How to Calculate Mortgage Payments (With Real Examples)

Your mortgage payment is the largest math problem most adults ever solve. A 1% rate difference over 30 years can mean $80,000+. Understanding the math saves you from being talked into bad loans, lets you compare offers, and helps you plan your real housing budget.

This guide shows you the formula, walks through real examples, and explains all the parts of “PITI” — the four pieces of a typical mortgage payment.

The Big Picture

Your monthly mortgage payment usually includes four things, summarized as PITI:

• Principal: the amount paying down your loan balance.
• Interest: the bank’s fee for lending you money.
• Taxes: property tax, paid into escrow.
• Insurance: homeowners insurance, paid into escrow.

Plus, in some cases:
– PMI (private mortgage insurance) if your down payment is under 20%.
– HOA fees (paid separately, not in the mortgage).

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The “principal + interest” math is what we’ll focus on — that’s the part dictated by the loan amount, rate, and term.

The Mortgage Payment Formula

\[M = P \times \frac{r(1+r)^n}{(1+r)^n – 1}\]

Where:
– \(M\) = monthly principal and interest payment.
– \(P\) = loan amount (after down payment).
– \(r\) = monthly interest rate (annual rate ÷ 12, as a decimal).
– \(n\) = total number of monthly payments (years × 12).

Yes, it looks ugly. You’ll never compute it by hand in real life — but knowing the formula explains what changes when each input changes.

Worked Example #1 — A Standard Mortgage

You buy a $400,000 home with 20% down ($80,000).
Loan amount: $320,000.
Rate: 7% (annual).
Term: 30 years.

• \(r = 0.07 / 12 = 0.005833\)
• \(n = 30 \times 12 = 360\)
• \(P = 320{,}000\)

\[M = 320000 \times \frac{0.005833(1.005833)^{360}}{(1.005833)^{360} – 1}\]

\[M \approx \$2{,}129/\text{month}\]

Over 30 years, you’ll pay:
– $2,129 × 360 = **$766,440 total.
– Of which: $320,000 principal + $446,440 interest.**

You’re paying the bank more in interest than the price of the house.

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Worked Example #2 — Same House, Lower Rate

Same $400,000 home, $80,000 down, $320,000 loan, but 5% rate instead of 7%.

$M \approx \$1{,}718/\text{month}$

• Monthly savings vs. 7%: $411.
• Total over 30 years: $618,480.
• Total interest savings vs. 7%: ~$148,000.

A 2% rate difference saves you almost half the home’s purchase price in interest. This is why “rate shopping” matters — even 0.25% adds up.

Worked Example #3 — 15-Year vs. 30-Year

Same $320,000 loan at 6%, two different terms:

30-year mortgage:
– Monthly payment: $1,919.
– Total interest: $370,840.

15-year mortgage:
– Monthly payment: $2,700.
– Total interest: $166,000.

The 15-year is $781 more per month** but saves **$204,000 in interest and you own the house in half the time.

Many financial planners recommend a 15-year mortgage if you can comfortably afford the higher payment. Most people choose 30-year for flexibility.

How Amortization Works

Each monthly payment splits between principal and interest — but the split changes over time.

Early in the loan

Most of your payment is interest. On a 30-year, 7% mortgage of $320,000:
– Month 1 payment: $2,129.
– Interest portion: $1,867.
– Principal portion: $262.

You barely chip at the balance.

Mid-loan (year 15)

• About half interest, half principal.

Final years

Most of your payment goes to principal. The interest portion is tiny.

Implication: If you sell the house in 5-7 years, you’ve barely paid down any principal. Most of your money went to interest. This is why “renting is throwing money away” is an oversimplification.

The Full PITI Math

A typical payment includes:

Principal & Interest (P&I)

From the formula above.

Property Tax (T)

Usually 0.5%-2.5% of home value annually, depending on state.
– $400,000 home × 1.2% = $4,800/year = $400/month.

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Homeowners Insurance (I)

Roughly $1,200-$2,000/year for a $400,000 home.
– $1,500/year = **$125/month.**

Total PITI (using our example)

• P&I: $2,129
• T: $400
• I: $125
• Total: $2,654/month.

The P&I number alone undercounts your real housing cost by $500+.

When PMI Kicks In

If your down payment is less than 20%, lenders charge Private Mortgage Insurance to protect themselves.

• Typical PMI: 0.5%-1.5% of loan balance per year.
• On a $320,000 loan at 1%: **$3,200/year = $267/month.**

You can usually drop PMI once you reach 20% equity, but you have to request it.

Smart Mortgage Math Strategies

1. Make one extra payment per year

On a 30-year mortgage, this cuts the term by ~4-5 years and saves tens of thousands in interest.

2. Round up to the nearest $50 or $100

Easy mental commitment. On a $2,129 payment, paying $2,200 each month chips at principal extra.

3. Use the biweekly trick

Pay half your monthly amount every 2 weeks instead of monthly. Result: 26 half-payments = 13 full payments per year — one extra.

4. Refinance when rates drop 0.75%+

The break-even point is usually 18-30 months. If you’ll stay longer, refinancing wins.

5. Don’t buy more house than you can comfortably afford

Lenders will approve you for more than you should borrow. Use the 28/36 rule: housing payment ≤ 28% of gross income; total debt payments ≤ 36%.

Common Mortgage Math Mistakes

Looking only at the monthly payment

A lower monthly payment from a 40-year mortgage costs you a fortune in interest over time.

Forgetting taxes and insurance

A $1,800 P&I payment can become a $2,500 PITI payment. Plan for the real number.

Ignoring closing costs

Closing costs are typically 2-5% of the loan amount — on a $320,000 loan, that’s $6,400-$16,000. Often rolled into the loan, adding interest.

Underestimating maintenance

A homeowner spends 1-3% of home value per year on maintenance and repairs. Budget for it separately.

Overweighting tax deductions

The mortgage interest deduction is real but smaller than people think — and only matters if you itemize. For most middle-income filers, the standard deduction is bigger.

How Much House Can You Afford?

Use the 28/36 rule:
– Total monthly PITI ≤ 28% of gross income.
– Total debt payments (PITI + cards + car + student loans) ≤ 36% of gross income.

Example

• Household income: $100,000/year = $8,333/month gross.
• 28%: $2,333/month max for PITI.
• 36%: $3,000/month max for all debt payments.

If you want a $2,500 PITI, you need ~$108,000 income at the 28% cap.

Adjustable vs. Fixed Rate

Fixed rate

• Rate locked for the full term.
• Predictable monthly payment.
• Higher initial rate than ARMs.

Adjustable Rate Mortgage (ARM)

• Lower initial rate (e.g., 5/1 ARM: fixed for 5 years, then resets annually).
• Risk: rate jumps later.
• Worth it only if you’re sure you’ll sell or refinance before the reset.

For most homeowners, fixed-rate is the safer math.

Free Resources

Effortless Math has financial math practice for all levels:

• Math Blog — financial math guides.
• Math Topics Library — exponents, percents, real-world math.
• SAT Math Resources — compound interest and mortgage math appear on standardized tests.

Frequently Asked Questions

What’s a “good” mortgage rate in 2026?
Depends on the market. In 2024-2026, “good” was anywhere from 5.5% to 7.5%. Watch the 10-year Treasury — mortgage rates roughly track it.

Should I do a 15- or 30-year mortgage?
30-year is safer (lower payment, more flexibility). 15-year saves massive interest if you can comfortably afford the higher payment.

How much should I put down?
20% avoids PMI. Less is fine for first-time buyers — just budget for PMI.

What’s the difference between APR and interest rate?
Interest rate = the rate on the loan. APR = interest rate + fees, annualized. Always compare APRs across lenders.

Can I pay off my mortgage early?
Yes, in most cases. Some loans have prepayment penalties — read the contract.

Is it better to invest or pay down the mortgage?
Depends on your rate and risk tolerance. At a 4% mortgage, investing (long-term ~7-10% returns) usually wins. At 8%+, paying down the mortgage often wins.

A Mortgage Is a Math Problem, Not a Marketing Pitch

Lenders are good at making bad loans sound good. The formula doesn’t lie. Run the numbers yourself. Compare PITI, not just P&I. Shop rates aggressively. Make at least one extra payment a year. Over 30 years, your willingness to do the math saves you the price of a second home.

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