How a Mortgage Works: Amortization, Interest & Monthly Payments Explained

When a lender quotes you a rate and a term, they're really handing you three inputs to a single formula. Everything else — your monthly payment, your total interest bill, your amortization schedule — falls out of that formula automatically. Once you understand it, a mortgage stops feeling like a black box.

The monthly payment formula

Your fixed monthly principal-and-interest (P&I) payment is calculated with the standard loan amortization formula:

M = P × r(1 + r)ⁿ / ((1 + r)ⁿ − 1)

Where:

• M = monthly payment
• P = principal (the amount borrowed)
• r = monthly interest rate = annual rate ÷ 12
• n = total number of monthly payments = years × 12

Worked example

Suppose you borrow $320,000 at 6.5% APR over 30 years:

• r = 0.065 ÷ 12 = 0.005417
• n = 30 × 12 = 360
• M = 320,000 × 0.005417 × (1.005417)³⁶⁰ / ((1.005417)³⁶⁰ − 1)
• M ≈ $2,022.62 per month

Over the full 30 years you'll pay $2,022.62 × 360 = $728,143 in total — more than twice the original loan. The extra $408,143 is pure interest.

Why early payments are mostly interest

Every month, your interest charge is your remaining balance × the monthly rate. In month one of the example above:

• Interest: $320,000 × 0.005417 = $1,733
• Principal: $2,022.62 − $1,733 = $290

Only $290 — about 14% of your payment — actually reduces the loan in month one. The rest goes to the lender as interest. This proportion flips gradually over time. By year 25 you'd be paying around $1,500 in principal and $500 in interest each month.

This is the essence of amortization: the payment stays constant, but its composition shifts. Early payments are heavy on interest because the balance is large; late payments are heavy on principal because the balance is small.

What "amortization schedule" means

An amortization schedule is simply a table showing, for every period, the split between principal and interest and the resulting balance. Looking at the yearly summary of our $320,000 loan:

• Year 1: you pay ~$20,800 in interest, ~$3,470 in principal. Balance falls to $316,530.
• Year 10: you pay ~$19,300 in interest, ~$4,970 in principal. Balance: $282,500.
• Year 20: you pay ~$16,300 in interest, ~$7,970 in principal. Balance: $213,000.
• Year 30: you pay ~$250 in interest and the remaining ~$2,000 in principal. Balance: $0.

Notice how slowly the balance falls at first. After ten years of payments you still owe about 88% of the original loan. This surprises many homeowners who expect to have made a bigger dent.

How a shorter term changes everything

Switching from a 30-year to a 15-year term on the same $320,000 at 6.5%:

• Monthly payment rises from $2,022 to $2,788 (+$766/month)
• Total interest drops from $408,143 to $181,740
• Lifetime saving: $226,400

You pay $766 more every month but save over $226,000 in interest. The break-even intuition: each extra dollar of principal you pay off today eliminates all future interest that would have compounded on it.

What your payment doesn't include

The formula gives you principal and interest only. Your actual monthly housing cost adds:

• Property tax — varies widely by location (typically 0.5–2% of assessed value per year)
• Homeowners insurance — typically $100–$200/month depending on property value and location
• PMI (private mortgage insurance) — required in the US when your down payment is under 20%; typically 0.5–1.5% of the loan per year until you reach 20% equity
• HOA fees — if applicable, can range from $50 to $1,000+/month
• Maintenance — a common rule of thumb is 1% of home value per year

A $320,000 loan at 6.5% has a $2,023 P&I payment, but total monthly housing costs on a $400,000 home might realistically be $3,000–$3,500 once you add the items above.

Making extra payments

Because interest accrues on the remaining balance, extra principal payments have an outsized effect. Adding $200/month extra to our example loan:

• Loan paid off in about 24 years instead of 30 (6 years early)
• Total interest saved: roughly $75,000

Many lenders allow one extra payment per year applied entirely to principal (a so-called "13th payment"). On a 30-year mortgage this alone typically shortens the loan by 4–5 years.

Check your mortgage agreement: some have prepayment penalties that cap extra payments or charge a fee. These are less common today but still exist, especially on certain fixed-rate products in the UK and Canada.

Interest rate vs. APR

The rate the formula uses is the nominal interest rate. Lenders are also required to disclose the APR (Annual Percentage Rate), which bundles in fees like origination charges, mortgage broker fees, and discount points. APR is always higher than (or equal to) the stated interest rate.

When comparing loan offers, compare APRs — not the headline rate — because a low-rate loan with high fees can be more expensive than a slightly higher-rate loan with no fees. The break-even depends on how long you plan to stay in the home.

Fixed vs. variable rate

Everything above applies to a fixed-rate mortgage where the rate stays constant for the full term. Variable-rate mortgages (ARM in the US, tracker/variable in the UK, ARM/HELOC in Canada, variable in Australia) use the same payment formula but recalculate M when the rate resets — typically annually or at the end of a fixed initial period.

A variable rate is a bet that rates fall or stay flat over your hold period. Fixed is a bet that current rates are a fair price for rate certainty. Neither is universally better; the right choice depends on your timeline and risk tolerance.

Common mistakes

• Comparing monthly payments, not total cost. A 40-year term at 6% looks cheaper monthly than a 25-year term at 6.5%, but total interest is far higher.
• Ignoring opportunity cost. A larger down payment reduces interest but ties up capital. If you can earn more in an index fund than the mortgage rate (after tax), it may not be optimal to pay down the mortgage aggressively.
• Buying at the lender's maximum. Qualifying for a large mortgage doesn't mean it's affordable. Lenders use debt-to-income ratios that don't account for childcare, retirement savings, or irregular expenses.
• Focusing only on the rate. A 0.1% lower rate saves about $6/month on a $320,000 loan. Closing costs of $3,000 take over 40 months to break even at that saving.