Mortgage Calculator — How It Works, the Formula, and a Worked Example
A mortgage is probably the biggest loan you will ever sign, and the monthly payment is the number your whole budget bends around. This calculator shows you that number honestly — not just the loan repayment, but the taxes, insurance and PMI that ride along with it — so you can test a home price, a down payment, or a rate and see the real monthly cost before a lender ever runs your credit.
Enter the home price, your down payment, the interest rate and the loan term. The calculator works out your monthly principal and interest, adds your estimated tax, insurance and PMI, and builds a full amortization schedule showing how much of every payment is interest in the early years — and how quickly that flips as the balance falls.
What your monthly payment actually includes
Lenders call it PITI + PMI. Four of the five parts are collected by your lender every month and held in an escrow account:
- Principal — the slice that actually pays down your loan balance. It starts small and grows every month.
- Interest — the lender's charge, worked out monthly on whatever balance remains. Early on, this is the bigger slice.
- Property taxes — set by your county or city, not your lender. Around 1.1% of the home's value per year is a fair US planning average, but it ranges from under 0.4% to over 2% depending on the state.
- Homeowner's insurance — required by every lender. Budget roughly $150–$250 a month for a mid-priced home.
- PMI (private mortgage insurance) — added when your down payment is under 20%. It protects the lender, not you, and it disappears once your balance reaches 80% of the home price.
The mortgage payment formula
Every fixed-rate mortgage on earth is priced with the same equation. If you have ever wondered where the number comes from, here it is:
- M — your monthly principal-and-interest payment
- P — the loan amount (home price minus down payment)
- r — the monthly interest rate: your annual rate divided by 12 (so 6.5% becomes 0.005417)
- n — the number of payments: years × 12 (360 for a 30-year loan)
The formula spreads the loan so every payment is identical, but the mix inside each payment changes: interest is charged on the remaining balance, so as the balance shrinks, less goes to interest and more to principal. That slow flip is exactly what the amortization schedule above shows.
A worked example, step by step
Say you are buying a $350,000 home with 20% down and a 6.5% fixed rate over 30 years:
- Loan amount: $350,000 − $70,000 down = $280,000 (that's P).
- Monthly rate: 6.5% ÷ 12 = 0.005417 (that's r). Payments: 30 × 12 = 360 (that's n).
- Apply the formula: M = 280,000 × 0.005417 × (1.005417)360 ÷ ((1.005417)360 − 1) = $1,769.79 a month in principal and interest.
- Add the escrow items: property tax at 1.1% of $350,000 is $320.83 a month, and insurance at $180 a month brings the true monthly cost to $2,270.62. At 20% down there is no PMI.
- The long view: over 360 payments you hand over $637,124.57 — meaning $357,124.57 of it is interest, more than the loan itself. That single figure is why the next section matters.
30 years or 15? The term changes everything
Keep the same $280,000 loan at 6.5% but halve the term. The 15-year payment jumps to $2,439.10 — $669.31 more each month — yet total interest collapses to $159,038.11. You save $198,086.46. A shorter term is the single most powerful lever most buyers have; the trade-off is simply whether the higher payment still leaves your monthly budget breathing room. For a quick feel of payment size at today's example rate:
| Loan amount | Monthly P&I (6.5%, 30 yrs) |
|---|---|
| $200,000 | $1,264.14 |
| $300,000 | $1,896.20 |
| $400,000 | $2,528.27 |
Five honest ways to lower the payment
- Raise the down payment. Every extra dollar down is a dollar you never pay interest on — and crossing 20% removes PMI entirely.
- Buy down the rate. Discount points (1% of the loan per point) typically cut the rate by about 0.25%. Worth it if you will stay past the break-even point — usually five to seven years.
- Extend the term. A 30-year loan costs far more overall but keeps the monthly number survivable. You can always pay extra later; you cannot pay less.
- Fix the credit score first. The gap between a 680 and a 760 score can be half a percent of rate — tens of thousands over the loan's life.
- Make one extra payment a year. Thirteen payments instead of twelve knocks years off a 30-year term. Use the extra-payment field above to see your own number.
Rule of thumb: lenders like your total housing cost under 28% of gross monthly income, and all debts together under 36%. If the payment above breaks that, the home — not the math — needs to change.
Half a percentage point, in dollars
Rate quotes get argued over in eighths of a point, which makes them sound like trivia. They aren't. Take the same $280,000 loan from the worked example and move only the rate. At 6.0% the payment is $1,678.74 a month and total interest lands at $324,346.93. At the example's 6.5% you already know the figures: $1,769.79 and $357,124.57. At 7.0% the payment climbs to $1,862.85, and the interest bill reaches $390,624.92. So half a point swings the monthly cost by about $91 to $93 either way — and swings the lifetime cost by roughly $33,000. That's a car, bought or not bought, hiding inside a fraction most people round away. It's also why an afternoon spent collecting three competing quotes pays better than almost any other afternoon in the whole buying process.
Why the payment creeps up over the years
Here's the part that surprises new owners: even with a fixed rate, the amount leaving your account usually rises. Your principal and interest never change — they're locked by the formula. But the escrow slice isn't locked. Property taxes get reassessed, insurance premiums climb, and once a year your servicer runs an escrow analysis and adjusts the monthly collection to match. A tax reassessment alone can add $40 or $60 a month without your loan changing at all. None of this means the calculator was wrong. It means the honest way to use it is to treat the tax and insurance lines as this year's estimate, not a lifetime promise — and to leave a little slack in the budget for the letter that arrives each spring.
Frequently asked questions
How much house can I afford?
Start with the 28/36 rule: housing costs below 28% of your gross monthly income, and all debt payments below 36%. On a $6,000 monthly income that points to roughly $1,680 for housing — then check the payment here against your real budget, because childcare, commuting and savings goals do not appear in any lender's formula.
Do I always have to pay PMI?
No. PMI applies to conventional loans with less than 20% down. It is charged monthly until your balance reaches 80% of the home's original value, you can ask for it to be removed at that point, and by law it terminates automatically at 78%. A 20% down payment avoids it from day one.
What is the difference between the interest rate and APR?
The interest rate prices the loan itself. The APR wraps that rate plus lender fees, points and closing costs into one number, so it is always a little higher — and it is the better figure for comparing two lenders' offers side by side.
Are property taxes and insurance included in the payment?
Usually, yes. Most lenders collect one-twelfth of your yearly tax and insurance bills with each monthly payment and hold the money in escrow, paying the bills when they fall due. This calculator includes both so the monthly figure matches what actually leaves your account.
What happens if I pay extra each month?
Extra money goes straight to principal, which cuts the interest charged on every future payment. Even $100 extra a month on a $280,000 loan at 6.5% saves tens of thousands in interest and clears the loan years early. Try it in the extra-payment field above — the schedule updates instantly.
Is a 15-year mortgage always better than a 30-year?
On total cost, yes — the worked example above saves $198,086.46 in interest. But the payment is $669.31 higher every month, and a payment you cannot sustain is worse than a slightly expensive one you can. Many buyers take the 30-year term and voluntarily pay it like a 15-year when times are good.
Looking for more like this? Every calculator on CalcMega gets a plain-English guide with the formula and a worked example — browse them all on the CalcMega Blog.
Estimates only — your lender's figures will vary with your credit profile, location and the loan program you choose. Nothing here is financial advice.