Compound Interest Calculator
See how your money grows — or find out how much to save for a goal.
How to Use This Calculator
Investment Growth tab
The default tab. Enter your initial deposit and monthly contribution — the calculator shows your total balance after compound interest. Expand "More options" to adjust the interest rate, compounding frequency, and time period. The default rate reflects a long-term stock market average. For savings accounts, lower it to 3–5%.
Savings Goal tab
Start with your target amount and years to reach it. The calculator reverse-solves: it tells you how much to save each month. If you already have savings, enter them under "More options" as starting balance — this reduces your required monthly contribution.
Compare tab
Enter two scenarios side by side: Option A and Option B. Each row takes an initial deposit, interest rate, and time period. The result shows which option earns more and by how much. Use this to compare a conservative savings account vs. an index fund, or a 10-year vs. 20-year horizon.
Share your result
Every input is encoded in the URL. Click Share, send the link to your partner — they'll open it and see your exact numbers. No re-entering, no screenshots.
The Formula
Compound interest is calculated using this formula:
Where:
- A = future value (what you end up with)
- P = initial deposit (principal)
- r = annual interest rate (as a decimal)
- n = compounding frequency per year (12 for monthly, 4 for quarterly, 1 for annually)
- t = time in years
- PMT = regular monthly contribution
The first part, P(1 + r/n)^(nt), calculates the growth of your initial deposit. The second part calculates the accumulated value of your regular contributions. When you move the rate slider in the calculator, you change r — and you can see how even a 1% difference compounds over decades.
The Savings Goal tab uses the same formula but solves for PMT — the monthly amount you need to reach your target.
Example
Alex — saving for a down payment
Alex has $5,000 in savings and wants $40,000 for a house down payment in 5 years. A high-yield savings account offers 4.5% APY, compounded daily.
Savings Goal tab
Result: Alex needs to save $527/month. Of the $40,000 total, $31,620 comes from deposits and $3,380 from interest — money earned while sleeping.
Compare tab
Alex wonders: what if they invest in an index fund at 7% instead of a savings account at 4.5%?
The index fund earns more — but comes with risk. The savings account is FDIC-insured. For a 5-year goal with a specific deadline, Alex chose the guaranteed option and opened a high-yield savings account.
Alex shared the link with their partner. Same numbers, same page, no explaining needed.