📈 Compound Interest Calculator

Compound interest is interest calculated on both your original principal and the interest you've already earned. Unlike simple interest — which only ever applies to your starting amount — compound interest means that every period, your interest earns interest of its own. This self-reinforcing cycle is what makes it so powerful over long time horizons.

Albert Einstein is often (likely apocryphally) credited with calling compound interest the eighth wonder of the world. Whether or not he said it, the math backs up the sentiment. A $10,000 investment at 8% simple interest for 30 years returns $34,000. The same investment with monthly compounding returns $109,357 — more than three times as much, with no additional contributions.

This compound interest calculator lets you enter any principal, rate, time period, and compounding frequency, then shows you the exact final balance and a year-by-year breakdown of how your money grows.

The standard compound interest formula is:

A = P × (1 + r/n)^n×t

• A — the final amount (principal + interest)
• P — the principal (your starting amount)
• r — the annual interest rate as a decimal (8% = 0.08)
• n — the number of times interest compounds per year (monthly = 12)
• t — the time period in years

Worked example: You invest $10,000 at an 8% annual rate, compounded monthly, for 10 years.

A = 10,000 × (1 + 0.08/12)^12×10 = 10,000 × (1.00667)¹²⁰ = $22,196

Your $10,000 nearly doubles in 10 years without a single additional deposit. The $12,196 in interest earned is 22% more than the $10,000 you started with — all of it generated by compounding, not contributions.

The more often interest compounds, the more you earn — because each compounding event increases the base on which the next calculation is made. The difference between annual and daily compounding is real, though it shrinks at lower interest rates.

Using the same $10,000 at 8% for 10 years:

• Annually (1×/year): $21,589
• Semi-annually (2×/year): $21,911
• Quarterly (4×/year): $22,080
• Monthly (12×/year): $22,196
• Daily (365×/year): $22,253

The gap between annual and daily compounding is about $664 on a $10,000 investment over 10 years. Over 30 years the same comparison grows to roughly $8,000. Compounding frequency matters more the longer your time horizon and the higher the rate.

Most savings accounts and bank products compound daily. Most investment accounts and index funds effectively compound annually (dividends reinvested once a year) or continuously across trading days.

The Rule of 72 is a quick mental shortcut for estimating how long it takes money to double at a given compound interest rate. Divide 72 by the annual interest rate and the result is roughly the number of years to double your money.

• At 4%: 72 ÷ 4 = 18 years to double
• At 6%: 72 ÷ 6 = 12 years to double
• At 8%: 72 ÷ 8 = 9 years to double
• At 10%: 72 ÷ 10 = 7.2 years to double
• At 12%: 72 ÷ 12 = 6 years to double

The rule is an approximation — the compound interest calculator gives you the exact figure — but it is useful for quickly evaluating whether an investment opportunity makes sense before running the full numbers.

What's the difference between compound interest and simple interest?

Simple interest is calculated only on the original principal: if you invest $1,000 at 10% simple interest for 5 years, you earn $100 per year for a total of $500 in interest. Compound interest is calculated on the growing balance — your $1,000 earns $100 in year one, but in year two it earns 10% of $1,100 ($110), and so on. Over 5 years at 10% compounded annually, the same $1,000 grows to $1,611, earning $111 more than simple interest would produce. The gap widens dramatically over longer periods.

What annual rate of return should I use?

The right rate depends on what you're modelling. The S&P 500 has returned approximately 10% per year on average before inflation over the long run, which makes 7–8% a common assumption after inflation. High-yield savings accounts currently pay 4–5%. Bonds have historically returned 3–5%. For any long-term projection, running three scenarios — a conservative 5%, a moderate 7–8%, and an optimistic 10% — gives a more useful range than a single number. The calculator lets you adjust the rate instantly to compare outcomes.

Does this calculator include regular contributions?

This compound interest calculator models a single lump-sum investment without ongoing contributions. If you want to include regular monthly deposits, use the SIP / Dollar Cost Averaging calculator, which uses the standard future value of an annuity formula to handle recurring contributions alongside an optional initial lump sum.

How long does it take to double my money?

Use the Rule of 72: divide 72 by your annual interest rate. At 8%, money doubles in approximately 9 years (72 ÷ 8 = 9). At 6%, it takes about 12 years. The compound interest calculator shows the exact balance at every year in the breakdown table, so you can see the precise doubling point for any combination of inputs.

Is compound interest always beneficial?

Compound interest works in your favour when you are the investor — it accelerates the growth of your savings and investments. It works against you when you are the borrower. Credit card debt, for instance, typically compounds daily at rates of 20–30%, which is why balances can grow so rapidly when only minimum payments are made. The same mathematical principle that builds wealth in an investment account can erode it in a debt account. The loan EMI calculator shows the total interest cost of a loan under its own compounding terms.