Simple Interest Calculator -- Step-by-Step

Calculate Simple Interest

Simple Interest Calculator computes interest using the formula I = P × r × t. Enter the principal, annual rate, and time to see the interest earned, total amount, and a step-by-step breakdown — all calculated in your browser.

Quick Examples

Simple Interest Formula

The Core Formula

I = interest earned, P = principal (starting amount), r = annual interest rate as a decimal, t = time in years.

To get the total amount at the end:

Worked Example

You deposit $8,000 at 4.5% per year for 3 years.

1. Convert rate: 4.5% → 0.045
2. Multiply: I = 8,000 × 0.045 × 3 = $1,080.00
3. Total: A = 8,000 + 1,080 = $9,080.00

The interest is the same every year: $360.00 in year 1, $360.00 in year 2, $360.00 in year 3.

Rearranged Formulas

You can solve for any variable:

Simple Interest for Periods Other Than Years

The formula always uses time in years. Convert shorter periods:

• Months: divide by 12. Example: 6 months = 6/12 = 0.5 years
• Days: divide by 365 (or 360 in some banking conventions). Example: 90 days = 90/365 ≈ 0.2466 years

Example: $2,000 at 8% for 90 days: I = 2,000 × 0.08 × (90/365) = $39.45

Simple Interest vs. Compound Interest

With simple interest, you earn the same dollar amount each year. With compound interest, earned interest is added to the principal, so each period earns more than the last.

Over short periods (under 2 years), the difference is small. Over longer periods, compound interest produces significantly higher returns. See the compound interest calculator for comparison.

When Simple Interest Is Used

• Auto loans: Many car loans calculate interest on the original loan amount
• Short-term personal loans: Payday or installment loans often use simple interest
• U.S. Treasury bonds: Pay fixed interest based on face value
• Trade credit: Net-30/60/90 terms often imply simple interest on overdue amounts
• Student loans: Federal student loans accrue simple interest (while in deferment)
• Academic contexts: Introductory finance courses teach simple interest before compound interest

Common Pitfalls

• Forgetting to convert the rate: 5% must become 0.05 in the formula. Using 5 instead of 0.05 gives a result 100× too large.
• Mixing time units: If the rate is annual, the time must be in years. 6 months = 0.5 years, not 6.
• Assuming all loans use simple interest: Credit cards, mortgages, and most savings accounts use compound interest. Check the terms.
• Ignoring fees: The interest calculation doesn't include origination fees, service charges, or other costs that affect the total cost of borrowing.
• Day-count conventions: Some institutions use a 360-day year (called "ordinary interest" or "banker's rule") instead of 365. This produces slightly higher interest: $10,000 × 0.05 × (90/360) = $125.00 vs. (90/365) = $123.29.

Frequently Asked Questions

What is simple interest?

Simple interest is interest calculated only on the original principal amount. The formula is I = P × r × t, where P is the principal, r is the annual rate (as a decimal), and t is the time in years. Unlike compound interest, earned interest is not added back to the principal.

How do you calculate simple interest for months?

Convert the months to a fraction of a year. For example, 6 months = 0.5 years, 9 months = 0.75 years. Then use I = P × r × t with that fraction. $10,000 at 5% for 6 months: I = 10,000 × 0.05 × 0.5 = $250.

What is the difference between simple and compound interest?

Simple interest is calculated only on the original principal and grows linearly. Compound interest is calculated on the principal plus all previously earned interest, so it grows exponentially. Over short periods the difference is small; over long periods compound interest produces significantly more.

When is simple interest used in real life?

Simple interest is commonly used for short-term personal loans, auto loans, some government bonds (like U.S. Treasury bonds), certificates of deposit interest calculations, and trade credit terms.

Can simple interest apply to periods shorter than a year?

Yes. Express the time as a fraction or decimal of a year. For days, divide by 365 (or 360 in some banking conventions). For example, 90 days = 90/365 ≈ 0.2466 years.

How do I find the interest rate from the interest amount?

Rearrange the formula to r = I / (P × t). For example, if $500 interest was earned on $10,000 over 2 years: r = 500 / (10,000 × 2) = 0.025, which is 2.5% per year.

What happens if the interest rate is 0%?

If the interest rate is 0%, no interest is earned. The total amount equals the original principal regardless of the time period.

Is simple interest always less than compound interest?

For the same principal, rate, and time (more than one compounding period), yes. Compound interest earns interest on interest, so it always produces a higher total. The exception is when the time period is exactly one compounding period — then both methods give the same result.

Related Tools

• Compound Interest Calculator — see how compounding affects growth
• Loan Calculator — calculate monthly payments for amortizing loans
• Loan Amortization Schedule — view a full payment-by-payment breakdown
• Savings Goal Calculator — figure out how much to save each month
• Percentage Calculator — quick percentage computations

Simple Interest Calculator FAQ