Mortgage Amortization Calculator

What this calculator does

This calculator builds the full repayment schedule for a fixed-rate mortgage. You enter the loan amount, the interest rate and the term in years. It reports the monthly principal-and-interest payment, the total interest paid over the life of the loan, and a year-by-year table showing how much of each year's payments went to interest, how much went to principal, and the balance left at the end of the year. It is for buyers comparing loan offers, owners checking how much equity their payments have built by a given year, and anyone deciding between a shorter and a longer term.

The payment shown is principal and interest only. Most monthly mortgage bills also collect property taxes and homeowners insurance into an escrow account, and some include mortgage insurance or HOA dues, so the amount a lender bills each month is usually higher than the figure here.

How the math works

The monthly payment comes from the standard fixed-rate formula, payment = L × r / (1 − (1 + r)⁻ⁿ), where L is the loan amount, r is the annual rate divided by twelve and n is the number of monthly payments. The payment is rounded to the cent. The calculator then steps through the loan one month at a time: interest for the month is the current balance times r, rounded to the cent, and the rest of the payment reduces the balance. The final payment is adjusted so the balance ends at exactly zero, and the months are grouped into years for the table.

This is the process the Consumer Financial Protection Bureau describes as amortization: early in the loan the balance is high, so most of each payment is interest; as the balance falls, less interest accrues and more of the same payment goes to principal. The payment itself never changes on a fixed-rate loan. Only the split does.

Worked example

These are the calculator's default inputs: a $350,000 loan at a fixed 6.5% for 30 years.

• Monthly rate: 6.5% ÷ 12 = 0.5417%, over 360 payments.
• Monthly payment: $2,212.24 for principal and interest.
• First month: $1,895.83 of interest ($350,000 × 0.5417%) and $316.41 of principal.
• First year: $22,634.81 of interest and $3,912.07 of principal, leaving a balance of $346,087.93.
• Whole loan: $446,404.05 of interest, so the payments total $796,404.05 on a $350,000 loan.

Principal first exceeds interest in month 233, in the twentieth year of the loan. Until then, more than half of every payment is interest.

What the schedule shows in selected years

The year-by-year table above is easier to read with a few rows pulled out. These are from the worked example. Cumulative interest is the total paid from the first payment to the end of that year.

Two points stand out. After seven years, the borrower has paid $152,284.78 of interest and reduced the balance by only $33,543.38, under 10% of the original loan. And the balance does not fall to half the original amount until month 257, more than 21 years in. Anyone who expects to sell or refinance within a decade builds most of their equity from the down payment and any change in the home's value, not from the scheduled principal. The balance column is also the payoff amount a refinance would start from, before any prepayment or fees.

How the rate and the term change the cost

The same $350,000 loan, run through the calculator at different rates and terms:

On a 30-year loan, each half point of rate changes the payment by roughly $110 to $116 a month and total interest by about $40,000 to $42,000. Shortening the term has a larger effect on total interest but raises the payment: at 6.5%, the 15-year loan costs $836.64 more a month than the 30-year loan and $247,606.90 less in interest over its life. The 15-year row at 6.0% shows the combined effect of a shorter term and a lower rate. Rates here are example inputs, not quotes. The payment a household can carry is a separate question, which the affordability calculator linked below addresses.

Mortgage interest and the tax deduction

The interest column matters for taxes only if you itemize. IRS Publication 936 (2025 edition, the most recent at this review) says home mortgage interest is deductible only if you itemize deductions on Schedule A, and only on the first $750,000 of home acquisition debt ($375,000 if married filing separately) taken on after December 15, 2017. Debt incurred before December 16, 2017 keeps the older $1 million limit ($500,000 if married filing separately).

Itemizing only helps when total itemized deductions exceed the standard deduction. For tax year 2026 the IRS set the standard deduction at $16,100 for single filers, $32,200 for married couples filing jointly and $24,150 for heads of household. In the worked example, first-year interest of $22,634.81 is above the single standard deduction but $9,565.19 below the joint one, so a married couple would need that much in other itemized deductions, such as state and local taxes or charitable gifts, before the interest reduced their tax at all. Because the interest column shrinks every year, the gap widens over time: by year 10 the interest is $19,535.84.

Frequently Asked Questions

Sources

• Consumer Financial Protection Bureau, How does paying down a mortgage work?
• IRS Publication 936 (2025), Home Mortgage Interest Deduction
• IRS, tax inflation adjustments for tax year 2026 (standard deduction)

Related Tools

• Mortgage Refinance Breakeven Calculator
• Mortgage Recast vs Refinance Calculator
• Home Affordability Calculator
• Physician Loan vs Conventional Mortgage Calculator
• Analysis: Fed Rate Cuts and the Refinance Window

Built and verified by The Breakeven Math — last reviewed September 18, 2026.