Compound Interest Calculator

Compound interest is the mechanism by which interest starts earning interest, and this calculator makes that effect visible in concrete numbers. You enter a starting amount, an optional monthly contribution, an annual interest rate, the investment period, and how often interest is compounded — yearly, quarterly, monthly, or daily — and the tool returns the final balance, the total of everything you put in, the interest earned, and a year-by-year breakdown showing the balance grow. Savers comparing accounts, investors testing scenarios, and anyone curious whether starting early really matters will get an immediate answer. No login or signup stands between you and the result, and every input can be changed freely to test another scenario.

What this tool can do

• 💰 An initial principal plus an optional regular monthly contribution.
• 🔄 Four compounding frequencies: yearly, quarterly, monthly, or daily.
• 📈 A year-by-year table: balance, cumulative contributions, and interest earned in each year.
• 🧮 Headline figures for final balance, total contributions, and total interest earned.
• 📊 A growth chart that draws the compounding curve.
• 🧾 Interest applied per compounding period rather than once per year.

When would you use it

• 🏦 Comparing a daily-compounding savings account with a yearly-compounding one.
• 🎓 Estimating how a monthly deposit into an index fund could grow by retirement age.
• 👶 Quantifying the difference between starting to save at 25 and starting at 35.
• 💸 Projecting what a one-time windfall might become over a fixed horizon.

All calculations happen in your browser; no financial figures leave your device and nothing is stored. The honest caveat is that a constant annual rate is assumed: real returns move up and down, and taxes, inflation, and fees are not modeled — treat the output as a projection of the math, not a forecast of reality.

Principal P, annual rate r, compounded n times a year for t years:

FV = P · (1 + r/n)^(n·t)

• Monthly compounding beats yearly — but past roughly quarterly the extra gain flattens fast.
• Rule of 72: doubling time ≈ 72 divided by the rate in percent — six percent doubles in about twelve years.
• Regular deposits turn this into a series: each contribution compounds from its own start date.