How to Calculate Mortgage Payments: Complete Guide (2026)

How Mortgage Payments Work

When you take out a mortgage, you borrow a large sum of money and repay it over time in equal monthly installments. Each payment covers two things: a portion of the principal (the amount you borrowed) and the interest (what the lender charges for lending you the money).

Early in your loan, most of each payment goes toward interest. Over time, the balance shifts — more goes to principal and less to interest. This process is called amortization, and it's why your loan balance barely seems to move in the first few years even though you're making full payments.

For example, on a $315,000 loan at 6.5%, your first monthly payment of $1,991 breaks down to roughly $1,706 in interest and only $285 toward principal. By year 15, that same $1,991 payment splits to about $1,050 in interest and $941 toward principal. By the final year, nearly the entire payment is principal.

The Mortgage Payment Formula

Lenders use a standard formula to calculate your monthly principal and interest payment. It looks intimidating, but it's straightforward once you break it down:

M = P [ r(1 + r)^n ] / [ (1 + r)^n - 1 ]

Where:

• M = monthly payment
• P = loan principal (amount borrowed)
• r = monthly interest rate (annual rate / 12)
• n = total number of payments (loan term in years x 12)

Worked Example

Say you're buying a $350,000 home with 10% down ($35,000), leaving a loan of $315,000. Your interest rate is 6.5% on a 30-year term.

• P = $315,000
• r = 6.5% / 12 = 0.005417
• n = 30 x 12 = 360 payments

Plugging those numbers into the formula gives a monthly principal and interest payment of $1,991. Over the full 30 years, you'll pay a total of $716,760 — meaning $401,760 goes to interest alone. That's more than the original loan amount, which is why the interest rate matters so much.

What Affects Your Monthly Payment

Your actual monthly housing cost goes beyond just principal and interest. Here's a breakdown of every component that determines what you'll pay each month.

Principal and Loan Amount

The more you borrow, the higher your payment. A $315,000 loan at 6.5% for 30 years costs $1,991/month, while a $250,000 loan at the same rate costs $1,580/month. Every $10,000 you can reduce from the loan amount saves roughly $63 per month.

Interest Rate

Even small rate differences have a huge impact over time. On a $315,000 loan over 30 years:

The difference between a 5.5% and 7.5% rate on the same loan is $414 per month and nearly $149,000 in total interest. Shopping around for the best rate is one of the most impactful financial decisions you can make.

Loan Term

Shorter loan terms mean higher monthly payments but dramatically less interest paid overall. On a $315,000 loan at 6.5%:

• 30-year term: $1,991/month, $401,760 total interest
• 20-year term: $2,349/month, $248,760 total interest
• 15-year term: $2,745/month, $179,100 total interest

The 15-year loan costs $754 more per month but saves you $222,660 in interest over the life of the loan. That's a tradeoff worth considering if your budget can handle the higher payment.

Down Payment

Your down payment directly reduces the loan amount. On a $350,000 home:

• 5% down ($17,500): $332,500 loan = $2,102/month
• 10% down ($35,000): $315,000 loan = $1,991/month
• 20% down ($70,000): $280,000 loan = $1,770/month

Beyond lowering your payment, putting down 20% or more eliminates the need for PMI, which saves even more.

Private Mortgage Insurance (PMI)

If your down payment is less than 20%, lenders require PMI to protect themselves in case you default. PMI typically costs 0.5% to 1.5% of the original loan amount per year. On a $315,000 loan, that adds roughly $131 to $394 per month.

PMI isn't permanent. Once your loan balance drops to 80% of the home's original value (or the home appreciates enough), you can request its removal. Many homeowners forget to do this and pay PMI longer than necessary.

Property Taxes

Property taxes vary widely by location, typically ranging from 0.5% to 2.5% of your home's assessed value per year. On a $350,000 home, that's anywhere from $146 to $729 per month. Your lender usually collects taxes as part of your monthly payment and holds them in an escrow account.

Homeowners Insurance

Lenders require homeowners insurance, which covers damage from fire, storms, theft, and liability. The average cost is roughly $1,200 to $2,000 per year ($100 to $167/month), though this varies significantly based on location, coverage level, and the home itself.

How to Lower Your Payment

If your projected payment feels too high, you have several strategies to bring it down:

• Increase your down payment: Every additional dollar you put down reduces both the loan amount and the monthly payment. Even a few thousand dollars makes a difference.
• Shop for a better rate: Get quotes from at least three lenders. A difference of even 0.25% saves thousands over the loan's life. Don't forget credit unions and online lenders.
• Improve your credit score: Borrowers with scores above 740 typically qualify for the best rates. Pay down existing debt and correct any errors on your credit report before applying.
• Choose a longer term: A 30-year term costs more in total interest than a 15-year, but the monthly payment is significantly lower. You can always make extra payments to pay it off faster.
• Buy less house: It sounds obvious, but buying below your maximum approval amount gives you a comfortable payment with room in your budget for savings and unexpected expenses.
• Consider an adjustable rate: If you plan to sell or refinance within 5 to 7 years, an ARM can offer a lower initial rate. Just understand the risk that rates may increase after the fixed period.

Extra Payments Save Thousands

Making extra payments toward your mortgage principal is one of the most effective ways to build equity and reduce total interest. Because interest is calculated on the remaining balance, every extra dollar you pay reduces the base that interest accrues on for the rest of the loan.

Here's what extra payments look like on a $315,000 loan at 6.5% over 30 years (base payment: $1,991/month):

An extra $100 per month — roughly $3.30 per day — saves you $68,000 and gets you mortgage-free three and a half years sooner. One practical approach is to divide your monthly payment by 12 and add that amount each month, which is equivalent to making 13 payments per year instead of 12.

Before making extra payments, confirm with your lender that there are no prepayment penalties and that extra payments are applied to the principal rather than future payments. Most conventional loans allow prepayment without penalty, but it's worth verifying.

Fixed vs. Adjustable Rate

When choosing a mortgage, you'll pick between a fixed-rate and an adjustable-rate mortgage (ARM). Each has clear advantages depending on your situation.

Fixed-Rate Mortgage

Your interest rate stays the same for the entire loan. Your principal and interest payment never changes, making it easy to budget. This is the right choice if you plan to stay in the home long-term or want predictability. The vast majority of homebuyers choose fixed-rate loans.

Adjustable-Rate Mortgage (ARM)

An ARM starts with a lower fixed rate for an introductory period (commonly 5, 7, or 10 years), then adjusts periodically based on market conditions. A 5/1 ARM, for example, is fixed for 5 years then adjusts every year after that.

ARMs make sense if you're confident you'll move or refinance before the adjustment period. The initial rate is typically 0.5% to 1% lower than fixed rates, which can save hundreds per month in the short term. However, if rates rise and you're still in the home, your payment could increase substantially.

Key Takeaways

• Your mortgage payment is calculated using the loan amount, interest rate, and loan term with the standard amortization formula
• On a $350,000 home with 10% down at 6.5% for 30 years, the principal and interest payment is roughly $1,991/month
• Your total monthly housing cost includes principal, interest, property taxes, homeowners insurance, and possibly PMI
• Even a 0.5% difference in interest rate can save or cost you tens of thousands over the life of the loan
• A 15-year mortgage costs more per month but saves over $200,000 in interest compared to a 30-year on the same loan
• Extra payments are powerful: just $100/month extra on a $315,000 loan saves roughly $68,000 in interest
• Put down 20% if possible to avoid PMI, which can add $130 to $400 per month
• Choose fixed-rate for long-term stability; consider an ARM only if you plan to sell or refinance within 5 to 7 years