Understanding Loan Interest: Simple vs Compound

Updated June 2026 - 6 min read

Interest is the cost of borrowing money. Whether you are taking out a mortgage, auto loan, student loan, or personal loan, understanding how interest is calculated helps you compare offers, estimate your true cost, and find strategies to pay less over time.

Simple Interest

Simple interest is calculated only on the original principal balance:

Example: A $10,000 loan at 5% per year for 3 years accumulates $10,000 x 0.05 x 3 = $1,500 in interest. Simple interest is common for short-term personal loans.

Compound Interest

Compound interest charges interest on both the principal and any accumulated interest. It grows faster than simple interest:

• A = final amount
• P = principal
• r = annual interest rate (decimal)
• n = compounding periods per year
• t = time in years

How Mortgage Amortization Works

Most mortgages and installment loans use amortization - fixed monthly payments that cover both interest and principal. Your monthly payment M is calculated as:

In early payments, most of your money goes to interest. Over time, more goes to principal. This is why paying even a small extra amount each month can save thousands in interest.

APR vs Interest Rate

The interest rate is just the cost of borrowing the principal. APR (Annual Percentage Rate) includes the interest rate plus fees, points, and other loan costs expressed as a yearly rate. Always compare APRs when shopping for loans - not just the interest rate.

Strategies to Pay Less Interest

• Make extra principal payments. Even $50 extra per month on a 30-year mortgage can save years of payments.
• Refinance when rates drop. A 1% rate reduction on a $300,000 mortgage saves roughly $170/month.
• Choose a shorter loan term. A 15-year mortgage carries a lower rate than a 30-year, and you pay far less total interest.
• Improve your credit score. Better credit means lower rates. Even a 0.5% improvement on a large loan saves thousands.
• Make biweekly payments. Paying half your monthly payment every two weeks results in one extra full payment per year.