The calculator subtracts your down payment from the home price to find the loan principal, then applies the standard fixed-rate amortization formula to compute a constant monthly payment. Each month's payment splits into interest (principal × monthly rate) and principal (the rest), and the schedule recomputes the remaining balance row by row. The full schedule shows where every dollar goes — early payments are mostly interest, late payments are mostly principal.


              M = P × (r × (1+r)^n) / ((1+r)^n − 1)
// P = principal (price − downPayment)
// r = annualRate / 100 / 12
// n = years × 12

Common use cases

Comparing 15-year vs 30-year fixed loans on the same home price.
Estimating how much a $50k down payment increase saves over the loan life.
Stress-testing affordability when interest rates change by 1–2 percentage points.

See also: Loan Calculator , Salary Calculator .

Frequently asked questions

Does this include property tax and insurance?

No — the monthly payment shown is principal + interest only (P&I). Real lenders also escrow property tax, homeowners insurance, and PMI (if down payment is under 20%). Add roughly 1–2% of home value per year for taxes + insurance to estimate true monthly cost.

What about adjustable-rate mortgages (ARMs)?

This calculator assumes a fixed rate. For an ARM, model the initial fixed period at the teaser rate, then re-run with a higher projected rate for the remaining term.

Why is total interest so much larger than the loan amount?

Compounding over 30 years is brutal. At 7% on $300k, you pay roughly $419k in interest — more than the loan itself. Even small rate decreases or extra principal payments save tens of thousands.

Can I add extra principal payments?

Not in this version — the schedule assumes the contractual minimum. To estimate the impact of extra payments, reduce the loan term and re-run; even adding 10% to each monthly payment can shave 5–8 years off a 30-year loan.

Embed this tool

Add this free mortgage calculator to your own website or blog — copy the code below and paste it into your page HTML. No signup, no API key.

<iframe src="https://otoolrun.com/embed/mortgage-calculator/" width="100%" height="560" style="border:0;border-radius:12px" loading="lazy" title="Mortgage Calculator"></iframe>
<p style="font-size:14px"><a href="https://otoolrun.com/tools/mortgage-calculator/">Mortgage Calculator by oToolRun</a></p>

🏠 Mortgage Calculator

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