Simple Interest Calculator
Solve the simple-interest formula I = P·r·t for any unknown — interest, principal, rate, or time — and see the total amount owed or earned. Ideal for short-term loans, promissory notes, and quick math homework where interest does not compound.
How to Use
- Choose what you want to solve for: interest, principal, rate, or time.
- Enter the three values you know — principal, annual rate, and time in years.
- When solving inversely (for principal, rate, or time), fill in the interest or total amount field.
- The result and step-by-step work update live as you type.
- Read both the interest (I) and the total amount A = P + I in the result panel.
Show Work
Formula
About the Simple Interest Calculator
The Simple Interest Calculator is a simple, free helper for personal-finance and money decisions that runs entirely on your own device. Solve the simple-interest formula I = P·r·t for any unknown — interest, principal, rate, or time — and see the total amount owed or earned. Ideal for short-term loans, promissory notes, and quick math homework where interest does not compound.
How it works
Enter your figures and the result appears instantly, updating the moment you change anything. There is no submit button and nothing to wait for, so it is easy to try a few what-if numbers and compare the results. Just check each box holds the kind of value it expects.
Want the deeper story? The Knowledge Base explains the ideas behind the tools in more detail.
Frequently Asked Questions
What is the simple interest formula?
Simple interest is I = P · r · t, where P is the principal, r is the annual rate as a decimal, and t is time in years. The total you repay or receive is A = P + I, which also equals P(1 + rt). Unlike compound interest, the rate is applied only to the original principal, never to accumulated interest.
How is simple interest different from compound interest?
Simple interest is charged only on the principal, so it grows in a straight line. Compound interest is charged on the principal plus all previously accrued interest, so it grows faster over time. For the same rate and term, compound interest always costs a borrower more (or earns a saver more) than simple interest.
When is simple interest actually used?
Many short-term and consumer loans use it — auto loans, some personal loans, promissory notes between individuals, and certain Treasury and corporate instruments. It is also the standard model taught in finance and math courses. Mortgages and credit cards, by contrast, compound.
How do I enter a partial year?
Express the time in decimal years. Six months is 0.5, ninety days is roughly 0.25, and eighteen months is 1.5. The formula scales linearly with time, so half the term produces exactly half the interest at the same rate.
Can I solve for the interest rate or term?
Yes — switch the "Solve for" selector to rate or time, then enter the principal, the interest (or total amount), and the remaining known value. The tool rearranges I = P·r·t to isolate the unknown and shows the algebra in the step-by-step work.
How do I use the Simple Interest Calculator?
Simply type your numbers and read the result, which refreshes the instant you change something. There is nothing to submit and nothing to wait for.
Does it cost anything or need an account?
No. The tool is completely free, there is no account to create, and it keeps working offline after the page first loads.
Is anything I type uploaded?
No. The tool works entirely on your device, so the values you enter never leave your browser.
Common Use Cases
Pricing a short-term personal loan
Enter principal, rate, and term to see the exact interest cost before you lend money to a friend or family member.
Checking a promissory note
Verify that the interest figure on a note matches the agreed principal, rate, and time.
Solving for an unknown rate
Given how much interest was charged, work backward to find the effective annual rate.
Homework and exam practice
Confirm answers to finance and algebra problems with a clear, shown-work breakdown of each step.
Comparing simple vs. compound
Compute simple interest here, then run the same numbers through the Compound Interest calculator to see the gap.
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