Simple Interest Calculator

What Is Simple Interest?

Simple interest is the most straightforward way to calculate the cost of borrowing money or the return on a savings deposit. Unlike compound interest, which charges interest on top of interest, simple interest only ever applies to the original principal. That means if you deposit $1,000 for five years at 6% simple interest, you earn exactly $60 per year every single year — not $60 in year one, $63.60 in year two, and so on. The growth is perfectly linear.

Knowing how to calculate simple interest is a foundational financial skill. Car loans in the United States almost always use a daily simple interest method. Short-term personal loans, Treasury bills, many savings bonds, and some business lines of credit all use simple interest. Understanding the formula means you can verify a lender's numbers in seconds, compare loan offers quickly, and know exactly how much your savings will earn at any given rate.

The Simple Interest Formula Explained

The formula has three variables and one output:

Breaking down each component:

Principal (P) is the starting amount — the loan amount you borrowed or the deposit you made. It does not change over the life of a simple interest agreement. Even if some interest has built up, the next period's interest is always calculated on this same original figure.

Rate (r) is the annual interest rate converted to decimal form. To convert a percentage to a decimal, divide by 100: 5% becomes 0.05, 12% becomes 0.12, 0.5% becomes 0.005. If a lender quotes you a monthly rate (common for some personal loans), multiply it by 12 to get the annual rate before using this formula.

Time (t) must be in years when using the standard formula. For six months, use 0.5. For 90 days, use 90 ÷ 365 ≈ 0.2466. Some commercial lenders use a 360-day year (called the "banker's rule" or "ordinary interest") — if that applies to your loan, divide by 360 instead of 365.

To get the total amount you will have at the end (or owe at the end), simply add the principal back:

This is called the "maturity value" or "future value" in finance. It represents the final amount that changes hands when the loan or investment period ends.

How to Use This Calculator

Using this tool takes under ten seconds:

1. Enter the principal. This is the starting amount — the amount borrowed or deposited, in dollars. Do not include any fees or existing interest; use only the raw principal figure.
2. Enter the annual interest rate as a percentage (enter 5, not 0.05). If your rate is quoted monthly, multiply by 12 before entering.
3. Enter the time period and choose the unit from the dropdown — years, months or days. The calculator converts automatically. For 18 months, enter 18 and select "Months."
4. Read the results. The blue card shows the total interest amount. The grey cards below show the total amount (principal + interest) and the effective total-period rate. Toggle "Compare with compound interest" to see how the two methods diverge for your specific inputs.

Results update instantly as you type — no button to click. If any field is empty or zero, the calculator waits for valid inputs before showing results.

Worked Examples

Example 1: Car Loan

You borrow $18,000 to buy a used car at 7% simple interest for 4 years.

Your monthly payment would be $23,040 ÷ 48 months = $480. Note that most real car loan agreements use daily simple interest — meaning interest accrues each day on the outstanding balance, so paying early reduces the total interest you pay.

Example 2: Short-Term Personal Loan

You need $3,500 quickly and borrow it for 90 days at a 15% annual simple interest rate.

Example 3: Savings Account or Certificate of Deposit

You deposit $5,000 in a fixed-term account paying 4.25% simple interest for 18 months.

Example 4: Treasury Bill

You purchase a 26-week (182-day) T-bill with a $10,000 face value at a 5.2% discount rate. T-bills use a 360-day year for discount calculations, but for a straightforward comparison:

Simple Interest vs Compound Interest

The difference between simple and compound interest is one of the most important concepts in personal finance. Understanding it determines whether you are making smart savings decisions and catching unfavorable loan terms before you sign.

Simple interest applies the rate to the original principal every period. The interest accrued in one period does not affect the base for the next period. Growth is perfectly linear. Over ten years, you earn exactly ten times what you earned in year one.

Compound interest applies the rate to the running balance, which includes all accumulated interest. Albert Einstein reportedly called compound interest the "eighth wonder of the world." Growth is exponential — slowly at first, then dramatically faster. Over ten years, you earn far more than ten times the year-one interest, because each year's interest is slightly larger than the last.

A concrete illustration: $10,000 at 6% for 20 years. Simple interest gives $10,000 × 0.06 × 20 = $12,000 in interest, for a total of $22,000. Compound interest (annual compounding) gives $10,000 × (1.06)^20 = $32,071 — more than double the simple interest result. That's the power of compounding over long time horizons.

Converting Time: Years, Months, and Days

The I = Prt formula requires time to be in years. The most common mistake beginners make is entering months or days without converting. Here are the conversion rules this calculator uses:

• Months to years: Divide by 12. So 6 months = 0.5 years, 9 months = 0.75 years, 36 months = 3 years.
• Days to years: Divide by 365. So 30 days ≈ 0.0822 years, 180 days ≈ 0.4932 years, 365 days = 1 year.
• Banker's Rule (360-day year): Some commercial and international loans divide by 360, not 365. This gives a slightly higher interest amount. Always check your loan agreement. This calculator uses 365 days per year.

If you know the exact start and end date of a loan, count the calendar days between them and enter that in the Days field. For example, from March 15 to September 1 is 170 days. Entering 170 days gives the most precise result.

Where Simple Interest Appears in Real Life

Auto Loans

Almost every car loan in the United States uses the daily simple interest method. Interest accrues each day on the outstanding principal balance. This means that if you make a payment earlier than scheduled, more of it goes toward principal, you reduce your balance faster, and the total interest you pay over the life of the loan decreases. Paying even $50 extra per month on a 48-month car loan can save $200–$400 in interest. Conversely, paying late means more interest has accrued since your last payment, so a larger portion of your late payment covers interest rather than principal.

Treasury Bills and Bonds

Short-term US government securities (T-bills, maturities of 4, 8, 13, 26, or 52 weeks) are typically priced using simple interest math. The government sells them at a discount from face value, and you receive the full face value at maturity. The difference is your interest. Medium-term notes and longer bonds pay semi-annual coupon interest, which functions similarly to simple interest on each coupon period, though the compounding occurs when coupons are reinvested.

Personal and Payday Loans

Many short-term personal loans and installment loans advertise simple interest rates. This is important because payday lenders often quote flat fee charges that are technically simple interest — but the annualized equivalent can be enormous. A $15 fee to borrow $100 for 14 days works out to: I = Prt → 15 = 100 × r × (14/365) → r = 15/(100 × 0.0384) = 391% annually. This calculation reveals predatory lending terms that might otherwise appear modest.

Certificates of Deposit

Some CDs, particularly short-term ones (under 12 months), pay simple interest at maturity rather than compounding monthly. A 6-month CD at 5.0% on a $20,000 deposit pays: I = 20,000 × 0.05 × (6/12) = $500. The bank writes you a $500 check at maturity, and if you roll it over, you start fresh with the original $20,000 principal. A compounding CD would roll both the principal and the interest into the new balance, giving you a slightly higher base for the next term.

Promissory Notes and Informal Loans

When individuals lend money to each other — family loans, business partner loans, startup founder notes — simple interest is the standard. It's easy to document, easy to verify, and minimizes disputes. The IRS also requires that loans between related parties charge at least the Applicable Federal Rate (AFR) to avoid them being reclassified as gifts.

Solving for Other Variables

The simple interest formula can be rearranged to solve for any of its four variables, not just I:

This is especially useful when you know what you want to earn (or pay) and need to back into one of the other variables. For example:

• Finding the rate: You lent $4,000 and received $4,480 back after 2 years. The interest is $480. r = 480 ÷ (4,000 × 2) = 0.06 = 6%.
• Finding the time: You want to earn $750 in interest on $5,000 at 6%. t = 750 ÷ (5,000 × 0.06) = 750 ÷ 300 = 2.5 years.
• Finding the principal: You need to earn $1,200 in interest in 3 years at 8%. P = 1,200 ÷ (0.08 × 3) = 1,200 ÷ 0.24 = $5,000 required deposit.

Common Mistakes to Avoid

• Forgetting to convert the rate to decimal. Entering 5 instead of 0.05 will give a result 100× too large. In this calculator, enter the percentage (5) and it converts automatically.
• Using months or days without converting to years. Entering 6 for six months without converting gives a result 12× too large. Use the unit selector (months/days) in this calculator to avoid this entirely.
• Confusing APR with simple interest rate. APR (Annual Percentage Rate) includes origination fees and other charges on top of the interest rate. For an accurate comparison between loans, always compare APRs — not just stated interest rates.
• Assuming loans use simple interest when they compound. Most mortgages, credit cards, and investment accounts use compound interest. Don't use the simple interest formula for those — the result will significantly underestimate the true cost.
• Not accounting for 360-day vs 365-day years. Commercial paper and some international bonds use 360-day conventions. The difference is small (about 1.4% of the interest amount) but can matter on large principal amounts.

Frequently Asked Questions