Simple Interest Calculator

Simple interest is a method of calculating interest where you only ever earn or pay interest on the original principal amount. The balance doesn't grow on top of itself over time, which is what makes it "simple" compared to compound interest.

It's commonly used for short-term loans, auto financing, personal loans, and some savings products. If you borrow $1,000 at 5% simple interest for two years, you pay interest on that $1,000 for both years. Not on $1,000 plus last year's interest. Just the original $1,000, every time.

That straightforward structure makes simple interest easy to predict. You always know exactly how much extra you're paying or earning, and there are no surprises from interest building on itself.

The formula is short and clean:

SI = P × R × T

• SI = Simple Interest (the dollar amount of interest earned or owed)
• P = Principal (the starting amount of money)
• R = Rate (the annual interest rate, expressed as a decimal)
• T = Time (the number of years the money is borrowed or invested)

One thing to watch: the rate needs to be in decimal form. So 6% becomes 0.06, and 12.5% becomes 0.125. If your time period is in months, divide by 12 to convert it to years before using the formula. For example, 9 months becomes 0.75 years.

That's really the whole thing. Three inputs, one multiplication, done.

Let's walk through it with a concrete example. Say you deposit $2,500 in a savings account at an annual interest rate of 4% for 3 years.

1. Identify your values: P = 2500, R = 4% = 0.04, T = 3
2. Plug into the formula: SI = 2500 × 0.04 × 3
3. Multiply P by R first: 2500 × 0.04 = 100
4. Then multiply by T: 100 × 3 = 300

Your simple interest is $300. That's the amount earned over those three years, separate from your original deposit.

The order of multiplication doesn't actually matter since it's just three numbers being multiplied together. But working left to right keeps things organized, especially when you're doing this by hand.

Simple interest alone tells you what you earned or owe in interest. But most of the time, you want the total amount, which means the principal plus the interest combined.

The formula for total amount is:

A = P + SI or written out in full: A = P(1 + R × T)

Using the example from the previous section: A = 2500 + 300 = $2,800. After three years, your account balance would be $2,800.

This is the number that matters most for planning. If you're taking out a loan, this tells you the total you'll repay. If you're investing, it's what you walk away with at the end of the term. Always calculate total amount when you need the full picture, not just the interest portion.

You don't always have all three inputs sitting in front of you. Sometimes you know the interest and need to back-calculate one of the other variables. The formula rearranges easily.

Say you paid $180 in interest on a $1,500 loan over 2 years and want to verify the interest rate. R = 180 / (1500 × 2) = 180 / 3000 = 0.06, or 6% per year. Easy check.

This kind of reverse calculation is useful when you're comparing loan offers, checking if a quoted rate adds up, or figuring out how long it'll take to earn a specific amount of interest on savings.

Simple interest and compound interest both use a principal and a rate, but they behave very differently over time. The core difference: compound interest charges (or pays) interest on accumulated interest, not just the original principal.

For short time frames, the difference between the two is small. Over decades, it can be enormous. A 5% compound interest investment will significantly outperform a 5% simple interest one after 20 or 30 years because the compounding keeps stacking.

When you're borrowing money, simple interest is generally the better deal. When you're growing money, compound interest works in your favor, assuming you're the one earning it.

Simple interest shows up more often in everyday financial life than most people realize.

• Auto loans: Many car loans use simple interest. You pay interest on the remaining principal balance, so making extra payments actually reduces your total interest cost.
• Personal loans: Short-term personal loans from banks or credit unions often use simple interest, making them more predictable than credit card debt.
• Short-term savings: Some certificates of deposit (CDs) and basic savings accounts, especially short-term ones, calculate returns using simple interest.
• Student loans: Federal student loans accrue simple interest while you're in school, though they may switch to compound once repayment begins depending on the loan type.
• Installment loans: Retail financing and buy-now-pay-later plans sometimes use simple interest structures.

The common thread in all of these is that the loan or investment has a defined term and the interest doesn't compound on a daily or monthly basis. If you're ever unsure which type applies to your situation, ask your lender directly or check the loan disclosure paperwork. It makes a real difference in how much you end up paying.

Here are a few worked examples to make the formula feel second nature.

Example 1: Short-term personal loan
You borrow $5,000 at 8% annual interest for 18 months.
Convert time: 18 months = 1.5 years
SI = 5000 × 0.08 × 1.5 = $600
Total repaid: $5,000 + $600 = $5,600

Example 2: Savings account
You deposit $10,000 at 3.5% simple interest for 4 years.
SI = 10000 × 0.035 × 4 = $1,400
Total balance: $10,000 + $1,400 = $11,400

Example 3: Finding the rate
You earned $250 in interest on a $2,000 deposit over 2.5 years. What was the rate?
R = 250 / (2000 × 2.5) = 250 / 5000 = 0.05 = 5% per year

Example 4: Finding the time
You borrowed $3,000 at 6% and paid $540 in interest. How long was the loan?
T = 540 / (3000 × 0.06) = 540 / 180 = 3 years

Each of these follows the same logic. Once you're comfortable with the base formula, the rearranged versions feel just as natural.