Compound Interest Calculator
Calculate compound interest growth on savings and monthly contributions. See total interest earned over time.
Your result will appear here.
Compound Interest Calculator online
See how your savings grow with compound interest. Enter your initial principal, annual interest rate, time period, and optional monthly contribution to see the future value, total contributions, and total interest earned.
How it works
Lump sum only: A = P(1 + r)^n where P = principal, r = annual rate (decimal), n = years.
With monthly contributions (ordinary annuity): A = P(1 + r)^n + PMT × [((1 + r)^n − 1) / r] where PMT = monthly contribution × 12, r = annual rate, n = years. Contributions are assumed at end of each period. The tool computes year-by-year for the growth breakdown.
Worked example
Start with $10,000, add $500/month, 7 % annual return, 30 years: lump sum grows to $10,000 × 1.07^30 ≈ $76,123. Monthly $500 × 12 = $6,000/year; annuity factor = (1.07^30 − 1)/0.07 ≈ 94.46; contribution portion = 6,000 × 94.46 ≈ $566,760. Total ≈ $642,883. Total contributed = 10,000 + 500 × 12 × 30 = $190,000. Interest earned ≈ $452,883 — more than 2× the contributions.
Practical tips
- Start early — the last 10 years of a 30-year horizon often contribute more than the first 20 combined.
- Even a 1 % higher return compounds dramatically over decades; compare 6 % vs 7 % vs 8 %.
- Inflation is not modeled here — subtract ~2–3 % from the nominal rate for real purchasing power.
- This assumes annual compounding. Real accounts may compound monthly/daily (slightly higher effective yield).
Privacy and browser processing
Whenever practical, DailyWebKit processes input locally in your browser. Tools that need network data can depend on external services, browser permissions and CORS. Do not enter confidential information into a third-party network service.
Frequently asked questions
What is the formula for compound interest with monthly contributions?
A = P(1+r)^n + PMT × [((1+r)^n − 1) / r] where PMT = monthly × 12, r = annual rate, n = years. This assumes contributions at period end.
How much will $500/month at 7 % grow to in 30 years?
Monthly $500 = $6,000/year. Annuity factor at 7 % for 30 years ≈ 94.46. Contribution portion ≈ 6,000 × 94.46 = $566,760.
Does this account for taxes and fees?
No. This is a nominal growth model. Real returns after taxes, fees and inflation will be lower.
What if I contribute at the beginning of each month (annuity due)?
The formula above assumes end-of-period (ordinary annuity). For beginning-of-period, multiply the annuity portion by (1+r). The difference is small for long horizons.
Is this tool free?
Yes. It is free and does not require an account.
Does DailyWebKit upload my input?
No. It runs locally in your browser and your input is never uploaded.