Compound Interest Calculator
Future value with monthly deposits, any compounding. Runs in your browser.
About this compound interest calculator
This compound interest calculator shows how much your money will grow when the interest itself earns interest. Enter a starting amount, a yearly rate, how often it compounds and how long you'll leave it, and you get the future value, the total interest and a year-by-year breakdown.
Most people don't save a lump sum and walk away. So you can add a monthly or yearly deposit, choose whether it goes in at the start or end of each period, and raise it a little every year. The formula is filled in with your own numbers, so you can check the result by hand.
Everything runs in your browser. Nothing you type is sent anywhere, and the link in your address bar keeps your inputs so you can share a scenario or come back to it later.
Features
Nine compounding options
Yearly, half-yearly, quarterly, monthly, semi-monthly, bi-weekly, weekly, daily and continuous. The effective yearly rate (APY) updates with each one.
Regular deposits that can grow
Add a monthly or yearly deposit, put it in at the start or end of the period, and raise it by a set percentage each year to match pay rises.
The formula with your numbers
A = P × (1 + r/n)^(nt) is shown with your P, r, n and t, plus how much of the balance comes from your deposits.
Compound vs simple interest
A highlighted figure shows how much extra compounding earns on your starting amount compared with plain simple interest.
Doubling time, exact and quick
See how many years it takes to double your money, worked out exactly, next to the Rule of 72 estimate.
Today's money view
Enter an inflation rate and the final balance is also shown in today's money, with a line on the chart and a column in the table.
Chart, table and CSV
A stacked yearly chart splits the balance into starting amount, deposits and interest. Switch to the yearly table or download it as CSV.
How to use the compound interest calculator
- 1Enter the starting amount, yearly rate, compounding and time.
- 2Add an optional monthly or yearly deposit, its timing and yearly raise.
- 3Read the future value, total interest and APY, then check the chart or yearly table.
When to use it
Checking a savings account or CD
Put in the bank's rate and compounding to see the real balance after one, three or five years. Then confirm the quoted yield with the APY calculator.
Planning retirement or index fund savings
Try 300 a month at an assumed 7% for 30 years, then see what starting five years earlier does. The gap is usually bigger than people expect. For a mutual fund plan, the SIP calculator adds step-up and goal modes.
Saving for a goal
Work out whether a monthly amount reaches a house deposit, school fees or an emergency fund in time, and in today's money.
How is compound interest calculated?
The standard formula is A = P × (1 + r/n)^(nt). P is the starting amount, r is the yearly rate as a decimal, n is how many times a year interest is added and t is the number of years. A is the balance at the end.
Take 10,000 at 7% compounded monthly for 10 years. That's 10,000 × (1 + 0.07/12)^120 = 20,096.61. Simple interest at the same rate would pay 700 a year, or 7,000 in total. Compounding earns 10,096.61, so about 3,097 of the interest is interest on interest.
Regular deposits use the future value of an annuity on top of that. With an extra 200 at the end of each month, the same example ends at 54,713.58. You put in 34,000 and earn 20,713.58 in interest.
Does compounding frequency matter?
It matters less than most people think. Here is 10,000 at 7% for 10 years with each option:
- Yearly: 19,671.51 (APY 7.00%)
- Quarterly: 20,015.97 (APY 7.19%)
- Monthly: 20,096.61 (APY 7.23%)
- Daily: 20,136.18 (APY 7.25%)
- Continuous: 20,137.53 (APY 7.25%)
How long does it take to double your money?
The Rule of 72 says divide 72 by the yearly rate. At 8% that's 9 years. The exact answer is ln 2 ÷ ln(1.08), which is 9.006 years, so the shortcut is very close in the 6% to 10% range.
At low rates it drifts. At 2% the rule says 36 years while the exact figure is 35.0. The calculator shows both, so you can see how far off the shortcut is for your rate.
Why does starting early make such a difference?
Time is the exponent in the formula, so extra years count more than extra money. Saving 200 a month at 7% for 40 years gives about 524,963. Stop at 30 years and you get about 243,994. Those last 10 years add only 24,000 of deposits but more than double the balance.
Timing within each period matters a little too. Moving the 200 deposit from the end of the month to the start lifts the 10-year example from 54,713.58 to 54,915.51, because each deposit earns one extra month of interest.
Why use Mavros
| Mavros | Typical upload sites | |
|---|---|---|
| Regular deposits with yearly raise | Yes, monthly or yearly, start or end | Monthly only, no raise |
| Compounding options | Nine, including continuous | Three to five |
| Formula with your numbers | Yes | Generic formula only |
| Inflation-adjusted result | Yes, with a chart line | Rarely |
| Exact doubling time | Yes, next to the Rule of 72 | Rule of 72 or nothing |
| Yearly table and CSV | Yes, free | Table only, or behind sign-up |
| Ads and tracking | None, runs in your browser | Ads and product offers |
Tips
- Use the APY, not the headline rate, when you compare accounts. A 4.5% rate compounded daily is a 4.60% APY.
- Set inflation to 2% or 3% for long plans. A balance of 54,714 in 10 years buys about what 40,712 buys today at 3% inflation.
- Market returns are not fixed. For stock funds, try a lower and a higher rate to see a range instead of one number.
- Interest on savings is often taxed. If yours is, lower the rate by your tax share for a rough after-tax figure.
- Compounding works against you on debt. Run a credit card or loan balance the same way to see the cost.
Questions
What is the compound interest formula?
A = P × (1 + r/n)^(nt). P is the starting amount, r the yearly rate as a decimal, n the number of compounding periods per year and t the time in years. For continuous compounding it becomes A = P × e^(rt).
How much will 10,000 grow in 10 years?
At 7% compounded monthly, 10,000 grows to 20,096.61 in 10 years. At 5% it would be about 16,470, and at 10% about 27,070. Change the rate in the calculator to see your own figure.
Is daily compounding better than monthly?
Yes, but only slightly. At 7%, monthly compounding gives an APY of 7.23% and daily gives 7.25%. On 10,000 over 10 years the difference is about 40. The rate itself matters far more.
What is the difference between simple and compound interest?
Simple interest is paid only on the original amount. Compound interest is also paid on interest already earned. At 7% for 10 years, 10,000 earns 7,000 simple interest but 10,096.61 when compounded monthly.
How does the Rule of 72 work?
Divide 72 by the yearly interest rate to estimate the years needed to double your money. At 6% that's about 12 years. It's accurate to within a few months for rates between about 4% and 12%.
Does this include taxes or fees?
No. The result is before tax and fees. If your interest is taxed every year, lower the rate by your tax share. For example, 5% taxed at 20% grows roughly like 4%.
Can I use it for stock market returns?
Yes, as a planning estimate. Enter an average yearly return and yearly compounding. Real markets go up and down, so the actual path will differ from a smooth line, and returns are not guaranteed.
How do I work out the percentage gain?
Divide the total interest by what you put in and multiply by 100. In the default example, 20,713.58 ÷ 34,000 is a 60.9% gain. The percentage calculator does this for any two numbers.