Mortgage Calculator: The 4 Numbers That Determine Your Monthly Payment
Reviewed by Jerry Croteau, Founder & Editor
Table of Contents
Most first-time homebuyers focus on the monthly payment number without understanding what drives it. That single number is actually the result of four variables working together — and changing any one of them changes both the payment and the total cost of the home in ways that are not always obvious.
Our mortgage calculator shows the full picture including payment breakdown, amortization schedule, and total interest paid. This guide explains the mechanics behind every input.
The four variables
- Home price — the purchase price you negotiate
- Down payment — how much you pay upfront (reduces the loan amount)
- Interest rate (APR) — the annual cost of borrowing
- Loan term — typically 15 or 30 years
The formula lenders use for a fixed-rate mortgage:
Monthly Payment = P x [r(1+r)^n] / [(1+r)^n - 1]
Where P = loan amount, r = monthly rate (APR / 12), n = total payments
What your payment actually covers: PITI
Most people think of the mortgage payment as principal and interest. Your actual monthly housing payment is usually PITI:
- Principal — reduces your loan balance
- Interest — cost of borrowing (the large portion in early years)
- Taxes — property taxes, collected by lender and paid to the municipality
- Insurance — homeowners insurance, and PMI if down payment is under 20%
When a lender quotes you a payment, clarify whether it includes taxes and insurance (TI). Many online calculators show only PI. The difference can be $400-800/month on a typical home.
How each variable affects your payment
Variable 1: Home price
The starting point. Every $10,000 in purchase price adds approximately $50-65 to your monthly payment at current rates (depending on term and rate). Negotiating $10,000 off the price of a home is worth about $50/month — not as dramatic as it sounds on a monthly basis, but it reduces your total interest paid by roughly $8,000-12,000 over 30 years.
Variable 2: Down payment
| Down payment | Loan amount | Monthly PI (30yr @ 7%) | PMI est. | Total monthly |
|---|---|---|---|---|
| 5% ($20,000) | $380,000 | $2,528 | ~$190 | ~$2,718 |
| 10% ($40,000) | $360,000 | $2,395 | ~$150 | ~$2,545 |
| 20% ($80,000) | $320,000 | $2,129 | $0 | ~$2,129 |
| 25% ($100,000) | $300,000 | $1,996 | $0 | ~$1,996 |
Based on a $400,000 home. PMI estimated at 0.6% of loan amount annually.
The 20% threshold eliminates PMI — a significant savings. PMI on a $380,000 loan at 0.6% annually is $2,280/year ($190/month) that builds no equity and drops off your balance sheet once you reach 20% equity.
Variable 3: Interest rate
| Rate | Monthly PI ($320K, 30yr) | Total interest paid |
|---|---|---|
| 5.5% | $1,817 | $333,944 |
| 6.5% | $2,023 | $408,280 |
| 7.0% | $2,129 | $446,458 |
| 7.5% | $2,237 | $485,474 |
| 8.0% | $2,348 | $525,368 |
The difference between 5.5% and 7.5% on the same $320,000 loan is $420/month and $151,530 in total interest over 30 years. Rate matters more than almost any other variable. A 1-point rate improvement on a $400,000 mortgage is worth roughly $180/month and $65,000 in lifetime interest.
Variable 4: Loan term
| Term | Rate (typical) | Monthly PI | Total interest |
|---|---|---|---|
| 30 years | 7.0% | $2,129 | $446,458 |
| 20 years | 6.75% | $2,418 | $259,929 |
| 15 years | 6.5% | $2,785 | $181,379 |
The 15-year costs $656 more per month but saves $265,079 in interest. Fifteen-year loans also typically carry rates 0.5-0.75% below 30-year rates, adding to the savings.
How much house can you actually afford?
Lenders use two ratios:
Front-end ratio (housing ratio): PITI / gross monthly income. Most conventional lenders want this under 28%.
Back-end ratio (DTI): (PITI + all other monthly debt) / gross monthly income. Most lenders want this under 43%.
Example: $95,000 household income
Monthly gross income: $7,917
Max PITI at 28% front-end: $7,917 x 0.28 = $2,217
With $400/month in car and student loan payments, max PITI at 43% back-end: ($7,917 x 0.43) - $400 = $3,004 - $400 = $2,604
The binding constraint is the front-end ratio: max housing payment of approximately $2,217.
At 7% / 30-year / 20% down, $2,217 monthly PI supports approximately a $333,000 loan, which means roughly a $415,000 purchase price. Use the for your specific income, debts, and local tax rates.
The total cost of homeownership beyond the mortgage
- Property taxes: national average around 1.1% of assessed value annually; ranges from 0.3% (Hawaii) to 2.5%+ (New Jersey)
- Homeowners insurance: typically $1,200-2,000/year for a median home
- Maintenance: the 1% rule — budget 1% of home value per year for maintenance ($4,000 on a $400,000 home)
- HOA fees: $0 to $1,000+/month depending on community
- Closing costs: 2-5% of purchase price, paid at closing
Run the full calculation with the mortgage calculator — enter your home price, down payment, rate, and term to see payment breakdown, amortization schedule, and total cost comparison between 15 and 30-year options.
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