Mortgage Calculator: How Monthly Payments and Amortization Work
Learn how your mortgage payment is calculated, what P&I and amortization mean, and how to use a free mortgage calculator to compare rates, terms, and extra payments. No account requiredโall math runs in your browser.
How the Monthly Payment Is Calculated
The Principal & Interest (P&I) Formula
- P (Principal):The amount you borrow: home purchase price minus down payment. This is the starting balance that gets paid down over the life of the loan.
- i (Monthly interest rate):Your annual interest rate divided by 12. For example, 6% per year โ i = 0.06/12 = 0.005 per month. Interest each month is computed as (current balance) ร i.
- n (Number of payments):Total number of monthly payments. A 30-year loan has n = 360; 15-year has n = 180. The formula solves for the fixed payment M that pays off the loan exactly after n months.
- What the formula does:It spreads the loan over n equal payments so that after each payment, the remaining balance is consistent with compound interest. Early on, most of M is interest; later, most of M is principal.
The calculator uses the standard formula for a fully amortizing fixed-rate loan. Your monthly P&I is:
where:
This formula assumes a fixed rate and level payments. Adjustable-rate mortgages (ARMs) use different mechanics and are not modeled here.
Adding Taxes, Insurance, and PMI
- Property tax:Entered as an annual percentage of home value (e.g. 1.2%). The calculator multiplies home price by this rate and divides by 12 for the monthly tax component.
- Homeowners insurance:Entered as an annual dollar amount. Divided by 12 and added to the monthly total.
- PMI (Private Mortgage Insurance):Required when the down payment is less than 20%. Typically 0.5โ1.5% of the loan amount per year, divided by 12. The calculator applies PMI until the loan balance would fall below 80% of the original home value (simplified).
- HOA, other costs, and extras:Any monthly HOA fee, other recurring costs (maintenance, utilities), and optional extra principal payments are added so you see the full impact on payoff date and total interest. Extra payments support a custom start date, and you can model up to two one-time lump sums applied to principal on a specific month.
The number you see as "Total Monthly" in the calculator is P&I plus the following (all converted to monthly amounts):
Understanding Amortization
Why Early Payments Are Mostly Interest
Each month, interest due = (current balance) ร (monthly rate). When the balance is largestโat the start of the loanโthe interest portion is largest, so only a small part of the payment reduces principal. As you pay down the balance, interest shrinks and more of each payment goes to principal. The balance-over-time chart above the amortization table visualizes this decline โ the curve is shallow early on (when payments are mostly interest) and steepens as more of each payment goes to principal.
Crossover Year and Payoff Term
The "crossover year" is the first year in which the total principal paid exceeds the total interest paid. After that, you are building equity faster. "Payoff term" shows how many years until the loan is paid offโit can be shorter than the selected loan term if you make extra principal payments or use biweekly payments (26 half-payments per year, equivalent to one extra full payment per year).
Who This Calculator Is For
Home buyers and refinancers
Use it to estimate monthly P&I and total housing cost (PITI + PMI) before you apply. Compare 15- vs 20- vs 30-year terms and different rates to see how term and rate affect payment and total interest.
Borrowers considering extra payments
Enter extra monthly or annual principal to see how much sooner you pay off the loan and how much interest you save. Set a future start date if the extra payments won't begin immediately. Model one-time lump sums (bonus, inheritance, tax refund) with a specific month and year. Turn on biweekly payments to model the effect of 26 half-payments per year.
Transparency and limits
All calculations run in your browser; no data is sent to servers. Results are estimates. Property tax and insurance vary by location; PMI rules vary by lender. For binding figures and loan approval, use a qualified lender or advisor.
Validation Checklist
Validate with Affordability
Before committing to a payment, use our House Affordability Calculator. Lenders may approve up to 43โ45% DTI; a common guideline for financial flexibility is keeping housing (PITI) under 28% of gross monthly income.
Compare Rent vs. Buy
In high-rate environments, renting can yield a higher net worth over 10 years due to the opportunity cost of the down payment and closing costs. Use our Rent vs. Buy Calculator to compare your scenario.
Opportunity cost of the down payment
Money put into a down payment could otherwise be invested. Use the Compound Interest Calculator to see what that sum could grow to over the same period.
Mortgage Cost FAQ
What is the monthly payment on a $400,000 mortgage?
At a 7% interest rate over 30 years, a $400,000 mortgage has a principal and interest payment of approximately $2,661/month. Add property taxes, homeowners insurance, and PMI (if applicable) for your true monthly cost. Use the calculator above with your exact rate and term for a precise figure.
What does a mortgage payment include?
A full mortgage payment typically has four components โ principal, interest, property taxes, and homeowners insurance, often called PITI. If your down payment is under 20%, PMI is added as a fifth component. This calculator includes all of these: enter your property tax rate and annual insurance to see total monthly housing cost including P&I, taxes, insurance, and PMI.
How does interest rate affect my monthly payment?
Rate is the biggest single lever. On a $400,000 30-year loan, the difference between 6% and 7% is roughly $240/month โ and over $86,000 in total interest. Even a 0.5% improvement is worth shopping for across multiple lenders.
What is an amortization schedule?
An amortization schedule shows every payment broken into principal and interest over the life of the loan. Early payments are mostly interest; later payments shift heavily toward principal. The schedule lets you see exactly how much equity you build each year.
How much do I need for a down payment?
The conventional minimum is 3โ5% for most loan programs, though 20% avoids PMI. FHA loans allow 3.5% down. A larger down payment reduces your loan balance, lowers your monthly payment, and reduces total interest paid โ the calculator lets you compare scenarios side by side.
What is PMI and how does it affect my payment?
Private Mortgage Insurance protects the lender if you default, and is typically required when your down payment is below 20%. PMI usually costs 0.5โ1.5% of the loan amount annually, added to your monthly payment. It cancels automatically once your loan-to-value ratio reaches 78%.
What is the difference between a 15-year and 30-year mortgage?
A 15-year mortgage carries a lower interest rate but a significantly higher monthly payment. On a $400,000 loan, you might pay $3,500/month at 6.5% vs. $2,528/month at 7% on a 30-year โ but save over $200,000 in interest. The right choice depends on your cash flow and financial goals.
How is mortgage interest calculated?
Mortgage interest is calculated monthly on your remaining balance. Each month: remaining balance ร (annual rate รท 12) = interest due. The rest of your payment reduces principal. As principal falls, the interest portion shrinks โ which is why extra payments early in the loan have an outsized impact.
How much house can I afford?
A common guideline is that total housing costs (PITI) should not exceed 28% of gross monthly income, and total debt payments should stay under 43% (the DTI limit most lenders use). Use the monthly payment result from this calculator alongside your income to check both thresholds before applying.
How do biweekly payments work?
Biweekly means you pay half your monthly P&I every two weeks. That results in 26 half-payments per yearโequivalent to 13 full monthly payments instead of 12. The extra payment goes to principal and shortens the loan and reduces total interest. This calculator can model biweekly payments and show the updated payoff term and interest savings.
What costs should I include in "Other Monthly Costs"?
Use this field for recurring housing expenses not covered by taxes, insurance, or HOA โ such as maintenance (a common rule of thumb is 1% of home value per year), utilities, lawn care, or pest control. Including these gives you a more realistic picture of total monthly housing outlay when comparing to your budget or rent.
Can I model a one-time lump sum payment?
Yes. The calculator supports up to two dated one-time extra payments โ useful for modeling a year-end bonus, tax refund, or inheritance applied directly to principal. Enter the amount and select the month and year. The amortization schedule and payoff date will update instantly to reflect the impact.