Mortgage Calculator

What this mortgage calculator estimates — and who uses it

A mortgage calculator is one of the most-used financial tools for homebuyers, homeowners considering a refinance, and real estate investors running numbers on rental properties. Enter a home price, down payment, loan term, and interest rate and you get an instant estimate of your monthly principal-and-interest payment, the total interest you will pay over the life of the loan, and a month-by-month amortization schedule for the first year.

First-time buyers typically use the calculator to figure out how much house they can afford before speaking with a lender. Existing homeowners use it to compare the cost of refinancing into a lower rate or switching from a 30-year to a 15-year term. Investors use it to model debt service on rental properties and ensure cash flow after expenses. In all three cases the goal is the same: understand the full cost of borrowing before signing anything.

Keep in mind that this tool calculates principal and interest only. Your real monthly housing cost will also include property taxes, homeowner's insurance, and — if your down payment is under 20% — private mortgage insurance (PMI). Those items are covered in the PITI section below.

How a mortgage payment is calculated

Mortgage lenders use standard loan amortization, which spreads your payments evenly over the loan term so that every payment is the same dollar amount. The formula that produces that fixed payment is:

Where P is the loan principal (home price minus down payment), r is the monthly interest rate (your annual rate divided by 12), and n is the total number of monthly payments (years multiplied by 12). For a 30-year loan, n = 360; for a 15-year loan, n = 180.

The result is a level payment that covers both interest for the current month and a portion of principal reduction. In the early years almost all of each payment is interest; by the final years almost all of it is principal. This is why the amortization table above shows a large interest component in month 1.

Understanding PITI

Lenders and housing counselors use the acronym PITI to describe the four components of a full monthly housing payment:

• Principal — the portion of your payment that reduces the outstanding loan balance.
• Interest — the cost of borrowing, paid to the lender and calculated on the current balance.
• Taxes — property taxes assessed by your local government, often collected monthly by the lender into an escrow account and paid annually on your behalf.
• Insurance — homeowner's insurance (hazard insurance), also typically escrowed. If your down payment is under 20%, PMI is an additional insurance cost added here. Some properties also carry HOA dues that function similarly to a monthly insurance or maintenance charge.

Worked example: $300,000 loan at 6.5% for 30 years

Suppose you buy a home for $340,000, put $40,000 (about 12%) down, and borrow $300,000 at a fixed rate of 6.5% for 30 years. Here is how the numbers work out step by step:

1. Convert the annual rate to a monthly rate: 6.5% ÷ 12 = 0.5417% per month, or r = 0.005417.
2. Determine the number of payments: 30 years × 12 months = n = 360.
3. Apply the formula: M = 300,000 · [0.005417 × (1.005417)³⁶⁰] / [(1.005417)³⁶⁰ − 1].
4. Result: (1.005417)³⁶⁰ ≈ 7.040, so M = 300,000 · (0.005417 × 7.040) / (7.040 − 1) = 300,000 · 0.038135 / 6.040 ≈ $1,896 per month in principal and interest.

Over 360 payments, the total amount paid is $1,896 × 360 = $682,560. Subtract the original $300,000 principal and you have paid roughly $382,000 in interest alone — more than the original loan amount. That figure is what makes the 15-versus-30-year tradeoff so significant: choose a 15-year term at a slightly lower rate and the total interest cost drops to roughly $160,000–$185,000 on the same loan, a difference of nearly $200,000.

None of that total-interest figure includes taxes, insurance, or PMI. Because this borrower put down less than 20%, they would also pay PMI — at 0.8% of the original loan, that is approximately $200 per month until the balance reaches 80% of the appraised value (typically around year 8 to 11, depending on the amortization pace). Adding typical taxes and insurance to this scenario could push the real all-in monthly payment to $2,500–$3,200 depending on location.

What drives your payment and total cost

Four levers control how much you pay each month and how much interest accumulates over the life of the loan:

Interest rate

The interest rate is the single largest driver of total cost. On a $300,000 loan, the difference between a 6.0% rate and a 7.0% rate is roughly $180 per month and over $65,000 in total interest over 30 years. Shopping lenders and improving your credit score before applying are the most reliable ways to secure a lower rate. Each 0.5-point improvement in credit score can translate to a quarter-point reduction in rate.

Loan term: 15 vs. 30 years

A 15-year mortgage carries a lower rate (often 0.5–0.75% below the 30-year rate) and cuts total interest roughly in half, but the monthly payment is 40–50% higher. A 30-year mortgage provides more monthly cash flow and can make sense if you invest the payment difference in retirement accounts or other appreciating assets. The right choice depends on your income stability, other debt obligations, and retirement timeline. Use the calculator to compare both scenarios side by side.

Down payment

A larger down payment reduces the loan principal (lowering monthly P&I), may unlock a better rate tier from the lender, and eliminates PMI once you reach 20%. However, a larger down payment means less cash available for an emergency fund, home repairs, or investment. Balance the cost of PMI against the opportunity cost of tying up additional capital in home equity.

Extra principal payments

Because mortgage interest is calculated on the remaining balance, any extra payment you make directly reduces future interest charges. Adding even $100–$200 per month to principal on a 30-year loan can shorten the payoff timeline by several years and save tens of thousands of dollars. Some borrowers make one extra payment per year (by dividing the monthly payment by 12 and adding that amount each month), which effectively shortens a 30-year loan by about 4–5 years. Pair this calculator with our loan calculator to model custom payoff scenarios.

Common mistakes to avoid when budgeting for a mortgage

• Budgeting only for P&I. The principal-and-interest payment shown here is the floor, not the ceiling. Always add estimated property taxes, homeowner's insurance, and PMI (if applicable) to get a realistic monthly commitment. Forgetting these can cause serious cash-flow problems after closing.
• Ignoring total interest paid. A low monthly payment on a 30-year loan can feel affordable, but the total interest cost is often staggering. Always check the total-interest figure and compare it against a shorter term or a higher monthly payment before deciding.
• Not comparing the 15-year and 30-year options. Most buyers default to a 30-year term without running the 15-year numbers. The higher monthly payment on a 15-year term is often offset by a lower rate and a dramatically lower total cost. If the 15-year payment fits your budget, it usually wins over time.
• Overlooking PMI. Private mortgage insurance is an invisible cost that many first-time buyers discover after closing. On a $300,000 loan at 0.8% PMI, that is $2,400 per year — roughly $200 per month added to your payment that builds zero equity. If you are close to 20% down, running the numbers on reaching that threshold is almost always worthwhile.
• Forgetting HOA dues and maintenance. Condos, planned communities, and many newer subdivisions carry monthly HOA fees ranging from $100 to over $1,000. These are not included in any mortgage calculator but are a real monthly cost. Budget at least 1% of the home's value per year for maintenance and repairs on top of your PITI.
• Not accounting for rate changes on ARMs. Adjustable-rate mortgages often start lower than fixed rates but can reset significantly after the initial fixed period ends. If you are evaluating an ARM, model the worst-case adjusted rate in this calculator to understand your maximum possible payment before committing.

Related financial calculators

Mortgage planning rarely happens in isolation. Once you know your monthly payment, you may want to compare the total cost of this loan against other debt payoff strategies, or model how your home purchase affects long-term wealth:

• Loan calculator — model any installment loan, including extra payment scenarios to see how quickly you can pay off your mortgage early.
• Compound interest calculator — compare the opportunity cost of a larger down payment against investing that money over time.
• Retirement calculator — understand how your housing costs interact with long-term retirement savings goals.