How a Mortgage Calculator Works
A mortgage calculator uses the standard amortization formula to determine how much you'll pay each month. The calculation takes four inputs: the loan amount (home price minus down payment), the annual interest rate, and the loan term in years. The formula converts those into a fixed monthly payment that covers both interest charges on the outstanding balance and a portion of the principal.
What makes the calculation more nuanced than it appears: your interest charge decreases each month as you pay down the principal. In month one of a $320,000 loan at 6.85%, roughly $1,827 of your payment goes to interest. By month 360 (year 30), only $12 goes to interest — because the balance is nearly zero. This gradual shift from interest-heavy to principal-heavy payments is what amortization describes.
The Mortgage Payment Formula
The formula used to calculate a fixed-rate mortgage payment is:
Where:
- M = Monthly payment
- P = Principal loan amount (home price − down payment)
- r = Monthly interest rate (annual rate ÷ 12 ÷ 100)
- n = Total number of payments (years × 12)
Example calculation: $400,000 home, $80,000 down (20%), 6.85% rate, 30-year term.
- P = $320,000
- r = 6.85% ÷ 12 ÷ 100 = 0.005708
- n = 30 × 12 = 360
- M = 320,000 × [0.005708 × (1.005708)^360] / [(1.005708)^360 − 1]
- M ≈ $2,099/month
You can verify this against any bank's mortgage calculator. The formula is universal across all fixed-rate mortgages — it's the same math your lender uses.
What's NOT Included in This Calculation
This calculator shows principal and interest (P&I) only. Your actual monthly payment to the lender is likely higher because most lenders require an escrow account that collects:
- Property taxes — Varies dramatically by location. New Jersey averages 2.47% of home value annually; Hawaii averages 0.29%. On a $400,000 home in New Jersey, that's ~$820/month added to your payment. In Hawaii, ~$97/month.
- Homeowner's insurance (HOI) — Typically $100–$200/month, depending on the home value, location, and coverage level. High-risk areas (hurricane, wildfire zones) can run higher.
- Private Mortgage Insurance (PMI) — Required if your down payment is less than 20%. Costs 0.5%–1.5% of the loan amount annually, divided into monthly installments. On a $320,000 loan, PMI adds roughly $133–$400/month. It cancels when your equity reaches 20%.
- HOA fees — If your property is in a homeowners association, monthly dues can range from $50 to over $1,000 depending on amenities.
To get your true all-in housing cost, add your P&I payment from this calculator to your estimated property taxes, insurance, PMI (if applicable), and any HOA fees. Lenders call this PITI: Principal, Interest, Taxes, and Insurance.
15-Year vs. 30-Year Mortgage: Full Comparison
The loan term is one of the most consequential decisions in a mortgage. Here's how the two most common terms compare on the same loan:
| Factor | 15-Year Mortgage | 30-Year Mortgage |
|---|---|---|
| Monthly payment (on $320K @ 6.35% / 6.85%) | ~$2,762 | ~$2,099 |
| Total interest paid | ~$177,000 | ~$435,000 |
| Equity after 5 years | ~$93,000 | ~$26,000 |
| Interest rate (typical) | ~0.5% lower | Higher |
| Break-even vs. renting (faster/slower) | Faster | Slower |
| Monthly cash flow flexibility | Lower | Higher |
The 30-year wins if: You're cash-flow constrained, you plan to invest the payment difference in the stock market (historically 7–10% annual return vs. 6.5–7% mortgage interest), or you expect to sell or refinance within 10 years.
The 15-year wins if: You want to own the home free and clear, you're within 15 years of retirement and want no mortgage payment in retirement, or you have stable high income and the higher payment isn't a burden.
How to Lower Your Monthly Mortgage Payment
There are five levers you can pull to reduce your monthly payment. Each has trade-offs:
- Increase the down payment. Every extra dollar of down payment reduces the principal dollar-for-dollar. Increasing from 10% to 20% on a $400,000 home eliminates $40,000 from the loan and — if it gets you below the 20% PMI threshold — also eliminates PMI entirely.
- Buy a less expensive home. The most direct lever. A $350,000 home vs. a $400,000 home with the same terms saves roughly $270/month.
- Extend the loan term. A 30-year term vs. 15-year reduces the monthly payment significantly, though you'll pay more in total interest.
- Reduce the interest rate. Even a 0.5% rate reduction saves meaningful money. On a $320,000 loan, dropping from 7.0% to 6.5% saves about $101/month and $36,000 in total interest. To get a lower rate: improve your credit score, buy mortgage discount points, or shop multiple lenders.
- Buy mortgage points. One discount point costs 1% of the loan amount and typically reduces the rate by 0.25%. On a $320,000 loan, one point costs $3,200 and saves ~$51/month. Break-even is about 63 months (5.25 years). Only worth it if you plan to stay in the home long-term.
Understanding Your Amortization Schedule
An amortization schedule is a month-by-month table showing how each payment is split between principal repayment and interest charges. In the early years of a 30-year mortgage, the split is heavily weighted toward interest — because you're paying interest on the full outstanding balance.
Here's how a $320,000 loan at 6.85% (30-year term) amortizes over key milestones:
| Year | Monthly Payment | Principal Paid | Interest Paid | Remaining Balance |
|---|---|---|---|---|
| 1 | $2,099 | $272 | $1,827 | $316,736 |
| 5 | $2,099 | $360 | $1,739 | $302,143 |
| 10 | $2,099 | $497 | $1,602 | $278,861 |
| 15 | $2,099 | $684 | $1,415 | $246,022 |
| 20 | $2,099 | $943 | $1,156 | $199,067 |
| 25 | $2,099 | $1,299 | $800 | $132,024 |
| 30 | $2,099 | $2,087 | $12 | $0 |
*Figures shown are approximate, representing the first payment of each year. Actual figures vary slightly due to rounding.
Notice that in year one, only $272 of your $2,099 payment reduces the principal — the other $1,827 is interest. By year 25, the split reverses: $1,299 goes to principal and $800 to interest. This explains why making extra principal payments early in the loan has an outsized impact on total interest paid.
Making Extra Principal Payments
One of the most effective ways to save on a mortgage is to make additional principal payments. Because each extra dollar paid toward principal reduces the balance on which future interest is calculated, the savings compound over the loan term.
On a $320,000, 30-year loan at 6.85%:
- Paying $200/month extra reduces the term by about 6 years and saves ~$96,000 in interest.
- Paying $500/month extra reduces the term by about 12 years and saves ~$192,000 in interest.
- Making one extra payment per year (biweekly payments) cuts roughly 4 years off the loan.
Most lenders accept extra principal payments without penalty (check your loan documents — some older loans have prepayment penalties, though these are rare today). When making an extra payment, specify it as a "principal-only" payment so the lender doesn't apply it to next month's payment.
Current Mortgage Rates (2025–2026)
Mortgage rates change daily based on Federal Reserve policy, bond market conditions, and lender competition. As of early 2026, 30-year fixed-rate mortgages have been ranging from 6.5% to 7.2% for well-qualified borrowers (720+ credit score, 20% down). Rates vary by:
- Credit score — A 760 score vs. a 680 score can mean a 0.5–1.0% rate difference.
- Down payment — Less than 20% typically means a higher rate and PMI.
- Loan type — Conventional, FHA, VA, and USDA loans have different rate structures.
- Lender — Rates vary 0.5–1.0% across lenders for the same borrower profile. Always get 3+ quotes.
- Loan size — Jumbo loans (above conforming limits) often carry slightly higher rates.
For current rates, check the Freddie Mac Primary Mortgage Market Survey (PMMS), published weekly on Thursdays. It's the most widely cited benchmark for national average mortgage rates.
Frequently Asked Questions
What does a mortgage calculator include?
This calculator computes principal and interest (P&I) only. Your real monthly payment is higher once you add property taxes, homeowner's insurance, and PMI (if your down payment is under 20%). Lenders call the full payment PITI: Principal, Interest, Taxes, and Insurance.
How accurate is this mortgage calculator?
Very accurate for fixed-rate mortgages. The formula used — M = P × [r(1+r)^n] / [(1+r)^n − 1] — is the same calculation every lender uses. The result matches what you'd see on a Loan Estimate from a lender for the principal and interest portion.
What is a good debt-to-income ratio for a mortgage?
Conventional lenders prefer a DTI below 36%, with housing costs (PITI) not exceeding 28% of gross monthly income. FHA loans allow up to 43% DTI (sometimes 50% with compensating factors). A lower DTI means better loan terms and more lender options.
Should I choose a 15-year or 30-year mortgage?
A 30-year gives lower monthly payments and more flexibility; a 15-year saves dramatically on interest (often $200,000+ on a large loan) and typically comes with a 0.5–0.75% lower rate. Choose 30 years if cash flow is tight or you plan to invest the payment difference. Choose 15 years if you want to pay off the home faster and interest savings are your priority.
What is PMI and how do I avoid it?
Private Mortgage Insurance (PMI) protects the lender — not you — if you default. It's required on conventional loans when your down payment is less than 20%. Cost: 0.5%–1.5% of the loan annually. To avoid it, put 20% down, use a piggyback loan (80-10-10), or wait until you've built 20% equity and request cancellation.
How do mortgage points work?
One discount point costs 1% of the loan amount and typically reduces your interest rate by 0.25%. On a $320,000 loan, one point = $3,200 upfront. At a $51/month savings, the break-even is ~63 months. Points make sense if you plan to stay in the home beyond the break-even period.
Methodology
This calculator uses the standard fixed-rate mortgage amortization formula as published by the Consumer Financial Protection Bureau (CFPB). All calculations are performed client-side in your browser — no data is transmitted to our servers. Property tax rates referenced on state-specific pages are sourced from ATTOM Data Solutions annual property tax report and the Tax Foundation. Median home price data is sourced from the National Association of Realtors (NAR) and Zillow Research, updated quarterly. Interest rate benchmarks are from the Freddie Mac PMMS. Last updated: May 2026.