Understanding Simple Interest
Simple interest is calculated only on the original amount of money, known as the principal. Unlike compound interest, previously earned or charged interest is not added to the balance when future interest is calculated.
This makes simple interest useful for estimating the basic cost of certain loans or the potential earnings from investments that do not compound.
How to Use the Calculator
Enter the principal amount and annual interest rate. Then enter the time period using years, months, days, or any combination of the three.
The optional start date is used only to estimate the ending date. It does not affect the amount of interest calculated.
The results include:
Total simple interest
Final amount after adding the interest
Entered time period
Time converted to years
Interest per year, month, and day
Estimated ending date
Simple Interest Formula
The calculator uses the standard simple interest formula:
Simple Interest = Principal × Annual Interest Rate × Time
The final amount is calculated as:
Final Amount = Principal + Simple Interest
For example, $5,000 at an annual simple interest rate of 6% for two years produces $600 in interest. The final amount would be $5,600.
How Months and Days Are Handled
Months are treated as 1/12 of a year, and days are calculated using a 365-day year. You can combine time units, such as 1 year, 6 months, and 15 days.
If you enter a start date, the calculator adds the entered calendar years, months, and days to estimate an ending date. When necessary, dates at the end of a month are adjusted to the last valid day of the resulting month.
Simple Interest Versus Compound Interest
With simple interest, interest is always based on the original principal. With compound interest, accumulated interest is periodically added to the balance, and future interest is calculated on the new total.
Because of this, compound interest generally grows faster than simple interest when the principal, rate, and time are the same.
Common Uses for Simple Interest
Simple interest may be used for certain personal loans, short-term loans, promissory notes, financing agreements, and basic investment estimates. It can also help compare different principal amounts, interest rates, and time periods.
Not every lender or financial institution uses the same calculation method. Some use daily balances, a 360-day year, compounding, fees, or changing interest rates. Use these results as an estimate and review the terms of the actual financial agreement.