📖 Guide

Mortgage Basics: How Home Loans Work

PITI, amortization, PMI, escrow, refinancing. The parts of a mortgage that most first-time-buyer articles skip or get wrong, explained with the actual math.

The Number Most Buyers Never Calculate

A 30-year mortgage on $320,000 at 6.5% sends $728,000 from your bank account to the lender over the life of the loan. The interest portion: $408,000. That is one of the most expensive purchases you will sign for in your life, and most buyers walk in without seeing that number.

Mortgages are not a scam. For most households, borrowing money to buy a house is the only path to ownership, and a paid-off home at 60 beats renting at 75. The trap sits in the math of how each monthly payment splits between interest and principal. Get that wrong and you can refinance at the wrong moment, stretch a 20-year budget into a 30-year loan, or walk away from a $1,200 closing-cost negotiation because the salesperson reframed it as $3 per month.

This guide covers three things: what your payment is made of, how the math works, and the structural traps your lender's brochure leaves out.

PITI: The Four Parts of Your Payment

When a lender quotes your monthly payment, the number bundles four separate items, often called PITI:

Principal. The portion that reduces what you owe. In year one of a 30-year loan, this slice is small.
Interest. What the lender charges for the loan. The lender calculates this each month on the remaining balance, so early payments are mostly interest.
Taxes. Property taxes, paid to your county. Your lender collects these into an escrow account and pays the tax bill on your behalf.
Insurance. Homeowner's insurance, also escrowed. Every lender requires it, since they own your collateral until you pay off the loan.

Two more parts can show up:

PMI (Private Mortgage Insurance). Required if your down payment is less than 20% of the home's value. Premiums run 0.3% to 1.5% of the loan amount per year. The lender drops PMI once you reach 78% loan-to-value.
HOA dues. Condos, townhomes, and many planned communities collect these directly. They do not flow through your lender.

Buyers who treat the mortgage as principal and interest get hit the first time their county reassesses property taxes 30% upward, or the first time their insurer raises premiums 18% after a regional weather year. Both happen. Budget the full PITI.

The Amortization Formula and What Falls Out of It

For a fixed-rate loan, your monthly principal and interest stay the same for the life of the loan. The formula that produces that constant number:

M = P × [ r(1+r)ⁿ ] / [ (1+r)ⁿ − 1 ]

M is the monthly payment, P is the loan amount, r is the monthly interest rate (annual rate / 12), n is the number of monthly payments (years × 12). The formula looks brutal. The idea is plain: find the constant monthly payment that, applied for n months, pays off a loan of P at rate r.

The surprise comes when you walk through the numbers. The payment stays the same. The split between principal and interest moves dramatically. In month one, the lender calculates interest on the full balance, so most of your payment goes to interest. As you knock down the principal, next month's interest is smaller, the principal portion grows, and the loan starts collapsing toward zero in the back half.

Worked Example: $400,000 Home, 20% Down, 6.5% APR

What your first payment buys you

Loan: $320,000. Rate: 6.5%. Term: 30 years.

Monthly principal + interest: $2,022. Across 360 payments, that totals $727,978. The lender keeps $407,978 of that as interest. More than the home's original price.

Look at month 1 against month 240 (year 20):

Month	Interest portion	Principal portion	% to principal
Month 1	$1,733	$289	14%
Month 60 (year 5)	$1,635	$388	19%
Month 120 (year 10)	$1,496	$526	26%
Month 180 (year 15)	$1,303	$719	36%
Month 240 (year 20)	$1,043	$979	48%
Month 300 (year 25)	$683	$1,339	66%
Month 359 (last)	$11	$2,011	99%

If you sell after four years, you built almost no equity through payments. Whatever equity shows up on paper came from price appreciation. A 2007-style price drop wipes out years of payments overnight, which is how a generation of 2008 buyers found themselves underwater.

15-Year vs 30-Year: The Trade-Off

The 30-year mortgage is the American default and one of the most expensive forms of debt most households carry. A 15-year mortgage usually comes with a rate 0.5 to 1.0 percentage points lower, and the shorter term combined with the lower rate slashes lifetime interest. Same $320,000 loan, three term options:

Term	Rate	Monthly P&I	Total interest
30-year	6.50%	$2,022	$407,889
20-year	6.25%	$2,339	$241,329
15-year	5.75%	$2,658	$158,392

The 15-year saves $250,000 in interest. The catch: the extra $636 each month has to come from somewhere. Over 15 years that adds up to $115,000 you cannot put into a 401(k), an emergency fund or a kid's college savings.

The honest framing: pick the shortest term you can carry without strangling the rest of your budget. For most middle-income buyers, that lands at 20 or 25 years.

The Extra-Payment Hack

You don't need a 15-year mortgage to get most of the interest savings. Any extra principal payment reduces the balance that every future month's interest sits on. The math compounds in your favor.

On the $320,000 / 6.5% / 30-year loan, sending an extra $200 per month to principal pays the loan off in 22 years and 6 months, and saves $117,000 in interest. The rate on those extra dollars is a guaranteed 6.5%. Most savings accounts and bonds cannot match it.

One real-world warning: tell your servicer the extra is "principal only." Some servicers default to applying it to next month's payment, which doesn't reduce interest at all. A short note on the check or memo field on a payment portal handles it.

PMI: Insurance You Pay for the Lender's Benefit

If your down payment is below 20%, the lender requires Private Mortgage Insurance. The premium protects the lender, not you. You pay it; they collect it.

PMI usually runs 0.3% to 1.5% of the loan amount per year. On a $400,000 loan at 0.6% PMI, you pay $200 per month, $2,400 per year, of pure cost with no equity built. The good news: federal law forces the lender to remove PMI at 78% loan-to-value, calculated against your original purchase price. You can request removal earlier at 80% LTV. Track this date. Lenders are slow about it unless prompted. A polite letter at the 80% mark is worth $200 per month.

A common question: should you put 20% down to avoid PMI? Sometimes yes, sometimes no. Parking $80,000 into a down payment means $80,000 that does not sit in an emergency fund or a brokerage account. If your alternative is a low-fee index fund averaging 7% real returns, that $80,000 grows to roughly $610,000 over 30 years. The PMI you would have avoided sums to $15,000 to $25,000. The 20% rule is conservative finance, not automatic optimization.

Refinancing: When the Math Works

Refinancing means paying off your current mortgage with a new one at a lower rate. Closing costs run 2% to 3% of the loan amount, so the break-even math matters.

Rule of thumb: refinance if the new rate is at least 0.75 to 1.0 percentage points lower than your current rate, and you will stay in the home long enough to recover the closing costs. Math: closing costs divided by monthly payment savings. If closing costs run $6,000 and the new payment drops $250 per month, you break even at month 24. Anything past that is savings.

The mistake people make: they refinance from a 30-year loan that is 8 years in, into a new 30-year loan. The monthly payment drops. The total term stretches to 38 years and lifetime interest goes up. If you refinance to a lower rate, also reduce the term. Refinance a 30 with 22 years left into a 20-year loan, not a fresh 30.

The 28/36 Affordability Rule

Lenders, financial planners and most realistic budget books point to the 28/36 rule:

28%. Total monthly housing payment (PITI + PMI + HOA) should stay below 28% of gross monthly income.
36%. Total monthly debt payments (housing plus car, student loans, minimum credit card payments) should stay below 36% of gross income.

A household earning $120,000 per year has $10,000 in monthly gross income. The 28/36 ceiling: $2,800 in housing, $3,600 in total debt. Lenders often pre-approve higher amounts than these rules suggest, because the lender's risk model optimizes for "what can you pay" rather than "what is sustainable." The gap between those numbers is where house-poor households live.

House-poor is not a metaphor. It is a budget state where the mortgage payment crowds out emergency savings, retirement contributions, normal life, and the small repair fund every house needs. The bank will lend at the edge. You don't have to borrow there.

Common Mistakes That Cost the Most

Treating the pre-approval as a target. The pre-approval is a ceiling, not a recommendation. Buy below it.
Skipping the inspection in a bidding war. Saves $500. Costs an average $4,500 in undetected issues, with worst-case six-figure surprises.
Comparing rates without comparing APR. APR folds in fees; the headline rate does not. Always compare lenders by APR.
Forgetting closing costs. 2% to 5% of the loan amount, paid at signing on top of your down payment. On a $320,000 loan, that lands at $6,400 to $16,000 you also need to bring.
Ignoring property tax differences. Two homes at the same price can have wildly different monthly costs because of school-district tax rates. A house with $9,000 annual taxes costs $375 more per month than the same house with $4,500 taxes.
Treating HOA dues as static. HOAs raise fees and can pass special assessments for big projects, like roofs and parking lots. Read recent HOA financials before buying any property with shared infrastructure.

Frequently Asked Questions

What's the difference between APR and interest rate?

Interest rate is the percentage you pay on the loan balance. APR (Annual Percentage Rate) folds in lender fees, origination charges, mortgage insurance and points, then expresses the true cost as a single annual percentage. Two loans at the same 6.5% rate can cost very different amounts. Compare lenders by APR.

Should I put 20% down to avoid PMI?

Sometimes. If 20% drains your emergency fund or stops your retirement contributions, the wiser move is 10-15% down, keep the cash reserves, and pay PMI until you reach 78% loan-to-value. PMI is an annoying cost. An empty emergency fund is a financial disaster.

Does the calculator include closing costs?

No. Closing costs are a one-time expense at signing, separate from the monthly payment. Plan for 2-5% of the loan amount on top of your down payment.

What about FHA, VA or USDA loans?

This guide covers conventional fixed-rate mortgages. FHA loans use Mortgage Insurance Premium (MIP) instead of PMI, often for the life of the loan if the down payment is under 10%. VA loans waive PMI for eligible veterans. USDA loans have geographic restrictions. The principal-and-interest math is identical. The insurance line changes.

Should I pay extra to principal or invest the money?

If your mortgage rate sits meaningfully higher than the after-tax return you would earn on the alternative, extra principal wins on guaranteed math. If you trust a diversified stock portfolio to return 7% real over the long term, investing wins on expected math. Most households split the difference. Don't over-optimize this past a few hundred dollars per month. The certainty of paying down a 6%+ mortgage has real psychological value.

What is escrow?

An account your lender maintains. You contribute to it each month, and the lender pays your property tax bill and insurance premium on your behalf when they come due. Most loans require escrow if your down payment is under 20%. Lenders refund over-collection. In practice this shows up as a small refund check each spring.

Can my property tax change after I buy?

Yes. Most US counties reassess every 1-5 years based on market value. Some states cap annual increases regardless of market changes. California's Prop 13 is the famous one. Look up your county's rules. A 30% jump in property tax after a fast-growing area's first post-purchase reassessment is not unusual.