How Your Mortgage Payment Is Calculated
Every fixed-rate mortgage payment is calculated using an amortization formula that determines how much of each payment goes to interest versus paying down the loan balance. The formula looks complex, but the logic is simple: at the beginning of the loan, you owe more principal, so more of each payment goes to interest. Over time, as the balance shrinks, more goes to principal. This is called amortization.
M = P × [r(1+r)^n] ÷ [(1+r)^n − 1] — where P is the loan amount, r is the monthly interest rate (annual rate ÷ 12), and n is the total number of payments (years × 12).
On a $320,000 loan at 6.5% for 30 years: the monthly rate is 6.5% ÷ 12 = 0.5417%, n = 360 payments, and the monthly P&I payment works out to $2,023. That payment never changes for the life of the loan — which is the defining feature of a fixed-rate mortgage. In month one, about $1,733 of that goes to interest and $290 goes to principal. By year 25, the split has flipped — mostly principal, very little interest — because the balance is so much smaller.
What Goes Into Your Monthly Payment (PITI)
When lenders talk about your mortgage payment, they use the acronym PITI — four components that make up what you actually owe each month. Understanding all four is important because lenders qualify you based on the total PITI payment, not just the principal and interest.
Principal
The portion of your payment that reduces your loan balance. In the early years of a mortgage, this is a small fraction of each payment. As the loan matures, principal makes up a larger and larger share. Each dollar of principal paid is a dollar of equity gained.
Interest
The cost of borrowing, paid to the lender. Calculated each month on the remaining loan balance. Because you owe the most at the start, you pay the most interest at the start. The total interest paid over 30 years on a $400,000 loan at 6.5% is approximately $511,000 — more than the original loan amount. This is why extra payments early in a mortgage are so powerful.
Taxes
Property taxes are collected by your lender monthly (1/12 of your annual tax bill) and held in an escrow account, then paid on your behalf when due. Property tax rates vary widely — from under 0.5% in Hawaii to over 2% in Illinois and New Jersey. On a $400,000 home in a state with a 1.2% rate, that's $4,800/year or $400/month added to your payment.
Insurance
Homeowners insurance is also typically escrowed by your lender. Annual premiums average $1,000–$2,000 for most single-family homes depending on location, home value, and coverage. Flood insurance (required in FEMA flood zones) is separate and can add significantly to this cost. If your down payment is less than 20%, PMI (Private Mortgage Insurance) is also added here — typically 0.5–1.5% of the loan amount annually.
Fixed Rate vs. Adjustable Rate Mortgages
The two main mortgage types differ in how the interest rate behaves over time. The right choice depends on how long you plan to stay in the home and your tolerance for payment uncertainty.
Fixed-Rate Mortgage
The interest rate is locked for the entire loan term — 15, 20, or 30 years. Your P&I payment never changes. This predictability is the primary advantage: you know exactly what your payment will be in year 1 and year 30. The tradeoff is that fixed rates are usually slightly higher than the initial rate on an ARM, because the lender is bearing the risk of rate changes.
Best for: Buyers who plan to stay in the home long-term, buyers who prioritize payment stability, and any environment where rates are near historic lows (locking in a low rate is a significant financial advantage over decades).
Adjustable-Rate Mortgage (ARM)
An ARM starts with a fixed rate for an initial period (commonly 5, 7, or 10 years), then adjusts periodically based on a market index. A "5/1 ARM" is fixed for 5 years, then adjusts every 1 year after that. ARMs typically offer a lower initial rate than a 30-year fixed — often 0.5–1.0% lower — which can mean meaningfully lower payments in the early years.
Best for: Buyers who plan to sell or refinance before the fixed period ends, buyers stretching to afford a home in a high-rate environment expecting rates to fall, and buyers confident their income will rise if the rate adjusts upward.
The risk: If rates rise significantly during the adjustment period, your payment can increase substantially — sometimes hundreds of dollars per month. Most ARMs have annual and lifetime caps on how much the rate can increase, but those caps can still allow significant payment increases.
🏡 $400,000 Loan — 30-Year Fixed vs. 5/1 ARM at 6.5% Fixed / 5.75% ARM
The Real Cost of a 30-Year vs. 15-Year Mortgage
The choice between a 15-year and 30-year mortgage is one of the most impactful financial decisions a homeowner makes. The monthly payment difference is significant — but the total cost difference is staggering.
📊 $400,000 Loan at 6.5% — 15 vs. 30 Years
The 15-year mortgage costs $957 more per month — but saves nearly $284,000 in total interest and has the loan paid off in half the time. The right answer depends on your income stability, other financial goals (maxing retirement accounts, for example), and how much flexibility you want in your monthly budget. Many financial planners suggest: if you can comfortably afford the 15-year payment with room to spare, the interest savings almost always justify it. If the 15-year payment would strain your budget, the 30-year gives you flexibility.
How Extra Payments Dramatically Reduce Your Loan
One of the most powerful and underused mortgage strategies is making extra principal payments — even small ones. Because of how amortization works, extra payments early in a loan eliminate future months of interest on that principal, compounding the savings over the remaining life of the loan.
On a $400,000 loan at 6.5% over 30 years, adding just $200/month in extra principal payments reduces the loan term by approximately 5 years and saves over $80,000 in total interest. Adding $500/month extra cuts about 9 years off the loan and saves roughly $160,000. The earlier in the loan you start making extra payments, the greater the impact — because you eliminate more future compounding of interest.
When making extra payments, always specify in writing (or online) that the extra amount should be applied to principal — not toward next month's payment. If you don't specify, some servicers will apply it as a prepayment of the next installment, which doesn't reduce your balance the same way.
How Much House Can You Afford?
Lenders use two ratios to determine how much they'll lend you. Understanding these before you shop prevents the disappointment of falling in love with a home you don't qualify for.
The 28% Front-End Ratio
Your total monthly housing payment (PITI — principal, interest, taxes, insurance) should not exceed 28% of your gross monthly income. If you earn $7,500/month before taxes, the maximum PITI payment most conventional lenders will approve is $2,100/month. Above this, you'd need compensating factors (large down payment, excellent credit, substantial reserves) to qualify.
The 36% Back-End Ratio (DTI)
Your total monthly debt payments — including the mortgage, car loans, student loans, credit cards, and any other recurring debt obligations — should not exceed 36% of gross monthly income. So if you earn $7,500/month and have $500/month in car payments and student loan payments, you have $7,500 × 36% = $2,700 total debt budget minus $500 existing debt = $2,200 maximum mortgage payment. Many lenders have relaxed the back-end ratio to 43% or even 50% for well-qualified borrowers, but keeping it under 36% gives you the most loan options and best rates.
Understand Your LTV Ratio
Your loan-to-value ratio determines whether you need PMI and affects your interest rate. Read our complete LTV guide to understand exactly how it works.
Read the LTV Guide →When Does Refinancing Make Sense?
Refinancing replaces your existing mortgage with a new one — typically to get a lower rate, change the loan term, or access equity. The decision comes down to one number: the break-even point.
Refinancing costs money upfront (typically 2–5% of the loan amount in closing costs). You need to stay in the home long enough for the monthly savings to exceed those upfront costs. If refinancing saves you $300/month but costs $9,000 in closing costs, your break-even is 30 months (2.5 years). If you plan to stay longer than that, the refinance makes financial sense. If you might sell before then, it likely doesn't.
The old rule of thumb was "refinance when rates drop 1%." The actual test is break-even analysis: calculate your monthly savings, divide the closing costs by the savings, and that's how many months until you break even. Use that number to decide.
Frequently Asked Questions
How is my monthly mortgage payment calculated?
Your base payment (principal + interest) uses the amortization formula: M = P × [r(1+r)^n] ÷ [(1+r)^n − 1], where P is the loan amount, r is the monthly rate (annual rate ÷ 12), and n is total payments (years × 12). The total monthly cost also includes property taxes, homeowners insurance, HOA fees, and PMI if applicable — all shown in this calculator.
Should I choose a 15-year or 30-year mortgage?
If you can comfortably afford the higher 15-year payment, you'll save enormously in interest and own the home outright in half the time. On a $400,000 loan at 6.5%, the 15-year saves nearly $284,000 in interest. If the higher payment would strain your budget or prevent you from building an emergency fund or maxing retirement accounts, the 30-year gives you flexibility — you can always make extra principal payments when cash flow allows.
What is PMI and when can I remove it?
PMI (Private Mortgage Insurance) is required when your down payment is less than 20%. It protects the lender — not you — and typically costs 0.5–1.5% of the loan amount annually. Federal law (Homeowners Protection Act) requires automatic PMI cancellation when your loan balance reaches 78% of the original purchase price based on the original amortization schedule. You can request cancellation at 80% LTV, or get a new appraisal if home values have risen enough to establish 80% equity based on current market value.
How much house can I afford?
The standard guideline is the 28/36 rule: total housing payment (PITI) ≤ 28% of gross monthly income, total debt payments ≤ 36%. At $7,500/month gross income, maximum PITI is $2,100. Use this calculator to find a home price and down payment combination that keeps your payment comfortably within your income. Most mortgage advisors also recommend keeping at least 3–6 months of expenses in liquid savings after your down payment — don't drain reserves to buy more house.
What credit score do I need to get a mortgage?
Conventional loans typically require a minimum score of 620. FHA loans can go as low as 580 (or 500 with a 10% down payment). However, the rate you get depends heavily on your score: a 760+ score can get you a rate 0.5–1.0% lower than a 660 score, which translates to tens of thousands of dollars in savings over 30 years. Before applying, it's worth spending 6–12 months improving your score if it's below 740.
What is an amortization schedule?
An amortization schedule shows every payment in the life of the loan, broken into how much goes to principal and how much goes to interest. In the early years, the vast majority of each payment goes to interest. By the final years, almost all of it goes to principal. The schedule above shows this year by year. It's useful for understanding exactly how much equity you build over time and how extra payments shift that balance.