How Loan Interest Really Works (Plain-English Guide)

The basic idea

When you borrow money, the lender charges interest as a percentage of the unpaid balance. Each month they calculate the interest you owe based on what's still outstanding. Your payment first covers that interest — anything left reduces the principal.

As you pay down the principal, the interest charge each month gets smaller. So later in the loan, more of each payment goes to principal.

How monthly interest is calculated

Most loans use a simple formula: monthly interest = (annual rate / 12) × current balance.

Example: $200,000 mortgage at 6% APR. Month 1 interest: 6%/12 × $200,000 = $1,000. If your monthly payment is $1,199, $1,000 goes to interest and $199 to principal. New balance: $199,801.

Month 2 interest: 6%/12 × $199,801 = $999. Your $1,199 payment now reduces principal by $200. The shift is tiny each month but compounds over years.

Why early payments are mostly interest

On a 30-year, $200K mortgage at 6%, the principal/interest split changes dramatically over time:

• Year 1 payment: ~$170 principal, ~$1,030 interest
• Year 10 payment: ~$310 principal, ~$890 interest
• Year 20 payment: ~$565 principal, ~$634 interest
• Year 30 payment: ~$1,193 principal, ~$6 interest

This is why making extra principal payments early in a loan saves much more than late payments — you're cutting future interest charges that would compound for decades.

Daily vs monthly interest accrual

Most mortgages and auto loans accrue interest monthly. Most credit cards and student loans accrue interest daily — meaning interest is calculated on yesterday's balance every single day.

Daily compounding makes credit card debt grow faster than people realize. A $5,000 balance at 22% APR with daily compounding effectively grows to ~$6,200 over a year if not paid.

Simple interest vs compound interest

Most installment loans (mortgages, auto, personal) use simple interest on a declining balance — you only pay interest on the remaining principal. Credit cards and some student loans use compound interest, where unpaid interest gets added to the balance and starts earning more interest.

What APR includes (and doesn't)

APR (annual percentage rate) is supposed to capture the true cost of a loan including most fees. But:

• Mortgage APR includes points and origination fees
• Credit card APR doesn't include annual fees
• Auto loan APR usually doesn't include extended warranties or GAP insurance

Always compare loans on APR, not just interest rate.

How to reduce total interest paid

• Choose the shortest term you can comfortably afford
• Make extra principal payments — especially in the first 5–10 years
• Refinance when rates drop 1%+ below your current rate
• Avoid restarting the clock with new loans on top of existing ones
• Pay off high-rate debt before low-rate (avalanche method)

Use the calculator

Frequently Asked Questions