Compound Interest Calculator
See what your savings or investments could grow into. Start with an amount, add a regular contribution, pick a rate and time horizon, and watch compounding do the work - with a year-by-year breakdown. Free, instant, and private - nothing you type leaves your browser.
What you already have saved or are investing today. Use $0 to start from nothing.
Depositing at the start of each period gives every contribution a little extra time to grow.
7% is an illustrative long-run stock-market average after inflation - not a promise. Use your savings account's rate for cash. Returns are not guaranteed.
How often interest is added back to your balance. More often = a bit more growth.
Future value in 20 years
$0
$0 of your own money +$0 in growth
- Starting amount
- $0
- Total contributions
- $0
- Total interest earned
- $0
- Total you put in
- $0
- Future value
- $0
Year-by-year growth (see the balance build)
| Year | Total contributed | Interest earned | Balance |
|---|
"Total contributed" includes your starting amount plus every deposit made by the end of that year.
Estimate only - not investment advice. The rate you enter is an assumption; real returns vary year to year and are not guaranteed. See notes below.
How this calculator works
Compounding is simple to state and powerful to watch: your money earns a return, that return is added to your balance, and next period you earn a return on the larger balance too. Do that for enough years and the growth starts to dwarf what you put in. This tool models every piece of that so the future value you see is the number the math actually produces.
- 1. Your starting amount grows for the full time horizon using the standard formulafuture value = P (1 + r/n)n×t, where P is the amount, r is the yearly rate, n is how many times a year it compounds, and t is the number of years.
- 2. Each regular contribution is added on your schedule (monthly or yearly) and then grows for however long it stays invested. A deposit you make in year 1 compounds for the whole horizon; one you make in the final year barely grows at all - which is exactly why starting early matters so much.
- 3. Compounding frequency - annual, semiannual, quarterly, monthly, or daily - decides how often interest is added back. More frequent compounding earns a little more on the same rate, but it is a small effect next to your rate, contributions, and time.
- 4. Contribution timing. Depositing at the start of each period gives every contribution one extra period of growth versus depositing at the end. It is a modest edge - the tool lets you see it both ways.
- 5. Total interest is simply the ending balance minus every dollar you put in (your starting amount plus all contributions). That gap is the compounding - money you never deposited.
The 7% default rate is an illustrative long-run figure: the US stock market has historically averaged about 7% a year after inflation over many decades. It is an assumption for planning, not a forecast - swap in your savings account's rate for cash, or a lower rate if you want a conservative view. Every input is editable.
What this estimate does not include
It assumes a steady rate and steady contributions. Real markets move up and down, not in a straight line, so an actual portfolio will not track a smooth curve. It also does not model taxes on your gains, investment fees, or inflation eroding what your money can buy - a dollar in 20 years buys less than a dollar today. Treat the result as a clear illustration of how compounding works over time, not a promise of a specific balance.
Frequently asked questions
What is compound interest?
Compound interest is interest you earn on your interest, not just on the money you first put in. Each period your balance grows, and the next period you earn a return on that bigger balance. Over a few years the effect is small; over decades it is the single biggest reason a modest, steady saver can end up with far more than they contributed. In the default example above, $49,000 of your own money grows into roughly $108,000 - more than half the ending balance is growth you never deposited.
How much does compounding frequency actually change my result?
Less than most people expect. Going from annual to daily compounding on a 7% rate over 20 years lifts the result by only a few percent - because the same total rate is just being sliced more finely. The three levers that really move the number are your rate of return, how much you contribute, and how many years you stay invested. Compounding frequency is real, but it is the small dial, not the big one.
Should I contribute at the start or the end of each period?
Contributing at the start of each period (the "start" setting) puts every deposit to work one period sooner, so it always ends a little higher than contributing at the end. On the default 20-year example the difference is a few hundred dollars. It is a genuine edge if you can set money aside early each month, but do not let it stop you from investing - a late deposit still beats no deposit.
What return rate should I use?
Use a rate that matches where the money actually sits. For cash in a savings account or CD, use that account's quoted rate. For long-term stock-market investing, the US market (S&P 500) has historically averaged roughly 10% a year before inflation and about 7% after inflation over many decades - so 7% is a common, honest planning assumption for real growth, and the default here. It is an assumption, not a guarantee: real returns swing widely year to year and the future can differ from the past. When in doubt, use a lower rate and let reality surprise you upward.
Why is time the most important ingredient?
Because growth compounds on growth, the dollars you invest earliest have the longest runway and do the heaviest lifting. A dollar invested at 7% roughly doubles every ten years, so money invested at 25 has far more time to double and re-double than the same dollar invested at 45. Starting earlier - even with smaller amounts - usually beats starting later with larger amounts. That is why "time in the market" tends to win.
Is the interest rate the same as APY?
Not quite. The interest rate is the nominal (before-compounding) rate, while APY - annual percentage yield - is the effective rate after compounding is applied, so APY is always a touch higher when interest compounds more than once a year. Banks advertise savings accounts in APY. If you have an APY, enter it here with annual compounding to avoid double-counting the compounding effect; if you have a nominal rate, enter it with the account's real compounding frequency.
Go deeper: saving & investing guides
Short, plain-English guides behind every number in this calculator.
Compound interest, explained
The snowball effect in plain English, with the formula and worked examples.
The Rule of 72
A 5-second way to estimate how long money takes to double.
Dollar-cost averaging
Why investing a fixed amount on a schedule smooths out the ups and downs.
High-yield savings accounts
Where to park cash you need soon, and how APY really works.
How to save for a house
Turning a down-payment goal into a monthly number that fits your budget.
How to save $1 million
What it actually takes each month, by the age you start.
Sources & last reviewed
Method last reviewed July 3, 2026 by Shivam Rai. Compound interest is a fixed formula; the only moving part is the rate you assume - always sanity-check it against where your money actually sits.
- U.S. Securities and Exchange Commission - compound interest concept and calculator (investor.gov/financial-tools-calculators/calculators/compound-interest-calculator)
- Standard formulas: lump-sum future value FV = P(1 + r/n)^(nt) and future value of a series (annuity) FV = C[((1 + r/n)^(nt) - 1) / (r/n)] - college-level finance references
- The 7% default is illustrative, not a forecast: the US stock market (S&P 500) has historically averaged roughly 10% a year nominal and about 6.5%-7% a year after inflation over the long run (100-year S&P 500 data via Macrotrends, through 2026; McKinsey analysis of equity returns since ~1800). Past performance does not guarantee future results.