Every loan you take out has a cost — and that cost is interest. Whether it is a mortgage, car loan, student loan, or personal loan, knowing how to calculate loan interest lets you compare lenders, make smarter borrowing decisions, and potentially save thousands of dollars.
Simple vs Compound Loan Interest
Most installment loans (personal loans, auto loans, mortgages) use simple interest amortization — interest is calculated on the remaining balance each period, not on previously accumulated interest.
Simple interest loans: Interest = Principal × Rate × Time. Each payment first pays the current month's interest, then reduces principal.
Compound interest (credit cards, some personal loans): Interest accrues on balance including unpaid interest, making debt grow faster if you carry a balance.
How Amortized Loan Interest Works
For most loans, payments are calculated using the amortization formula:
Example: $20,000 car loan at 7% for 60 months:
Total paid = $396.02 × 60 = $23,761. Total interest = $3,761 (18.8% of the loan amount).
→ Calculate your exact loan payment with our Loan Calculator.
How to Calculate Interest on Each Payment
In an amortized loan, each payment pays a different mix of interest and principal. Early payments are mostly interest; later payments are mostly principal.
For the $20,000 car loan above, the first and last payments look like this:
| Payment # | Balance | Interest | Principal |
|---|---|---|---|
| 1 | $20,000 | $116.67 | $279.35 |
| 30 | $10,752 | $62.72 | $333.30 |
| 60 | $394.32 | $2.30 | $393.72 |
Notice how payment #1 is 29.5% interest, while payment #60 is only 0.6% interest. This is why making extra principal payments early saves the most interest.
→ View your full amortization schedule with every payment broken down.
APR vs Interest Rate: What's the Difference?
The interest rate is the base cost of borrowing. APR (Annual Percentage Rate) includes the interest rate plus all fees (origination fees, mortgage points, closing costs) expressed as an annual rate.
APR is always higher than the stated interest rate. A mortgage with 6.5% interest and $4,000 in fees on a $200,000 30-year loan might have an APR of 6.73%. When comparing loans, always compare APRs — not just interest rates.
| Loan | Interest Rate | Fees | APR |
|---|---|---|---|
| Lender A | 6.5% | $2,000 | 6.65% |
| Lender B | 6.75% | $0 | 6.75% |
Lender A is cheaper despite the higher stated rate because the APR accounts for lower fees.
How to Reduce Total Loan Interest
Five proven strategies to minimize the total interest you pay:
- Make extra principal payments — even $50/month extra on a mortgage saves thousands and cuts years off the loan
- Choose a shorter term — a 15-year mortgage at 6.5% saves $100,000+ in interest vs a 30-year, though monthly payments are higher
- Refinance when rates drop — refinancing a 7% mortgage to 5.5% on a $300,000 balance saves ~$280/month
- Make biweekly payments — paying half your monthly payment every two weeks results in 26 half-payments (13 full payments) vs 12/year, cutting years off a mortgage
- Shop lenders — comparing at least 3-5 lenders typically saves 0.5-1% on mortgage rates, worth tens of thousands over the loan life
Mortgage Interest: A Special Case
Mortgages work the same way as other amortized loans, but the large amounts make the interest numbers dramatic:
A $350,000 30-year mortgage at 7%:
- Monthly payment: $2,329
- Total paid over 30 years: $838,440
- Total interest: $488,440 — more than the loan itself
The same mortgage at 5.5% (1.5% lower rate):
- Monthly payment: $1,987 ($342 less per month)
- Total interest: $365,320 — saving $123,120 in interest
A 1.5% rate difference saves $123,120 on a $350,000 mortgage. This is why mortgage rate shopping is the single highest-impact financial negotiation most people will ever do.
→ Calculate your full mortgage payment including taxes, insurance, and PMI.