About Compound Interest Calculator

Future value with configurable compounding + monthly contributions. Part of the DevTools Surf developer suite. Browse more tools in the Calculators collection.

Use Cases

• Project retirement savings with monthly contributions
• Compare compounding frequencies (monthly vs annual)
• Plan long-term investment growth against a target
• Visualize the cost of delaying 401k contributions by a year

Tips

• n is the number of compounding periods per year (12 = monthly, 365 = daily)
• Monthly contributions are added at the start of each month
• The earnings line separates principal + contributions from pure growth

Fun Facts

• The 'Rule of 72' is a shortcut: divide 72 by your annual rate to get doubling time. At 8% a year, money doubles every 9 years.
• Albert Einstein allegedly called compound interest 'the eighth wonder of the world' — though there's no verified source for this quote, it appeared in print as early as 1916.
• Continuous compounding (the mathematical limit as compounding periods approach infinity) uses the formula Pe^(rt), where e is Euler's number ~2.71828.

FAQ

What's 'n' (compounding periods)?

How many times per year interest is calculated: 1 (annual), 12 (monthly), 365 (daily), 4 (quarterly). More frequent = slightly higher effective rate.

Why do monthly contributions grow so much?

Each contribution starts earning interest immediately. The earliest contributions compound for the entire period. A year-early contribution at 7% becomes ~2× after 10 years.

What's APR vs APY?

APR: annual rate ignoring compounding. APY: effective annual yield including compounding. For comparison, always use APY.

Can I model withdrawals?

Negative monthly contributions simulate drawdowns. Useful for retirement-withdrawal planning. Break-even analysis: when does the portfolio run out?