Compound Interest Calculator

How to use this calculator

1. Starting amount: what you already have saved or invested. Leave it at $0 if you're starting from scratch.
2. Monthly contribution: what you'll add every month. $25 a week is about $108 a month; $100 a month is about $3.30 a day.
3. Average yearly return: the growth rate you expect, on average. We use 7% in our lessons: roughly the long-run US stock market average after inflation. For a high-yield savings account, try about 4%; for an average savings account, about 0.4%.
4. Years: how long the money stays invested.

The results update as you type: your final balance, how much of it you put in, how much is growth, a chart and a year-by-year table.

How the math works

The calculator compounds monthly: each month, the balance grows by one-twelfth of the yearly rate, and then your contribution is added at the end of the month. That's the same method we use in every Money, Drawn Out lesson, so the numbers match the videos.

For a single starting amount, the compound interest formula is:

A = P × (1 + r/12)^(12 × t)

where P is the starting amount, r is the yearly rate as a decimal (7% = 0.07) and t is the number of years. Regular monthly contributions are added on top, month by month. Lesson 7 walks through the formula step by step.

A worked example

Put $100 a month into an investment earning 7% a year from age 25 to 65 (40 years):

The first year earns only about $39 of growth. By the last year, the balance grows by more than $17,000 in a single year. That's the "slow, slow, then sudden" curve we draw out in Lesson 1.

Try these comparisons

• Start 10 years later. Change 40 years to 30. At the same $100 a month, the final balance falls from about $262,000 to about $122,000, less than half, even though you only invested $12,000 less.
• Lower the fee. A 1% yearly fee turns a 7% return into roughly 6%. Over 40 years that cuts $262,000 to about $199,000.
• Savings vs investing. Try 4% (a high-yield savings account) and 0.4% (an average savings account) to see why where you keep your money matters.

The Rule of 72

Divide 72 by your yearly return to estimate how many years it takes your money to double. At 7%, that's about 10 years; at 4%, about 18 years; at 0.4%, about 180 years. The calculator shows your doubling time under the results.

Important limits

• Returns aren't constant. Real markets go up and down, and some years are negative (the S&P 500 lost about 37% in 2008). An average return only shows up for people who stay invested through the bad years.
• Taxes and inflation aren't modeled. Using 7% gives a result in roughly today's dollars. Inside a 401(k) or IRA, growth is tax-advantaged; in a regular account, you may owe tax on gains.
• This is an illustration, not a promise or personal advice.

Frequently asked questions

How much will $100 a month be worth in 40 years?

At a 7% average annual return compounded monthly, about $262,000. You contribute $48,000, so roughly $214,000 is growth. Real returns vary year to year.

What return should I use in a compound interest calculator?

For long-term stock market investing, many people plan with about 7% a year, roughly the historical US average after inflation. For a high-yield savings account, use the current rate (around 4% in 2026). Using a conservative rate means good years become a bonus rather than a plan.

Does this calculator compound monthly or yearly?

Monthly, with contributions added at the end of each month. Compounding monthly gives a slightly higher result than compounding yearly at the same rate.

How long does it take to double my money?

Use the Rule of 72: divide 72 by your yearly return. At 7%, money doubles in about 10 years; at 4%, about 18 years.

Why does compound interest feel so slow at the start?

Because in the early years your contributions are most of the balance and growth is small. Growth builds on growth, so the curve bends upward later. In our $100-a-month example, growth passes everything you've put in around year 19.