To calculate the interest portion of a mortgage payment, multiply your loan balance by your annual interest rate and divide by 12. The principal portion is whatever remains from your fixed monthly payment after that interest is subtracted. On a $300,000 mortgage at 4%, your first month’s interest would be $1,000 ($300,000 × 0.04 ÷ 12), and the rest of your payment chips away at the balance.
That simple formula repeats every month, but because your balance shrinks with each payment, the split between principal and interest constantly shifts. Here’s how the full calculation works, how to build it in a spreadsheet, and what it means for the total cost of your loan.
The Monthly Interest Formula
Every mortgage payment starts with one calculation: how much interest you owe on your current balance. The math is straightforward.
- Monthly interest = Outstanding balance × (annual interest rate ÷ 12)
- Principal portion = Total monthly payment − monthly interest
Say you have a $300,000 loan at a 4% fixed rate. In month one, your interest charge is $300,000 × 0.04 ÷ 12 = $1,000. If your total monthly payment is $1,432, then $432 goes toward principal. Your new balance drops to $299,568. In month two, you recalculate interest on that lower balance: $299,568 × 0.04 ÷ 12 = $998.56, so $433.44 now goes to principal. Each month, interest shrinks by a little and principal grows by a little.
This process repeats for all 360 payments on a 30-year loan (or 180 on a 15-year). By the final months, nearly your entire payment is principal because the remaining balance is so small that it barely generates any interest.
How to Calculate the Fixed Monthly Payment
Before you can split principal from interest, you need to know the total monthly payment itself. Lenders use a standard amortization formula that keeps the payment constant over the life of the loan:
Monthly payment = P × [r(1 + r)^n] ÷ [(1 + r)^n − 1]
In that formula, P is the loan amount (principal), r is the monthly interest rate (annual rate divided by 12), and n is the total number of monthly payments. For a $300,000 loan at 4% over 30 years: r = 0.04 ÷ 12 = 0.003333, and n = 360. Plugging those in gives you roughly $1,432 per month for principal and interest.
You don’t need to do this by hand. Online mortgage calculators handle it instantly, and spreadsheets have built-in functions that do the same thing.
Using a Spreadsheet to Run the Numbers
Excel and Google Sheets both include a PMT function that calculates your fixed monthly payment in one step. The syntax is:
PMT(rate, nper, pv)
- rate: Your monthly interest rate. If your annual rate is 6%, enter 0.06/12.
- nper: Total number of payments. For a 30-year mortgage, enter 360.
- pv: The loan amount (present value). Enter it as a positive number.
For a $400,000 loan at 6.8% over 30 years, you’d type: =PMT(0.068/12, 360, 400000). The result comes back negative (indicating money going out) at roughly $2,608 per month.
To see exactly how much of any single payment is interest or principal, you can build a simple amortization table. In column A, list the month number. In column B, start with your loan balance. In column C, calculate that month’s interest (balance × rate ÷ 12). In column D, subtract the interest from your fixed payment to get the principal. In column E, subtract the principal from the previous balance to get your new balance. Copy those formulas down for 360 rows, and you have a complete picture of every dollar across the life of the loan.
Why the Split Between Principal and Interest Shifts
At the start of a 30-year fixed loan, when the balance is at its highest, the majority of each payment covers interest. Over time, as the balance decreases, the interest portion shrinks and more of each payment goes toward principal. This gradual rebalancing is called amortization.
On that $400,000 loan at 6.8%, your first payment sends about $2,267 to interest and only $341 to principal. Halfway through the loan (around year 15), the split is closer to even. By the final years, almost the entire $2,608 payment is principal. Over the full 30 years, you’d pay roughly $538,772 in total interest on top of the original $400,000, which is why even small changes to the interest rate or loan term can have enormous dollar consequences.
How Extra Payments Change the Math
Because interest is recalculated on the outstanding balance each month, every extra dollar you put toward principal immediately reduces the interest charged in every future month. The effect compounds over time.
Using the same $400,000 loan at 6.8% as an example, here’s what different extra payment strategies would save:
- One extra full payment per year ($2,608): Pays off the loan in 24 years instead of 30, saving about $126,000 in interest.
- $100 extra per month: Cuts roughly 3 years off the loan and saves about $69,183 in interest.
- $50 extra per month: Shaves about 20 months off the term and saves around $37,413 in interest.
If you make extra payments, tell your loan servicer to apply the additional amount to principal. If you don’t specify, the servicer may apply it as an early payment of next month’s bill, covering both principal and interest. That still helps, but not as much as a direct principal reduction.
Principal and Interest vs. Your Full Payment
The principal and interest calculation gives you only part of your actual monthly mortgage bill. Most homeowners also pay property taxes and homeowners insurance, often collected by the lender and held in an escrow account. These four components together, principal, interest, taxes, and insurance, are commonly referred to as PITI.
Your lender estimates your annual tax and insurance costs, divides by 12, and adds that amount to your monthly payment. That escrow portion doesn’t affect the principal and interest math at all. It simply sits alongside it. When you see a mortgage calculator quoting “P&I” versus “total payment,” the difference is taxes and insurance. If you carry private mortgage insurance (required on many loans with less than 20% down), that premium is typically rolled into the monthly bill as well.
Understanding the P&I portion separately matters because it’s the piece driven entirely by your loan amount, interest rate, and term. Those are the variables you can negotiate or change through refinancing, while taxes and insurance are set by your local government and your insurer.