The Basic Formula: Principal Times Rate Divided by 12

Your lender calculates the interest you owe each month by taking the remaining balance of your loan, multiplying it by your annual interest rate, and dividing by 12. That number is the interest portion of your monthly payment. The rest of your payment goes toward principal — the actual loan amount you borrowed.

Here's a concrete example: if you owe $300,000 on a mortgage with a 6% annual interest rate, the calculation is $300,000 × 0.06 ÷ 12 = $1,500 in interest for that month. If your total monthly payment is $1,799, then $1,500 goes to interest and $299 goes to principal.

This matters because the interest you pay shrinks as your balance shrinks. Next month, if your balance is now $299,701, the interest calculation starts with that smaller number. Over time, more of each payment chips away at principal and less goes to interest — but only because the balance keeps falling.

Key Takeaways

  • Monthly interest is calculated by multiplying your loan balance by your annual rate and dividing by 12.
  • The interest portion of your payment decreases over time as your balance decreases, even though your monthly payment stays the same.
  • Early in the loan, most of your payment covers interest; late in the loan, most covers principal.
  • Your interest rate is locked in at closing for a fixed-rate mortgage, but adjustable-rate mortgages change the rate on a set schedule.
  • The amortization schedule your lender provides shows exactly how much interest and principal you pay each month for the entire loan.

Why Your Interest Payment Changes Every Month

The balance used in the calculation is the one that exists on the day your payment is processed, not the balance from the previous month. When you make a payment, the principal portion reduces the balance immediately. The next month's interest calculation uses this new, lower balance.

This is why a 30-year mortgage doesn't cost the same amount in interest every month. In month one, you might pay $1,500 in interest and $299 in principal. By month 180 (halfway through), you might pay $900 in interest and $900 in principal. By month 359, you might pay $20 in interest and $1,779 in principal. Your total payment stays the same, but the split shifts.

Your lender provides an amortization schedule at closing — a table showing every payment for the entire 15, 20, or 30 years, with the interest and principal breakdown for each one. This schedule is calculated at closing and assumes you make every payment on time and that your rate does not change.

How Your Interest Rate Affects the Calculation

The interest rate you lock in at closing is the annual percentage rate, or APR. For a fixed-rate mortgage, this rate never changes. A 6% rate stays 6% for the entire loan, whether rates in the market rise to 8% or fall to 3%.

If you have an adjustable-rate mortgage (ARM), the rate is fixed for an initial period — often 3, 5, 7, or 10 years — and then adjusts annually or semi-annually based on a market index plus a margin set by your lender. When the rate adjusts, the calculation changes. A rate that rises from 6% to 7% increases your monthly interest payment, though your lender usually recalculates your payment to keep it level (which means more of it goes to interest and less to principal).

Points paid at closing can lower your interest rate. One point costs 1% of the loan amount and typically reduces your rate by 0.25%. If you pay points, the calculation uses the lower rate, which means lower monthly interest — but you paid cash upfront to get there.

The Difference Between Simple and Compound Interest

Mortgages use simple interest, not compound interest. Simple interest means you pay interest only on the balance you currently owe, not on interest you've already paid. This is the more favorable method for a borrower.

Compound interest — where you pay interest on interest — would make mortgages far more expensive. Credit cards often use compound interest, which is one reason credit card debt grows so quickly. Mortgages do not work this way. You pay interest on principal only, calculated fresh each month based on what you still owe.

What Happens If You Pay Extra Principal

If you send an extra $200 with your regular payment and specify that it goes to principal, your balance drops by $200 immediately. The next month's interest calculation uses this lower balance, so you pay less interest that month. Over the life of the loan, extra principal payments reduce the total interest you pay and shorten the loan term.

This is why paying extra principal early in the loan saves more interest than paying extra late. In month one, an extra $200 to principal might save you $1,200 in interest over the remaining 360 months. In month 300, the same extra $200 might save you only $30 in interest over the remaining 60 months.

Always confirm with your lender that extra payments are applied to principal, not held as a credit toward next month's payment. Some servicers require a written request or a specific payment method to ensure extra funds reduce the balance.

How Taxes and Insurance Affect Your Total Payment

Your monthly mortgage payment often includes more than just principal and interest. If you have an escrow account, your payment also covers property taxes and homeowners insurance, divided into 12 monthly portions. These are not interest calculations — they are set amounts based on your property tax bill and insurance premium.

Some lenders also add mortgage insurance (PMI) if you put down less than 20%. PMI is a fixed monthly fee, not calculated the same way as interest. It does not decrease as your balance decreases, though you can request removal once you reach 20% equity.

Your loan estimate and closing disclosure break down exactly what portion of your payment is interest, principal, taxes, insurance, and any other fees. These documents show the calculation for your specific loan.

Why the First Years Feel Like You're Paying Mostly Interest

On a 30-year mortgage, the first payment might be 80% interest and 20% principal. This feels unfair, but it's how the math works. You borrowed a large amount, so the interest on that large amount is large. As you pay down the balance, the interest shrinks automatically.

This is not a trick or a penalty — it's the nature of how simple interest works on a large balance. If you wanted to pay principal faster early on, you would need to send extra principal payments, which is always an option. But the standard amortization schedule front-loads interest because the balance is highest at the start.

Frequently Asked Questions

Does my interest rate change if market rates drop?

Not on a fixed-rate mortgage — your rate is locked in at closing and never changes. On an adjustable-rate mortgage, your rate is fixed for an initial period (3, 5, 7, or 10 years) and then adjusts based on market conditions. You can refinance a fixed-rate mortgage to a new loan with a lower rate, but that requires a new application and closing costs.

What if I make a payment late — does interest accrue differently?

Late payments do not change how interest is calculated, but they may trigger a late fee and damage your credit. Interest continues to accrue on the balance owed. If you miss a payment entirely, interest keeps building on the unpaid amount. Contact your lender immediately if you cannot make a payment on time.

Can I see how much total interest I'll pay over the life of the loan?

Yes. Your amortization schedule shows every payment and the cumulative interest paid. You can also multiply your monthly payment by the number of payments and subtract the original loan amount. On a $300,000 loan at 6% over 30 years, the monthly payment is roughly $1,799; multiply by 360 payments to get $647,640 total paid, minus $300,000 borrowed, equals about $347,640 in total interest.

Does paying biweekly instead of monthly save interest?

Yes, but only slightly. Biweekly payments mean you make 26 half-payments per year instead of 12 full payments, which equals 13 full payments annually instead of 12. The extra payment each year goes to principal, reducing the balance and the interest owed. Over 30 years, this can save tens of thousands in interest, but it requires discipline and confirmation that your lender accepts biweekly payments without penalty.

What's the difference between APR and interest rate?

The interest rate is the percentage used to calculate your monthly interest payment. The APR includes the interest rate plus other costs like points and fees, expressed as an annual rate. For interest calculation purposes, your lender uses the interest rate, not the APR. The APR is useful for comparing loans because it shows the true cost.