Calculate interest, ending balance, principal, rate, or time using the simple interest formula. The result updates instantly and shows the formula step used.
Formula focusI = P x r x t
Simple interest uses the original principal only. It does not compound interest on prior interest.
Mode
Choose What You Want to Calculate
Simple Interest Inputs
End balance$0
Results update automatically as you edit the inputs.
Total interest$0
Principal$0
Rate used0%
Time used0 years
Principal vs. Interest
Principal$0
Interest$0
Formula Used
I = P x r x t
Simple Interest Schedule
Simple interest grows in a straight line because each period uses the original principal.
Principal Interest
Period
Interest this period
Total interest
End balance
Method
Methodology: how this simple interest calculator works
The calculator uses the simple interest formula: interest equals principal multiplied by rate multiplied by time. If the rate is annual and the time is in months, the calculator converts months to years. If the rate is monthly and the time is in years, it converts years to months.
Unlike compound interest, simple interest does not add prior interest back into the principal. That is why the schedule rises by the same interest amount each equal period.
Data and source notes: No live bank, lender, or market data is used. The result is pure arithmetic based on the principal, rate, and time entered by the user. Real contracts may use daily accrual, amortization, compounding, fees, or penalty rules that are outside this simple formula.
Practical use
When Simple Interest Is Useful
Short-term loan estimate
Estimate interest on a note where interest is based only on the original principal.
Classroom formula check
Quickly solve for principal, rate, time, interest, or ending balance while showing the formula.
Simple comparison
Compare a simple-interest result against a compound-interest result to see the difference.
Interest-only estimate
Calculate how much interest a fixed principal generates over a stated time period.
Simple Interest Formula
I = P x r x t
I is total interest, P is principal, r is the interest rate per period, and t is the number of periods. End balance equals principal plus interest.
To solve for principal: P = I / (r x t). To solve for rate: r = I / (P x t). To solve for time: t = I / (P x r).
What is the difference between simple interest and compound interest?
Simple interest is calculated only on the original principal. Compound interest is calculated on principal plus previously accumulated interest.
Can this calculator solve for rate or time?
Yes. Choose Rate or Time mode and enter the other known values. The calculator uses the rearranged simple interest formula.
Why does the schedule grow in a straight line?
Each period uses the same original principal. Because the interest base does not change, equal periods produce equal interest.
Can I use monthly rates?
Yes. Select Per month for the rate period. The calculator will match the time unit so the formula uses the correct number of periods.
Is this how every loan calculates interest?
No. Many real loans use amortization, daily interest accrual, fees, compounding, or other contract terms. Use this page for simple-interest estimates and formula checks.
How do I calculate simple interest on a loan?
Multiply the principal by the interest rate per period and the number of periods. For an annual rate over years, the formula is interest = principal x annual rate x years.
Does simple interest include fees or penalties?
No. The formula only calculates interest from principal, rate, and time. Loan fees, late fees, origination charges, and prepayment rules must be checked separately.
When should I use a compound interest calculator instead?
Use a compound interest calculator when interest is added back to the balance, such as many savings, investment, or compounding growth scenarios.
Disclaimer
This calculator is for general informational purposes and simple arithmetic checks. It is not a loan quote, bank offer, legal advice, tax advice, or financial advice. Verify all contract terms before making borrowing, lending, or savings decisions.