 = $20,096.61 — the same figure this calculator reports.
Is daily compounding much better than monthly?
Barely. On $10,000 at 7% over 10 years, yearly compounding gives $19,671.51, monthly gives $20,096.61, and daily gives $20,136.18 — the monthly-to-daily jump is only $39.57. Pick a higher rate or a longer horizon instead; that is where the real difference comes from.
What is the rule of 72?
A quick mental shortcut: divide 72 by the annual rate to estimate the years needed to double your money. At 7%, 72 ÷ 7 ≈ 10.3 years — which matches the worked example above, where $10,000 at 7% compounded monthly becomes roughly $20,000 in 10 years. It is an approximation, not a substitute for the calculator.
Why does my monthly contribution get split across compounding periods?
The calculator spreads your monthly contribution evenly across the compounding periods and adds it at the end of each period, so the maths stays consistent whatever frequency you pick. That matches most savings accounts; some investment products credit contributions differently, which is one reason this tool is an estimate, not a bank statement.