The Complete Guide to Mortgage Calculations
A mortgage is likely the largest financial commitment you will ever make. On a $400,000 30-year mortgage at 7%, you pay $558,000 in total interest over the life of the loan — nearly 1.4× the original loan amount. Understanding exactly how your mortgage works, where every dollar goes, and how small changes dramatically alter the total cost is the foundation of smart homebuying.
Understanding PITI: The True Cost of Your Monthly Payment
Your mortgage payment is more than just principal and interest. A complete PITI payment includes: Principal (reducing your loan balance), Interest (the lender's cost), Taxes (property taxes held in escrow by your lender), and Insurance (homeowner's insurance and PMI if applicable). This calculator shows all four components so you see your real monthly obligation, not just the advertised P&I figure.
The Amortization Tipping Point: When You Start Building Real Equity
Here is the number most lenders never show you: on a 30-year mortgage at 7%, your payments are front-loaded with interest. In your very first payment, approximately 80 cents of every dollar goes to interest, not principal. The tipping point — when more of each payment goes toward principal than interest — does not occur until around year 18–19 (month 223 of 360). This is why refinancing in the last decade of a 30-year mortgage rarely makes financial sense: you have already paid most of the interest.
PMI: What It Costs and When It Goes Away
Private Mortgage Insurance (PMI) is required when your down payment is less than 20% of the home's purchase price. It typically costs 0.5%–1.5% of the loan amount annually — on a $400,000 loan, that is $2,000–$6,000 per year added to your payment. The good news: by federal law (the Homeowners Protection Act), PMI must be automatically canceled when your loan-to-value ratio reaches 78%. You can proactively request removal at 80% LTV. Our calculator shows the exact month when PMI drops off.
The Power of Extra Payments
Extra principal payments have an outsized effect because they reduce the balance on which future interest is calculated. An extra $200/month on a $400,000 loan at 7% saves approximately $84,000 in total interest and cuts 5+ years off your loan term. Even a single extra payment per year (making 13 instead of 12 annual payments) saves roughly $55,000 and shortens a 30-year loan by over 4 years.
15-Year vs. 30-Year Mortgage: The Real Numbers
A 15-year mortgage at 6.5% on $400,000 has a monthly P&I of approximately $3,488 vs. $2,528 for 30 years at 7%. The 15-year costs $227,840 in total interest vs. $510,000 for 30 years — a savings of $282,160. The tradeoff: $960 more per month. If you can comfortably afford the higher payment, the 15-year mortgage is mathematically superior. Use the amortization table to compare both scenarios.
The 28/36 Rule for Mortgage Affordability
Most US mortgage lenders use the 28/36 rule: your monthly housing payment (PITI) should not exceed 28% of gross monthly income, and your total debt payments (housing + car + student loans + credit cards) should not exceed 36%. On a $100,000 annual salary ($8,333/month), the maximum recommended housing payment is $2,333 and total debt obligations are capped at $3,000. See the Affordability tab for your personalized assessment.
When Should You Refinance?
Refinancing makes sense when: (1) rates drop at least 0.75–1% below your current rate, (2) you plan to stay in the home long enough to recoup closing costs (typically 2–3 years), and (3) you are not deep into your loan term where most interest is already paid. Use the Extra Payments analyzer to compare accelerating your current mortgage vs. the cost of refinancing.