Mortgage Calculator: See What a Home Loan Really Costs

By ZeroFee Tools · October 4, 2026 · 7 min read

A mortgage quote tells you the monthly payment, but it hides the two numbers that matter most: how much interest you will pay over the life of the loan, and how little of your early payments actually reduce the balance. On a typical 30-year loan, your first payments are almost entirely interest, which is why the balance barely moves in year one.

The free Mortgage Calculator exposes all of it. Enter the home price, down payment, interest rate and loan term to get the monthly principal-and-interest payment, the total interest paid over the life of the loan, and a year-by-year amortization schedule showing how each payment splits between interest and principal. One honest note: this is P&I only, not property tax, insurance or HOA dues, which lenders usually bundle into the real monthly bill. Everything runs in your browser with no signup. For other loans, the loan calculator covers the same math; to weigh buying against renting, run the numbers through the rent vs buy calculator; and to see what your money could earn instead, try the compound interest calculator.

How is a monthly mortgage payment calculated?

Lenders use M = P x r(1+r)^n / ((1+r)^n - 1), where P is the loan amount, r the monthly rate and n the number of payments. A $280,000 loan at 6.5% for 30 years gives about $1,770/month in principal and interest, with early payments mostly interest.

Lenders use the amortization formula: M = P x r(1+r)^n / ((1+r)^n - 1), where P is the loan amount, r the monthly interest rate and n the number of payments. For a $280,000 loan at 6.5% over 30 years: the monthly rate is 0.065 / 12 = 0.0054167, and n = 360 payments. The formula gives about $1,770 per month in principal and interest.

Here is the part the quote hides: the first month's interest alone is 280,000 x 0.0054167 = $1,516.67. That is roughly 86% of the $1,770 payment going to interest, with only about $253 reducing the balance. The calculator's year-by-year schedule shows this shift clearly: early years are mostly interest, and the principal share grows with every payment until the final years are nearly all principal.

This front-loading is why extra payments early are so powerful. An extra $200 a month in year one attacks principal when the balance is highest, and every dollar of principal removed early also removes all the future interest that dollar would have accrued. The schedule makes this visible: compare the balance after five years with and without the extra payment to see compounding work in your favor for once.

How much interest will you pay over the life of the loan?

Multiply the monthly payment by the number of payments and subtract the loan amount. On a $280,000 loan at 6.5% for 30 years, total interest is about $357,000. A 15-year term on the same loan costs only about $159,000 in interest, saving roughly $198,000.

Multiply the monthly payment by the number of payments and subtract the loan amount. At $1,770 a month for 360 payments, total paid is about $637,128; minus the $280,000 borrowed, total interest is roughly $357,128. On a 30-year loan, total interest often approaches or exceeds the amount borrowed, and here it clearly does.

Now compare terms. The same $280,000 at 6.5% over 15 years: the payment rises to about $2,439 a month, but total paid is 2,439 x 180 = $439,020, so total interest is only about $159,000. Choosing 15 years instead of 30 saves roughly $198,000 in interest. The catch is the monthly budget: $2,439 versus $1,770 is a real lifestyle difference. Larger down payments cut interest the same way, by shrinking the loan from the start.

Rate shopping deserves the same comparison. A half-point rate difference on this loan moves the monthly payment by roughly $90, which is about $32,000 over 30 years. That is why lenders quote rates to three decimal places and why a single afternoon of rate shopping can be the highest-paid work of your year. Test 6.0%, 6.5% and 7.0% in the calculator and watch total interest swing.

How does the down payment change the picture?

A bigger down payment shrinks the loan amount, lowering both the monthly payment and total interest. On a $350,000 home, 20% down means borrowing $280,000 at about $1,770/month P&I at 6.5% for 30 years. Add local tax and insurance for the true monthly cost.

Take a $350,000 home with 20% down: $70,000 down, $280,000 borrowed. At 6.5% for 30 years that is the $1,770 monthly P&I from our example. Put only 10% down instead and you borrow $315,000; the payment rises to about $1,991 and total interest climbs by roughly $45,000. That is the leverage of a down payment: every extra dollar down saves more than a dollar in interest over 30 years.

But remember what the calculator does not include: property tax, homeowners insurance and possibly HOA dues, which lenders usually roll into an escrowed monthly payment. Add your local tax and insurance estimates to the P&I figure for the true monthly housing cost. And since this is P&I math only, pairing it with the rent vs buy calculator gives the full ownership picture before you sign anything.

One more number worth modeling: the break-even on buying points. Paying $2,800 (one point) to drop the rate from 6.5% to 6.25% saves about $45 a month on this loan, so the payback period is roughly 62 months. If you will sell or refinance before then, the points lose money. The calculator gives you the payment at each rate; the division is yours.

How to use the Mortgage Calculator in 4 steps

1. Enter the home price and down payment. The loan amount is the price minus your down payment.
2. Enter the rate and term. Type the interest rate (APR) and pick the loan term, such as 15 or 30 years.
3. Read the headline numbers. See the monthly P&I payment, total interest and total cost of the loan.
4. Scroll the yearly schedule. Watch how each payment splits between interest and principal as the years pass.

6 practical tips

• Compare 15 vs 30 years on total interest. The monthly difference is real, but so is saving roughly $198,000 in interest.
• Watch year one of the schedule. Seeing how little the balance moves early on makes extra payments feel worth it.
• Model a bigger down payment. Even 5% more down on a $350,000 home saves tens of thousands over 30 years.
• Add tax and insurance yourself. The P&I figure is not your full bill; budget the escrowed costs on top.
• Test rate changes. A half-point rate difference moves the monthly payment by hundreds on a large loan.
• Do the points math. Divide the cost of discount points by the monthly savings to find the break-even month.

Frequently asked questions