Mortgage Calculator
Calculate monthly mortgage payments and total interest
This free online mortgage calculator provides instant results with no signup required. All calculations run directly in your browser — your data is never sent to a server. Enter your values below and see results update in real time as you type. Perfect for everyday calculations, homework, or professional use.
$80,000 (20.0%)
Loan Principal
$320,000
Total Interest
$446,428
Total Paid
$766,428
How to Use This Calculator
Enter your input values
Fill in all required input fields for the Mortgage Calculator. Most fields include unit selectors so you can work in your preferred unit system — metric or imperial, whichever matches your problem.
Review your inputs
Double-check that all values are correct and that you have selected the right units for each field. Incorrect units are the most common source of calculation errors and can produce results that are off by factors of 2, 10, or more.
Read the results
The Mortgage Calculator instantly computes the output and displays results with units clearly labeled. All calculations happen in your browser — no loading time and no data sent to a server.
Explore parameter sensitivity
Try adjusting individual input values to see how the output changes. This is a quick and effective way to develop intuition about how different parameters influence the result and to identify which inputs have the largest effect.
Formula Reference
Mortgage Calculator Formula
See calculator inputs for the governing equation
Variables: All variables and their units are labeled in the calculator interface above. Input fields accept values in multiple unit systems — select your preferred unit from the dropdown next to each field.
When to Use This Calculator
- •Use the Mortgage Calculator when comparing financial options side-by-side — such as different loan terms or investment returns — to make more informed decisions.
- •Use it to quickly estimate costs or returns before making purchasing, investment, or borrowing decisions.
- •Use it for financial education and planning to understand how compound interest, fees, or tax affects the real value of money over time.
- •Use it when building or reviewing a budget to verify that projections and calculations are mathematically correct.
About This Calculator
The Mortgage Calculator computes monthly payment amounts, total interest paid, and amortization schedules for fixed-rate home loans. Enter the loan principal, annual interest rate, and loan term to see a complete breakdown of your mortgage costs. Understanding the relationship between loan amount, interest rate, term, and monthly payment is essential for homebuyers evaluating affordability. This calculator is also useful for comparing refinancing scenarios and estimating how extra payments reduce the total interest paid over the life of the loan.
The Theory Behind It
A mortgage is an amortizing loan secured by real estate. The monthly payment for a fixed-rate mortgage is calculated with the standard amortization formula PMT = P × r × (1+r)^n / ((1+r)^n − 1), where P is the principal (loan amount), r is the monthly interest rate (annual rate divided by 12), and n is the total number of monthly payments (years × 12). This formula is derived by requiring that the present value of the payment stream, discounted at the monthly rate, equals the original principal — it is the unique payment that pays off the entire loan over the specified term while charging interest on the outstanding balance each month. Each monthly payment is split between interest (computed as the current balance × monthly rate) and principal (payment minus interest). In the early years of the loan, the balance is high and most of each payment goes to interest; as the balance shrinks, the interest portion decreases and the principal portion grows. This creates the characteristic amortization curve that shows interest and principal crossing at roughly the 2/3 mark of the loan term for typical rates. The total interest paid over the life of the loan can be surprisingly large: on a 30-year mortgage at 6.5%, you pay about 1.3 times the original principal in interest — meaning a $300,000 loan costs you roughly $383,000 in interest on top of the $300,000 principal, for a total of $683,000 paid back over 30 years. Shortening the term to 15 years dramatically reduces total interest (often by 60–70%) but increases the monthly payment by roughly 40–50%. Making additional principal payments has a cascading effect: each extra dollar of principal reduces all future interest, shortens the loan duration, and accelerates equity buildup. Biweekly payment schedules (26 half-payments per year, equivalent to 13 monthly payments) effectively make one extra payment per year and can shave 4–7 years off a 30-year mortgage.
Real-World Applications
- •Home affordability analysis: before house-shopping, enter different loan amounts, interest rates, and terms to see what monthly payment fits your budget. A common rule of thumb is that housing costs (principal + interest + taxes + insurance) should not exceed 28% of gross monthly income — use the calculator to work backward from your income to a maximum home price.
- •Refinance break-even analysis: when rates drop, compare your current monthly payment with the new payment at a lower rate. Divide the closing costs by the monthly savings to get the break-even period in months; if you plan to stay in the home longer than that, refinancing makes sense.
- •Term comparison (15-year vs 30-year): see how much more principal you pay each month on a 15-year loan but how much less total interest you pay over the life of the loan. The trade-off is cash flow flexibility versus interest savings.
- •Extra-payment strategy: add a fixed extra amount to your monthly payment and see how many years it cuts off the loan. A $200 extra payment on a $300,000 30-year mortgage at 6.5% can shorten the loan by roughly 7–8 years and save $80,000+ in interest.
- •ARM vs fixed decision: compute the fixed-rate monthly payment and compare with the ARM teaser-rate payment plus the possible post-adjustment payment at higher rates. Stress-test whether your budget can absorb the worst-case ARM reset before choosing variable-rate financing.
Frequently Asked Questions
How is my monthly mortgage payment calculated?
The formula is PMT = P × r × (1+r)^n / ((1+r)^n − 1), where P is the loan amount, r is the monthly interest rate (annual rate ÷ 12), and n is the number of months (years × 12). For a $300,000 loan at 6.5% for 30 years: monthly rate = 0.065/12 = 0.00542, n = 360, PMT = $1,896/month. This is the 'P&I' (principal + interest) payment only. Your full housing payment will also include property taxes, homeowner's insurance, and potentially PMI or HOA fees — collectively called PITI (principal, interest, taxes, insurance). This calculator shows the P&I portion; add your local tax and insurance costs for the total housing cost.
How much house can I afford?
A traditional rule of thumb is that your total housing cost (PITI) should not exceed 28% of your gross monthly income, and your total debt payments (PITI + car loan + credit cards + student loans) should not exceed 36% — the '28/36 rule.' For a household earning $100,000/year ($8,333/month gross), that means maximum PITI of $2,333 and maximum total debt of $3,000. Work backwards from this PITI cap using your local tax and insurance estimates to determine maximum P&I, then use this calculator to find the loan amount. Lenders may approve you for more, but sticking to the 28/36 rule gives you margin for unexpected expenses.
Should I pay extra principal or refinance?
It depends on the rate differential and your time horizon. Paying extra principal reduces total interest without any closing costs or rate risk — you save exactly what the amortization table shows. Refinancing to a lower rate saves more per dollar but incurs 2–5% of the loan amount in closing costs, which must be recovered from monthly savings before you break even. Rule of thumb: if you can drop your rate by 1% or more AND plan to stay in the home for at least 3 more years, refinancing usually wins. If you're staying put but can't refinance, extra principal is the best lever.
What's the difference between a 15-year and 30-year mortgage?
On a $300,000 loan at 6.5%: the 30-year monthly payment is $1,896 and total interest is $383,000. The 15-year at roughly 0.25% lower (6.25%) is $2,572/month and total interest is $163,000 — $220,000 less. The 15-year saves enormous interest but requires a 36% higher monthly payment. Choose 15-year if the higher payment fits your budget with margin and you value being debt-free faster. Choose 30-year for lower monthly obligations and more cash flow flexibility (you can always make extra principal payments on a 30-year to replicate a faster payoff without being locked into it).
Why does my mortgage payment seem mostly interest at first?
In an amortizing loan, each month's interest is calculated on the current outstanding balance at the monthly rate. When the balance is near the original principal (early in the loan), most of the payment covers interest. As you pay down principal, the interest portion shrinks and the principal portion grows. For a $300,000 30-year at 6.5%, the first month's payment is about $1,625 interest and $271 principal. By year 15, the split is roughly 50/50. By the final year, almost all of each payment is principal. This is why making extra principal payments early in the loan has such a large impact on total interest — every dollar you pay off early avoids years of compounding interest.
Does this calculator include taxes and insurance?
No — this calculator shows principal and interest only (the 'P&I' portion of your mortgage payment). Your full monthly housing cost also includes property taxes (typically 0.5–2.5% of home value annually depending on location), homeowner's insurance (~0.3–0.5% of home value annually), private mortgage insurance or PMI if your down payment is less than 20% (~0.5–1.5% of loan amount annually), and potentially HOA fees. To estimate your total PITI, compute P&I here and add 1/12 of your annual tax, insurance, and PMI estimates.