Compound Interest
Calculator

The standard formula is A = P(1 + r/n)^(nt) where A = final amount, P = principal, r = annual interest rate as a decimal, n = number of times interest compounds per year, t = years. For continuous compounding: A = Pe^(rt). Example: $10,000 at 8% compounded monthly for 10 years → A = $10,000 × (1 + 0.08/12)^(12×10) = $22,196.

Simple interest: calculated only on the original principal — SI = P × r × t. For $10,000 at 8% for 10 years = $8,000 interest, total $18,000. Compound interest: interest is added to the principal each period, and future interest is calculated on the new total. Same inputs with monthly compounding = $12,196 interest, total $22,196 — $4,196 more. Over 30 years, the gap grows to over $75,000. This is why compounding is described as exponential growth — it accelerates over time.

The Rule of 72 is a mental shortcut: divide 72 by your annual interest rate to estimate how many years it takes to double your money with compound interest. At 8% → 72 ÷ 8 = 9 years. At 6% → 12 years. At 12% → 6 years. At 4% → 18 years. It works best for rates between 4% and 15% and uses the mathematical fact that ln(2) ≈ 0.693, and 72/100 ≈ 0.72 approximates this for whole-number rates.

Use the Monthly Contribution field in the calculator. Enter your starting amount (e.g., $5,000), set a monthly SIP amount (e.g., $300), and choose your rate and period. The tool uses: FV_contributions = PMT × [(1 + r)^n − 1] / r × (1 + r) added to the compounded principal. Example: $5,000 starting + $300/month at 9% for 15 years = $117,482 total, of which $60,000 is invested and $57,482 is pure compound growth.

APR (Annual Percentage Rate) is the stated interest rate without accounting for within-year compounding. APY (Annual Percentage Yield) is the true effective annual return: APY = (1 + r/n)^n − 1. A savings account offering 8% APR compounded monthly actually delivers 8.30% APY. Always compare savings accounts and investments using APY — not APR — to get an accurate like-for-like comparison. The calculator displays APY automatically.

More frequent compounding is always better for savers. Daily gives slightly more than monthly, which beats quarterly, which beats annual. However, the difference is smaller than most people expect. On $10,000 at 8% for 10 years: daily gives $22,253 vs monthly at $22,196 — just $57 more over 10 years. The choice of rate and time period matters far more than compounding frequency. Most bank accounts compound monthly or quarterly — both are excellent.

₹1,00,000 at 7% compound interest for 10 years: with annual compounding → ₹1,96,715. With quarterly compounding (typical for Indian FDs) → ₹2,00,160. With monthly compounding → ₹2,00,967. This models a standard Indian Fixed Deposit. Switch currency to ₹ INR in the calculator and enter these values to see the full year-by-year breakdown and confirm the exact figure with your bank's rate.

Yes, 100% free — no signup, no subscription, no ads. All calculations run entirely in your browser — no data is ever sent to any server, so your financial figures stay completely private. The calculator supports USD, INR, GBP and EUR, and works on all devices including mobile phones, tablets, and desktop computers.