Simple Interest Calculator - Free & Instant

Calculate simple interest with the I = P × r × t formula. See total repayment and how it compares to compound interest.

Currency

Principal Amount

Annual Interest Rate

Time Period

Results

Simple Interest Earned

$1,500.00

Principal

$10,000.00

Total Amount

$11,500.00

Comparison vs compound interest

With annual compounding over the same period you would earn $1,576.25 in interest, which is $76.25 more than simple interest.

Estimates for information only, not financial advice.

Simple Interest Formula

I = P × r × t

I = Interest earned

P = Principal = $10,000.00

r = Annual rate = 5%

t = Time in years = 3.00

Simple Interest Calculator Online

Calculate simple interest instantly with the classic I = P times r times t formula. Enter your principal, annual interest rate, and time period in years or months, and the calculator shows the interest earned and the total amount at the end of the term. Everything runs in your browser, so your figures never leave your device and results update as you type.

Simple interest is used for many short-term loans, car loans, certificates of deposit, treasury bills, and informal lending between individuals. Students learning finance basics, borrowers checking a loan quote, and savers comparing fixed-income products all rely on this calculation because it is transparent and easy to verify by hand.

Unlike compound interest, simple interest is charged only on the original principal, never on accumulated interest. A 10,000 loan at 5% for 3 years costs exactly 1,500 in simple interest, while the same loan with annual compounding costs about 1,576. The gap widens dramatically with longer terms and higher rates, which is why the calculator shows a side-by-side comparison line.

A practical rule: for terms under one year the difference between simple and compound interest is usually small, often under 0.2% of the principal at typical rates. Over 10 years at 7%, however, compound interest roughly doubles the earnings compared with simple interest, so always check which method a contract uses before signing.

If your savings or investment compounds, use our Compound Interest Calculator to model realistic growth, and try the Investment Calculator to include monthly contributions. Both tools share the same free, browser-based approach as this one.

How the Simple Interest Calculator Works

01Choose your currency and enter the principal amount you are lending, borrowing, or depositing.
02Enter the annual interest rate as a percentage.
03Set the time period and toggle between years and months to match your contract.
04Read the interest earned, the total amount, and the comparison line showing what annual compounding would have produced.

Using Simple Interest Correctly

Simple interest applies the rate to the original principal only, every period, without compounding. That makes it the standard for many short-term products: car loans, personal loans quoted with flat rates, certificates of deposit that pay out interest instead of reinvesting it, treasury bills, and informal loans between friends or family. To use the calculator well, first confirm which method your contract actually uses, because a flat 5% simple rate and a 5% compounding rate are different prices for the same money. Enter the principal, the annual rate, and the term. For terms shorter than a year, switch the toggle to months so you do not have to convert 9 months into 0.75 years by hand. The comparison line matters most for longer terms: over 1-2 years the difference from compounding is small, but over 10 years it becomes substantial. If a lender quotes a flat rate on a loan you repay monthly, be aware the effective annual rate is roughly double the flat rate, because your average outstanding balance is about half the original principal.

When to Use the Simple Interest Calculator

Use this tool when checking a loan or deposit that explicitly uses simple interest: car loans, short-term personal loans, CDs with paid-out interest, bonds trading on accrued interest, or a loan to a friend. It is also the right starting point for finance students verifying homework, since I = P times r times t is the foundation formula. If your product reinvests interest, switch to a compound calculator instead, because simple interest will understate the outcome.

Common Use Cases

Checking the total cost of a car loan or short-term personal loan quoted with a simple or flat interest rate.
Calculating the payout of a certificate of deposit that pays interest out instead of reinvesting it.
Agreeing fair terms for a private loan between family members or friends, with a number both sides can verify.
Verifying finance homework and textbook exercises built on the I = P r t formula. → Percentage Calculator
Comparing a simple-interest offer against a compounding savings account before choosing where to put money. → Compound Interest Calculator

Expert Tips

Always confirm whether a quoted rate is simple or compound before comparing offers; the same percentage can mean very different costs.
For flat-rate loans repaid in monthly installments, the effective annual rate is roughly double the flat rate, because you do not keep the full principal for the whole term.
Use the months toggle for short terms instead of converting to decimal years by hand; it avoids the most common source of calculation errors.

Frequently Asked Questions

What is the formula for simple interest?→

I = P times r times t, where P is the principal, r is the annual interest rate as a decimal, and t is the time in years. The total amount at the end is the principal plus that interest. For example, 10,000 at 5% for 3 years earns exactly 1,500.

What is the difference between simple and compound interest?→

Simple interest is charged only on the original principal for the whole term. Compound interest is charged on the principal plus previously accumulated interest, so it grows faster. The calculator shows both so you can see the gap for your exact numbers.

How do I calculate simple interest for months instead of years?→

Switch the time toggle to months. The calculator divides the number of months by 12 and applies the annual rate to that fraction of a year, so 6 months at 8% on 5,000 gives 200 in interest.

Is my data uploaded anywhere?→

No. All calculations run in your browser using JavaScript. Nothing you type is sent to a server, which makes the tool safe for private financial figures.

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