Calculate simple interest and compound interest. See exactly how your savings or debt grows over time.
An Interest Calculator computes the interest earned or owed on a principal sum of money, supporting both simple interest and compound interest calculations. This fundamental financial tool helps with understanding loan costs, investment growth, and the significant long-term difference between simple and compound interest structures across various savings and borrowing scenarios.
Simple Interest = Principal × Rate × Time, where the interest is calculated only on the original principal for the entire duration. Compound Interest uses the formula Amount = Principal × (1 + Rate/n)^(n×Time), where n is the number of times interest compounds per year, with the key difference being that compound interest is calculated on both the original principal and any previously accumulated interest.
Enter the principal amount, interest rate, time period, and select whether you want simple or compound interest (and if compound, the compounding frequency). The calculator returns the total interest earned or owed, along with the final amount including the original principal.
Example 1: A principal of 1,00,000 at 8% simple interest for 5 years earns 1,00,000 × 0.08 × 5 = 40,000 in interest, for a total amount of 1,40,000.
Example 2: The same 1,00,000 principal at 8% compound interest (compounded annually) for 5 years grows to 1,00,000 × (1.08)^5 ≈ 1,46,933, earning approximately 46,933 in interest, noticeably more than the simple interest scenario due to interest earning interest on itself.
Compound interest calculates new interest on the growing total (principal plus previously earned interest), while simple interest only ever calculates on the fixed original principal, meaning compound interest accelerates growth over time as the interest-earning base keeps expanding.
More frequent compounding (daily or monthly versus annually) results in slightly higher total interest for the same nominal rate, since interest starts earning its own interest sooner, though the practical difference between common compounding frequencies is usually relatively modest compared with the effect of the rate itself.
Most modern loans, credit cards, and savings accounts use compound interest, since it's the standard in contemporary banking, while simple interest is more commonly seen in specific short-term loans or certain bonds where the calculation is explicitly structured that way.
Since compound growth builds on itself exponentially while simple interest grows linearly, the gap between the two widens significantly the longer the time period extends, which is why compound interest's advantage becomes especially pronounced over decades rather than just a few years.
Knowing exactly how compound interest accrues on credit card balances or loans helps illustrate why unpaid balances can grow so quickly, motivating faster repayment, while also helping identify which debts carry the highest effective cost when prioritizing which to pay off first.
The Rule of 72 is a quick mental shortcut for estimating how many years it takes an investment to double at a given compound interest rate, by dividing 72 by the interest rate percentage, providing a fast approximation without needing the full compound interest formula.
APR (Annual Percentage Rate) typically represents a simple annualized rate without accounting for compounding effects, while APY (Annual Percentage Yield) reflects the actual total return including compounding, which is why APY is usually slightly higher than APR for the same nominal interest rate.
Yes, converting different offers into a standardized comparison, such as calculating total interest paid or earned over the same time period and principal amount, allows for an apples-to-apples comparison even when the offers have different stated rates or compounding structures.
Different financial markets and instruments use varying conventions for how many days are assumed in a year (such as 360 versus 365) for calculating daily interest, which can lead to small but real differences in total interest across seemingly identical rate agreements.
In rare economic conditions, central banks have implemented negative interest rates, meaning depositors effectively pay to hold money rather than earn interest, an unusual policy tool aimed at encouraging spending and lending rather than saving.
Daily interest accrual, common in credit cards and some lines of credit, means the balance is recalculated every single day rather than once a month, which can result in slightly higher effective interest compared with simple monthly compounding at the same nominal rate.