The Complete Guide to Mortgage Calculations (2026)

What a Mortgage Actually Is

A mortgage is not a single product — it's a contract that bundles a loan, a lien on your home, an interest rate, an amortization schedule, and (usually) an escrow account for taxes and insurance. When you pay your monthly bill you are paying four distinct things, often called PITI: Principal, Interest, Taxes, and Insurance. Understanding which part of your payment goes where is the foundation of every other calculation in this guide.

The Monthly Payment Formula (and Why It Looks Scary)

The standard fixed-rate mortgage payment formula is 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 number of monthly payments (years × 12). It looks intimidating but it's just compound interest in reverse: the bank is solving for the constant payment that, with interest, exactly drains the loan to zero by month n. A $400,000 loan at 6.5% for 30 years gives M ≈ $2,528.27/month — and over the life of the loan you will pay roughly $510,000 in interest alone.

Amortization: Why Your Early Payments Barely Touch Principal

In the first month of that $400,000 loan, your $2,528 payment splits into about $2,167 of interest and only $361 of principal. By year 15, the split is roughly 50/50. By the final year, almost the entire payment is principal. This is why making one extra principal payment per year can shave 5–7 years off a 30-year mortgage — every dollar of extra principal eliminates all the future interest that would have compounded on it.

APR vs. Interest Rate (the Most Misunderstood Number)

The interest rate is what the bank charges on the balance. The APR (Annual Percentage Rate) is the interest rate plus most fees — origination, discount points, mortgage insurance — expressed as an annualized cost. A 6.25% rate with $8,000 in fees can have a higher APR than a 6.5% rate with no fees. Always compare loans by APR, not by the headline rate, and ask the lender for the Loan Estimate (page 3) where APR is disclosed.

PMI, MIP and the 20% Down Myth

If your down payment is under 20%, conventional lenders charge Private Mortgage Insurance (PMI), typically 0.3%–1.5% of the loan annually, paid monthly. On a $400,000 loan at 0.6% PMI, that's $200/month — pure cost, building no equity. PMI auto-cancels at 78% loan-to-value on conventional loans, but FHA mortgage insurance (MIP) usually stays for the life of the loan unless you refinance. Calculate the breakeven: sometimes a slightly higher rate with no PMI beats a lower rate with years of insurance.

Escrow, Property Tax and Homeowners Insurance

Most lenders require an escrow account that collects 1/12 of your annual property tax and homeowners insurance with each payment. On a $500,000 home with 1.2% property tax ($6,000/yr) and $1,500/yr insurance, that's $625/month on top of principal and interest. Escrow accounts are reanalyzed yearly — when tax assessments rise, your monthly payment can jump by hundreds of dollars even on a fixed-rate loan.

Refinancing: The Real Breakeven Math

The right way to evaluate a refinance is breakeven months = total closing costs ÷ monthly savings. If refinancing saves you $220/month and closing costs are $6,600, breakeven is 30 months. If you plan to stay in the home longer than that, refinancing wins. Watch out for two traps: (1) restarting a 30-year clock erases years of amortization progress, and (2) rolling closing costs into the loan inflates the breakeven calculation.

Fixed vs. Adjustable Rate (ARM): When Each Wins

A 30-year fixed gives certainty; an ARM (commonly 5/1, 7/1, or 10/1) offers a lower initial rate that adjusts after the intro period. ARMs make sense when (a) you're confident you'll sell or refinance before the adjustment, or (b) the spread between fixed and ARM is wider than ~0.75%. They become dangerous when buyers stretch into a home they can only afford at the teaser rate.

How Much House Can You Actually Afford?

The classic rules: housing costs (PITI) under 28% of gross income, total debt under 36% (the '28/36 rule'). Lenders will often approve more — sometimes much more — but approval ≠ affordability. Build the full monthly picture: PITI + HOA + maintenance (estimate 1% of home value per year) + utilities. A $4,000 mortgage payment usually means a $5,200 true monthly housing cost.

Common Mistakes I See Every Week

1) Comparing rate instead of APR. 2) Ignoring the impact of points. 3) Forgetting that property taxes will rise. 4) Refinancing into a new 30-year term and celebrating a lower payment while extending total interest. 5) Putting exactly 20% down and draining emergency savings — keeping 6 months of reserves is almost always worth a small PMI premium for a year or two.

The Formula

Fixed-Rate Mortgage Payment Formula

Example Calculation

You borrow $400,000 at a 6.5% fixed rate for 30 years.

1. 1Monthly rate r = 0.065 / 12 = 0.0054167
2. 2Number of payments n = 30 × 12 = 360
3. 3(1+r)^n ≈ 7.0466
4. 4M = 400,000 × (0.0054167 × 7.0466) / (7.0466 − 1) ≈ $2,528.27
5. 5Total paid over 30 years ≈ $910,178 → interest ≈ $510,178

Use the Calculator

Put this knowledge into practice with our free calculators:

• 🏠 Mortgage Calculator
• 📈 Compound Interest Calculator
• 📜 Amortization Calculator