To calculate a mortgage payment, use the formula payment = P × r ÷ (1 − (1 + r)^−n), where P is the loan amount, r is the monthly interest rate (the annual rate divided by 12) and n is the number of monthly payments — or let the Mortgage Calculator run it instantly from a home price, down payment, rate and term. On a $300,000 home with 20% down, a $240,000 loan at 6.5% over 30 years costs $1,516.96 a month in principal and interest, and $306,107 of interest in total. This guide works that example in full, compares a 20-year term, and states plainly what the figure leaves out. General information only — not financial advice.
The Formula, in Plain Language
A mortgage payment has to do two jobs at once: pay the interest the lender charges on the balance each month, and chip away at the principal so the balance reaches exactly zero on the final payment. The formula balances those two jobs:
Monthly payment = P × r ÷ (1 − (1 + r)^−n)
- P — the principal: the amount actually borrowed, which is the home price minus your down payment.
- r — the monthly rate: the annual percentage rate divided by 12 and by 100. A 6.5% annual rate gives r = 0.065 ÷ 12 ≈ 0.0054167.
- n — the number of payments: years × 12. Thirty years is 360 payments; twenty years is 240.
One special case is worth knowing: at a 0% rate the formula divides by zero, so the payment is simply P ÷ n — the loan sliced into equal pieces with no interest at all. Every real mortgage sits between that floor and whatever your rate adds on top.
How to Use the Mortgage Calculator, Step by Step
- Enter the home price. The full purchase price, not the amount you want to borrow.
- Enter the down payment. The tool subtracts it: loan = price − down payment. On $300,000 with 20% down, the down payment is $60,000 and the loan is $240,000.
- Enter the annual interest rate and the term in years. The tool converts the rate to monthly and the term to a payment count.
- Read all four outputs, not just the payment. Monthly payment, total interest, total cost and payoff date — plus a yearly amortization table showing how the balance falls year by year.
- Re-run with one change at a time. A shorter term, a bigger down payment, a lower rate: seeing each lever's effect separately is how the calculator teaches, not just tells.
Worked Example: $300,000 Home, 20% Down, 6.5%, 30 Years
This is the worked example from the tool page, with figures you can verify against the formula. Price $300,000; down payment 20% = $60,000; loan P = $240,000. Rate 6.5% gives r ≈ 0.0054167; term 30 years gives n = 360. The formula returns a monthly payment of $1,516.96. Multiply back to check the totals: 360 payments × $1,516.96 ≈ $546,107 of total cost, and $546,107 − $240,000 principal = $306,107 of total interest (the tool page rounds this to roughly $306,000). The loan clears in month 360 — exactly 30 years after the first payment, which is what the payoff date shows.
Worked example: $240,000 loan at 6.5% — principal vs total interest, 30 years vs 20 years — hover or tap each bar for details.
| Figure | 30-year term | 20-year term | Difference |
|---|---|---|---|
| Loan amount | $240,000 | $240,000 | Same loan |
| Monthly payment (principal & interest) | $1,516.96 | $1,789.38 | +$272.42 a month |
| Number of payments | 360 | 240 | 120 fewer |
| Total interest | $306,107 | $189,450 | −$116,657 |
| Total cost of the loan | $546,107 | $429,450 | −$116,657 |
The 20-year comparison is the most useful second calculation most buyers can run. The same $240,000 at the same 6.5%, repaid over 240 payments instead of 360, costs $1,789.38 a month — $272.42 more each month — and $189,450 of interest in total. The trade in one line: pay about 18% more per month, pay about 38% less interest overall (116,657 ÷ 306,107 ≈ 0.381), and own the home free and clear ten years sooner. Whether that trade suits a household depends on cash flow, other debts and job security — questions a calculator frames honestly but cannot answer for you.
What the Payment Figure Does NOT Include
The calculator's payment is principal and interest only. A real monthly housing cost is higher, and a serious plan adds the missing pieces separately rather than pretending the formula covered them:
| Cost | In the calculator? | Note |
|---|---|---|
| Principal and interest | Yes | The formula's whole job |
| Property taxes | No | Vary widely by location; often collected monthly with the payment |
| Homeowner's insurance | No | Required by lenders; priced by the property and area |
| Mortgage insurance (PMI) | No | A deposit under 20% usually adds mortgage insurance until equity builds |
| Association / HOA fees | No | Apply to many condos and planned communities |
The tool also assumes a fixed rate for the whole term and no overpayments or fees. An adjustable-rate loan, or a habit of paying extra each month, changes both the total interest and the payoff date — the amortization table shows the fixed-rate path so you can see the baseline those changes would improve on.
Five Mistakes People Make Reading a Mortgage Calculation
- Borrowing the home price instead of the loan amount. The formula's P is price minus down payment. Run $300,000 through it instead of $240,000 and every figure that follows is fiction.
- Comparing loans by monthly payment alone. A lower payment bought with a longer term can cost far more in total interest — $306,107 versus $189,450 on the very same loan above. Compare total interest and payoff date too.
- Treating principal-and-interest as the whole housing cost. Taxes, insurance, mortgage insurance and association fees sit on top. A payment that looks affordable in the calculator can strain a real month once the rest arrives.
- Putting down a tiny deposit without pricing the insurance. Under 20% down usually means mortgage insurance added to the monthly cost. Sometimes that is still the right choice — but price it knowingly, with the calculator showing what the larger loan itself costs.
- Assuming the rate is the only negotiable number. Term, down payment and points all move the totals. Re-run the calculator changing one input at a time before deciding which lever matters most for your situation.
General information, not financial advice. This guide explains how mortgage maths works using one verified example. It is not a recommendation to borrow, a quote, or a substitute for advice from a licensed mortgage professional who knows your full circumstances.
Related Tools and Guides
- Mortgage Calculator — the tool used in this guide: payment, total interest, total cost, payoff date and a yearly amortization table.
- Percentage Calculator — for the deposit percentages and rate changes behind the mortgage figures.
- Discount Calculator — the same percentage discipline applied to prices and offers.
- How to calculate percentages — the plain-language foundation for deposits, rates and comparisons.
- How to calculate a discount — final prices, savings and stacked offers, worked in full.
Building a property, finance or booking website where visitors need honest calculators like this one built in? Book a free consultation with Asad Webs
Frequently Asked Questions
How is a mortgage payment calculated?
With the formula payment = P × r ÷ (1 − (1 + r)^−n): P is the loan amount (price minus down payment), r is the monthly rate (annual rate ÷ 12), and n is the number of monthly payments. At 0% interest the payment is simply P ÷ n.
What is the payment on a $240,000 loan at 6.5% over 30 years?
$1,516.96 a month in principal and interest, with total interest of $306,107 over the 360 payments and a total loan cost of $546,107.
How much interest do I save with a 20-year term instead of 30?
On the same $240,000 at 6.5%: the 20-year term costs $1,789.38 a month and $189,450 of interest in total — $116,657 less interest than the 30-year term, in exchange for $272.42 more per month.
Does the calculator include taxes and insurance?
No. It calculates principal and interest only. Property taxes, homeowner's insurance, mortgage insurance and association fees are extra and vary by property and location, so add them separately when planning a monthly cost.
What happens if my down payment is under 20%?
Your loan is larger, so every figure grows — and a deposit under 20% usually adds mortgage insurance to your monthly cost until you build enough equity. The calculator shows the loan itself; ask the lender for the insurance figure on top.
What does the amortization table show?
Year by year, how each year's payments split between interest and principal and how the remaining balance falls — early payments are mostly interest, later ones mostly principal, and the balance reaches zero in the final month of the term.
Can I use the calculator for an adjustable-rate mortgage?
Only as a fixed-rate snapshot: the tool assumes one fixed rate, no overpayments and no fees for the whole term. For an adjustable loan, re-run it at each rate you want to test to see that scenario's payment and totals.
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