How to calculate simple interest
Start with the three values you know. Choose the missing value under Solve for, fill in Principal ($), Maturity value ($), Annual interest rate (%), or Term as needed, and select Calculate. The result includes the formula, the values used, and an interest schedule.
- Choose Maturity value, Principal, Rate, or Time under Solve for.
- Enter the other three values in Principal ($), Maturity value ($), Annual interest rate (%), and Term.
- Choose Years, Months, or Days under Term unit.
- Select Calculate to show the result, formula, assumptions, and interest schedule.
How the Simple Interest Calculator works
The calculator uses I = P x r x t. P is the principal, r is the annual rate as a decimal, and t is the term in years. Maturity value adds that interest to the original principal with A = P + I. Each year uses the original principal again, so earlier interest is not added to the next year's calculation.
Principal: $10,000
Annual interest rate: 6%
Term: 5 Years
Simple interest: $3,000.00
Maturity value: $13,000.00What Solve for changes
Solve for marks the field the calculator will fill. Choose Maturity value for the ending amount, Principal for the starting amount, Rate for the annual rate, or Time for the term. Entered Principal ($) and Maturity value ($) values must be greater than 0. An entered Annual interest rate (%) must be between 0 and 100, and Term must be greater than 0.
To solve for Rate or Time, Maturity value must be greater than Principal. Time also needs a rate greater than 0. A calculated Rate can be above 100%, because the calculator applies that limit to a rate you enter, not to the result it calculates.
How term units affect the result
Years stay as entered. Months are divided by 12, and Days are divided by 365 before the formula runs. The result shows the converted term in years, along with simple interest and Maturity value.
The calculator builds the interest schedule after a successful calculation. It rounds the converted term to a whole number, shows at least one row, and limits the table to 600 rows. When the rounded term exceeds 600 years, the table says that it is showing the first 600.
Check a calculation and save the result
For example, enter $10,000 as Principal ($), 6% as Annual interest rate (%), and 5 under Term with Years selected. The calculator returns $3,000.00 in simple interest and a $13,000.00 Maturity value.
To work backward, choose Rate or Time under Solve for and enter the other three values. After the result appears, select Copy result for a plain-text summary or Download result (.txt) to save it.
When another financial calculator fits better
Choose Compound Interest Calculator when interest compounds and you need simple interest, compound interest, APY, or APR in one calculation. Choose Amortization Calculator for a loan schedule that lists principal, interest, and balance by period. Choose APR Calculator when upfront fees, payment frequency, and payment timing are part of the rate calculation. Financial Calculators links to the other loan, pay, debt, investing, housing, and savings calculators.
Frequently asked questions
How do I calculate simple interest with this calculator?
Choose Maturity value under Solve for, enter Principal ($), Annual interest rate (%), and Term, then select Calculate. The result uses I = P x r x t and shows Maturity value as Principal ($) plus simple interest.
Can I use this as a maturity value calculator?
Yes. Leave Maturity value selected under Solve for, enter Principal ($), Annual interest rate (%), and Term, and select Calculate. The calculator returns the ending amount and total simple interest.
How does the simple interest formula handle months and days?
Months are divided by 12 and Days by 365 to convert Term to years before applying the formula. The interest schedule then rounds that converted term to a whole number of rows.
Why does the calculator reject my simple interest inputs?
Principal ($) and Maturity value ($) must be greater than 0, Term must be greater than 0, and an entered Annual interest rate (%) must be from 0 to 100. Solving for Rate or Time also requires Maturity value to be greater than Principal, while solving for Time requires a rate above 0.