How to Calculate Compound Interest
Learn the compound interest formula, understand how compounding works, and calculate how an investment can grow over time.
Compound interest is interest earned on the original amount of money as well as the interest that has already accumulated. Because previously earned interest can become part of the balance, the amount earning interest can increase over time.
Compound interest is commonly used when estimating the future value of savings and investments. The result depends on the starting principal, interest rate, investment period and how frequently the interest is compounded.
Quick answer
To calculate compound interest, use the compound amount formula and subtract the original principal from the final amount.
A = P × (1 + r ÷ n)nt
Compound Interest = A − P
What Is Compound Interest?
With simple interest, interest is calculated only on the original principal. With compound interest, accumulated interest is added to the balance used for future interest calculations.
For example, if money earns interest annually, the first year's interest is added to the account balance. In the following year, interest can be calculated on that higher balance. Repeating this process allows the balance to grow through compounding.
Compound Interest Formula
A = P × (1 + r ÷ n)nt
Compound Interest = A − P
The variables in the formula are:
How to Calculate Compound Interest Step by Step
Identify the principal
Find the original amount invested or deposited.
Convert the interest rate
Convert the annual percentage rate into a decimal before using the formula.
Determine the compounding frequency
Use the number of compounding periods per year, such as 1 for annual or 12 for monthly compounding.
Enter the investment period
Use the number of years represented by the investment period.
Calculate the final amount
Apply the formula to find the compounded amount, then subtract the principal to find compound interest earned.
Compound Interest Example
Suppose you invest ₹10,000 at an annual interest rate of 8%, compounded annually, for 3 years.
Calculation
A = 10,000 × (1 + 0.08 ÷ 1)1 × 3
A ≈ ₹12,597.12
Compound Interest ≈ ₹12,597.12 − ₹10,000 = ₹2,597.12
This example assumes a fixed annual rate and annual compounding. Actual investment returns can differ when rates, fees, taxes, deposits or withdrawals vary.
How Compounding Frequency Affects Growth
The compounding frequency determines how often accumulated interest is added to the balance. Depending on the product, interest may be compounded annually, semi-annually, quarterly, monthly or at another frequency.
When the nominal rate and other assumptions remain the same, more frequent compounding generally produces a higher final amount because interest is incorporated into the balance more often.
Common compounding frequencies
Compound Interest vs Simple Interest
The key difference is how the interest base changes. Simple interest uses the original principal for the interest calculation, while compound interest allows accumulated interest to become part of the balance used for future calculations.
Simple Interest
Interest is calculated on the original principal.
Compound Interest
Interest can be calculated on the principal plus previously accumulated interest.
Common Mistakes When Calculating Compound Interest
- Using 8 instead of 0.08 for an 8% annual rate when the formula expects a decimal.
- Using the wrong number of compounding periods per year.
- Mixing months and years without converting the time period consistently.
- Confusing the final amount with the interest earned.
- Assuming an advertised rate guarantees the same investment return when the underlying rate or conditions can change.
How to Use a Compound Interest Calculator
A compound interest calculator can perform the formula automatically and make it easier to compare different principal amounts, interest rates and time periods. Enter the required values, select the relevant compounding frequency and review the calculated amount and interest.
Calculate your estimate
Use the Numorify Compound Interest Calculator to estimate the future value and compound interest from your inputs.
Open Compound Interest Calculator