Your monthly mortgage payment is the fixed monthly cost of repaying a fixed-rate home loan, calculated by the standard amortization formula M = P·r(1+r)^n / ((1+r)^n − 1), where P is the loan principal, r is the monthly interest rate, and n is the total number of monthly payments. The four inputs that feed that formula are the home price, the down payment, the annual interest rate, and the loan term in years. The calculator subtracts your down payment from the home price to produce the loan principal, then converts the annual rate into a monthly rate and the term into months before plugging everything into the formula. Once those values are entered, the tool returns your principal-and-interest payment, the total interest you will pay over the life of the loan, and a year-by-year amortization schedule. Adding the optional property tax, insurance, and HOA fields converts that P&I output into a PITI total that reflects the housing payment you will actually send at closing. The walkthrough below shows how to feed each input into the mortgage payment calculator and read the result so you can size a home to your real budget.
The Formula the Calculator Is Built On
The fixed-rate mortgage formula looks intimidating on paper, but every input has a plain-English meaning. P is the loan principal, the dollar amount you actually owe the bank after the down payment is applied. r is the monthly interest rate, which the calculator derives by taking your annual percentage rate, dividing by 100 to convert the percent into a decimal, and then dividing by 12 to spread it across the months. n is the total number of monthly payments, simply the loan term in years multiplied by 12. Once those three values are known, the formula returns M, the fixed monthly principal-and-interest payment that fully amortizes the loan by the final payment.
To see how the formula behaves, take a $300,000 home with a $60,000 down payment, a 6% annual rate, and a 30-year term. The loan principal is P = $300,000 − $60,000 = $240,000. The monthly rate is r = 6 / 100 / 12 = 0.005. The number of payments is n = 30 × 12 = 360. Substituting those values, (1 + r)^n = (1.005)^360 ≈ 6.0226, so the monthly payment works out to M = 240,000 × 0.005 × 6.0226 / (6.0226 − 1) ≈ $1,438.92. With that single payment figure, the total paid across 360 months is $1,438.92 × 360 = $518,011.20, and the total interest paid over the life of the loan is $518,011.20 − $240,000 = $278,011.20. That is the magnitude of cost the calculator is designed to expose before you ever talk to a lender.
When the annual interest rate is exactly 0%, the formula simplifies to M = P / n, because the (1 + r)^n term collapses to 1 and the numerator becomes zero. In that case the calculator just splits the principal evenly across every month of the term, total interest is $0, and the amortization schedule is a straight line of equal principal reductions. This special case is useful for sanity-checking the calculator against zero-rate promo loans or for seeing how a typical schedule would look without interest layered on top.
Calculate Your Mortgage in the Calculator
The mortgage payment calculator walks you through the four inputs in the same sequence as the formula, then shows the headline number before letting you expand into the optional cost fields. The exact operating steps are:
- Enter the home price and your down payment. The calculator subtracts the down payment from the home price automatically, and the resulting loan principal is what every downstream number is based on. A $400,000 home with an $80,000 down payment gives the calculator a $320,000 principal to work with.
- Type your annual interest rate and pick a loan term. Use the standard 15-year or 30-year buttons, or type any custom number of years if your situation calls for something else. The calculator converts the annual rate into a monthly rate internally, so you do not need to do that math yourself.
- Read your monthly principal-and-interest payment, total interest, and total paid as soon as the inputs are valid. Those three figures are the headline output and update live as you change any field.
- Expand the amortization schedule to see how each payment splits between principal and interest over the life of the loan. Early payments are mostly interest; the principal share grows month by month until the balance is zero at the final payment.
- Add property tax, homeowners insurance, and monthly HOA dues in the advanced section. Property tax is computed as home value × tax rate / 12, insurance as the annual premium / 12, and HOA is added as a flat monthly amount. The calculator then shows a PITI total that represents your true monthly housing cost.
Because every step happens locally in your browser, none of the numbers you type are uploaded or stored anywhere. You can adjust the home price, drop the rate by a quarter point, switch from a 30-year to a 15-year term, and watch the monthly figure update instantly without ever creating an account.
Reading Your Monthly Number: P&I vs PITI
The headline figure the calculator shows by default is P&I — Principal and Interest only. P&I is the amount that goes directly toward repaying the loan itself, and it is what the amortization formula is built to compute. PITI is what you actually send to your mortgage servicer each month once you add property tax, homeowners insurance, and (optionally) HOA dues to the base. The table below summarizes the difference:
| Component | What it covers | When it is used |
|---|---|---|
| P&I | Principal repayment + interest on the loan balance | Headline loan payment; what the amortization formula returns |
| PITI | P&I + property tax + homeowners insurance | What lenders use to qualify you for the loan |
| PITI + HOA | PITI plus monthly HOA dues | True monthly housing cost after every recurring housing bill |
The reason lenders look at PITI rather than P&I is that property tax and insurance are part of the cost of owning the home, and escrow accounts typically bundle them with the loan payment. The calculator's optional fields let you preview that PITI number so you can compare it to your actual monthly take-home pay before committing to a home price. If you want to compare scenarios — say, a 15-year versus a 30-year term, or a higher down payment versus a lower one — the qualitative direction is what matters: shorter terms raise the P&I but cut total interest paid, while bigger down payments lower both. Longer terms stretch the loan and increase total interest paid. The exact figure for each scenario should come from the calculator rather than from a quick mental estimate.
What the Calculator Does Not Include
The calculator is built for fast planning, which means a handful of real-world costs are intentionally excluded. The table below lists what the tool accounts for and what it leaves out:
| Included in the calculation | Not included — confirm with your lender |
|---|---|
| Loan principal (home price minus down payment) | Private mortgage insurance (PMI) when down payment is below 20% |
| Annual interest rate, fixed for the full term | Discount points paid to lower the rate |
| Loan term in years (15, 30, or custom) | Closing costs and origination fees |
| Monthly compounding of interest | Adjustable-rate (ARM) payment changes after the fixed period |
| Principal-and-interest monthly payment | Extra payments or prepayment of principal |
| Full year-by-year amortization schedule | Escrow shortfalls or surpluses year over year |
| Optional PITI total with tax, insurance, HOA | HOA special assessments or one-time fees |
According to standard amortization conventions, the rate is treated as fixed for the entire term and interest is assumed to compound monthly. Any change to either of those assumptions — for example, an adjustable-rate mortgage that resets after five years, or extra payments that accelerate the payoff — falls outside the calculator's scope and must be modeled separately. The same applies to PMI: if your down payment is less than 20% of the home price, expect a separate PMI premium that the calculator does not show.
Verifying Your Estimate Before You Commit
Once the calculator gives you a P&I figure, the right next step is to compare it with what a licensed lender would quote. Lenders run the same amortization formula and then layer on the excluded costs above — PMI, points, closing costs, taxes, insurance, HOA — so the loan estimate they hand you will almost always be higher than the calculator's P&I number. Bring your calculator result, the property's tax rate, your annual insurance quote, and any HOA dues to that conversation, and ask the lender to walk you through the gap line by line.
You can also use the calculator to pressure-test a few what-if scenarios without entering any real personal information. Drop the rate by half a point and see how the payment shifts; increase the down payment by $20,000 and see how the total interest falls; shorten the term from 30 to 15 years and compare the monthly delta to the lifetime interest savings. Because everything runs in the browser, none of those scenarios are stored, logged, or tied to your identity — the calculator simply re-applies the formula each time you change a field. Treat the output as a planning estimate, not a loan offer, and confirm every final figure with the lender before you sign.
For a fuller picture of the math behind the monthly payment, including how the compounding convention is defined and why fixed-rate amortization behaves the way it does, the standard reference is the amortization calculator description on Wikipedia, which documents the same annuity formula this calculator uses.