Compound Interest Calculator
What your savings grow to, with the year-by-year table.
What you put in on day one. Leave blank to start from zero.
Added at the end of every month, after that month's interest. Leave blank for none.
The yield before tax and fees. Pick below whether it is per year or per month.
Per year (effective): 12% a year compounds to exactly 12% in twelve months — a monthly 0.9489%. Compounded monthly (APR): 12% becomes 1% a month, which compounds to 12.68% a year. US savings rates are usually APY (effective); loan rates are APR.
How long the money stays invested.
Fractions of a year are rounded to whole months.
Fill in the fields and the answer appears here.
How this was worked out
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Guide: How compound interest grows3 min read
How compound interest grows
Simple interest is paid on what you put in. Compound interest is paid on the whole balance, including the interest already earned, so every month the base is a little bigger than the last. In the first year the difference is small. Over decades it takes over: 10,000 at 12% a year with 500 added every month becomes 142,023.50 in ten years — 70,000 of deposits and 72,023.50 of interest.
The formula is balance = initial × (1 + i)n + deposit × ((1 + i)n − 1) ÷ i, where i is the monthly rate and n the number of months. The page works it month by month and shows the monthly rate it used beside the answer, so the figure can be checked rather than trusted.
APY, APR and a monthly rate
The most common mistake is dividing a yearly rate by twelve. 1% a month compounds to 12.68% a year, not 12%. A savings account that advertises 12% APY is paying 0.9489% a month — the rate that compounds to exactly 12% over twelve months. A loan quoted at 12% APR charges 1% a month, and costs more than 12% over the year.
That is why the rate has a setting. Per year (effective) reads it as an APY; compounded monthly reads it as an APR; per month takes it as it is.
What the balance leaves out
Tax, fees and inflation. Interest in an ordinary account is usually taxable; retirement accounts may defer or remove that. Fund fees come off the return every year. And inflation shrinks what the final number buys: to see growth in today's money, subtract inflation from the rate — 7% with 3% inflation is roughly 4% of real growth.
The table
Up to two years the table runs month by month; beyond that, year by year, with the total put in, the interest so far and the balance. Save it as a CSV to open in a spreadsheet. Nothing you type leaves this tab, and the page works with the connection off.
Frequently asked questions
What is compound interest?
Interest earned on interest. Each month the rate is applied to the whole balance — what you put in plus the interest already earned — so the growth speeds up the longer the money stays. 10,000 at 12% a year with 500 a month becomes about 142,000 in ten years, and more than half of that is interest.
Is 12% a year the same as 1% a month?
No. 1% a month compounds to 12.68% a year. A true 12% a year is 0.9489% a month. Savings accounts usually quote the effective yearly figure (APY); loans quote APR, which is the monthly rate times twelve. The rate setting on the page decides which one you mean, and the working shows the monthly rate it used.
When are the monthly deposits added?
At the end of each month, after that month's interest — the usual convention for a regular transfer. A deposit at the start of the month would earn one month more of interest each time, so the balance here is the slightly conservative one.
Does it include tax and inflation?
No. The balance is before tax, fees and inflation, because all three depend on where you live and what you hold. Subtract inflation from the rate to see the growth in today's money: 7% with 3% inflation is roughly 4% of real growth.
Is anything sent anywhere?
No. Everything is computed in this tab, the numbers are never sent, and the page works offline. The table can be saved as a CSV.