About the Mortgage Calculator
A mortgage calculator estimates the monthly payment for a home loan and shows how much of each payment goes to principal versus interest over time. Enter the home price, down payment, interest rate, and term — the calculator returns the monthly principal and interest, and lets you fold in property tax, homeowners insurance, PMI, and HOA dues to produce a complete PITI estimate.
What a mortgage payment actually pays for
When most people say "mortgage payment" they mean the single monthly figure they send to their lender. In practice that figure is a bundle: principal (paying down the loan balance), interest (the cost of borrowing), property taxes (collected by the lender and forwarded to your local government), homeowners insurance (collected and forwarded to the insurer), and — if your down payment was less than 20% on a conventional loan — private mortgage insurance (PMI). Lenders use the acronym PITI for the first four; some break out PMI and HOA dues separately.
Only the principal-and-interest portion is fixed for the life of a 30-year fixed mortgage. Tax assessments and insurance premiums tend to rise over time, which is why many borrowers see their escrow payment drift up by 2–5% per year even when their rate is locked. Budgeting around principal-and-interest alone is one of the most common ways first-time buyers underestimate the true cost of homeownership.
How rate, term, and down payment interact
The three biggest levers are price, rate, and term. Price drives the loan amount; rate drives the cost of borrowing per dollar; term distributes that cost over more or fewer months. A 1-percentage-point change in rate moves the monthly P&I by roughly 10–12% on a 30-year loan, which is why even modest rate moves matter more than most people expect.
Term has the opposite intuition for many buyers. A 15-year mortgage usually carries a rate 0.5–0.75 percentage points lower than the 30-year for the same borrower, and because the principal pays down twice as fast, the total interest paid over the life of the loan is often less than half. The trade-off is a 30–50% higher monthly payment, which can crowd out other goals (retirement contributions, an emergency fund, college savings).
A larger down payment reduces the loan amount and may push you past the 80% loan-to-value threshold that eliminates PMI. On a $400,000 home, going from 10% down to 20% down typically saves $150–$300 per month in PMI alone, on top of the lower payment from a smaller principal.
The amortization curve: why early payments are mostly interest
An amortizing loan keeps the monthly payment constant but shifts the split between interest and principal over time. In month one of a 30-year loan, often 70–80% of the payment goes to interest; by year 20, that ratio has reversed. This is why making extra principal payments early in the loan has an outsized effect — every dollar of extra principal in year one saves roughly 30 years of compounded interest on that dollar.
Use a mortgage payoff calculator or amortization schedule to see this directly. A single extra $200/month on a $300,000, 30-year loan at 7% can shave roughly 7 years off the term and save tens of thousands in interest.
When to refinance — and when not to
The rough rule is to refinance when the new rate is at least 0.75–1 percentage point lower than your current rate and you plan to stay in the home long enough to recoup closing costs. Closing costs typically run 2–5% of the loan amount; divide that by your monthly savings to get the break-even point in months.
Less obvious considerations: a refinance restarts amortization, so even at a lower rate, you may pay more total interest if you reset to another 30-year term late in your existing loan. A shorter refinance term, or aggressive extra principal payments after refinancing, neutralizes that risk.
How it works
- Determine the loan amount. Home price minus down payment. The loan amount is what's actually borrowed and is what gets amortized.
- Apply the amortization formula. The standard mortgage formula converts principal, monthly rate (annual ÷ 12), and number of payments (years × 12) into a fixed monthly principal-and-interest payment.
- Add recurring housing costs. Property tax (typically 0.5–2.5% of home value annually, varying widely by state) and homeowners insurance (typically 0.3–0.8% of home value) are divided by 12 and added. PMI (if applicable) and HOA dues are added on top.
- Verify with an amortization schedule. Generate the full month-by-month schedule to see how each payment splits between principal and interest, and how the balance falls over time.
Formula
- M = Monthly principal & interest payment
- P = Loan amount (home price − down payment)
- r = Monthly interest rate (annual rate ÷ 12)
- n = Total number of payments (years × 12)
Worked examples
$400,000 home, 20% down, 7% rate, 30-year fixed
Loan amount = $320,000. Monthly P&I ≈ $2,129. With $4,800/year property tax and $1,200/year insurance, PITI ≈ $2,629. Total interest paid over 30 years ≈ $446,000 — more than the original loan.
Same home, 15-year term at 6.25%
Loan amount = $320,000. Monthly P&I ≈ $2,743 — about $614 more per month than the 30-year. Total interest ≈ $173,800, saving roughly $272,000 in interest versus the 30-year scenario.
Rate sensitivity: 1 point higher
Same $320,000 loan at 8% (instead of 7%) on a 30-year: monthly P&I jumps from $2,129 to $2,348 — an extra $219/month, or $78,840 more interest over the life of the loan.
Frequently asked questions
How is a mortgage payment calculated?
The standard amortization formula takes the loan amount, the monthly interest rate (annual rate ÷ 12), and the total number of monthly payments (years × 12) and produces a fixed monthly principal-and-interest payment. Property tax, homeowners insurance, PMI, and HOA dues are added separately to get the full PITI payment.
What is PITI?
Principal, Interest, Taxes, and Insurance — the four core components of a total monthly housing payment that lenders use to assess affordability against your income. Most lenders cap total housing costs around 28% of gross monthly income.
How much house can I afford?
A common rule (the 28/36 rule) is that total housing costs should stay under 28% of gross monthly income, and total debt payments under 36%. A house affordability calculator converts your income, debts, and down payment into a recommended home price range.
Do I need to pay PMI?
Private Mortgage Insurance is generally required on conventional loans when the down payment is under 20% of the home's value. It typically adds 0.3%–1.5% of the loan amount per year. By federal law (Homeowners Protection Act of 1998), PMI on most conventional loans is automatically removed once the loan-to-value ratio reaches 78%, and you can request removal at 80%.
Is a 15-year or 30-year mortgage better?
A 15-year term usually has a rate 0.5–0.75 points lower than a 30-year, and total interest is often less than half. The trade-off is a monthly payment 30–50% higher, which can crowd out retirement savings or an emergency fund. A 30-year with disciplined extra principal payments is a flexible middle ground.
How do property taxes affect the payment?
Property tax rates vary widely — from under 0.5% of home value annually in Hawaii to over 2% in New Jersey, Illinois, and parts of Texas. On a $400,000 home, that's a difference of $500/month. Always check the assessed value and effective rate for the specific property, not just the state average.
What's the difference between interest rate and APR?
The interest rate is the cost of borrowing the principal. APR (annual percentage rate) folds in lender fees, points, and certain closing costs to give a more comparable cost-of-borrowing figure across loans. APR is typically 0.1–0.4 points higher than the rate.
Should I pay points to lower my rate?
Each discount point usually costs 1% of the loan amount and lowers the rate by about 0.25 points. The break-even is typically 4–6 years; if you'll keep the loan longer than that, points usually pay off. If you might refinance or sell sooner, they often don't.