Free online calculator · Financial
Mortgage Calculator
Estimate monthly principal and interest from home price, down payment, interest rate and loan term. Compare the lifetime interest cost and use results as estimates, not lending offers.
Mortgage amortization and extra-payment explorer
Optional detailed model. Enter loan amount, APR, term, optional extra principal and estimated housing expenses. Compare estimated payoff time and cumulative interest; the original Mortgage Calculator remains at the top of this page.
Estimate only. Review the assumptions and limitations on this page before making financial or academic decisions.
Formula • Worked example • Interpretation
Mortgage payment formula and amortization
A fixed-rate mortgage divides loan principal and interest into equal scheduled payments. Use the calculator to explore what happens when the price, down payment, interest rate, or loan term changes.
Calculation method
Loan principal = purchase price − down payment. Monthly rate r = APR ÷ 12; payment count n = 12 × term in years. Payment = P × r / (1 − (1 + r)⁻ⁿ). If r = 0, payment = P / n.
Worked example
A $300,000 mortgage at 6% interest for 30 years costs about $1,798.65 per month in principal and interest. Over 360 payments that is approximately $647,515 paid, including roughly $347,515 in interest.
What your result means
A smaller monthly payment does not necessarily mean a cheaper loan. A longer term generally reduces the required payment but can increase lifetime interest. Compare the total interest as well as the monthly amount.
Tips for better calculations
Try the same loan with 15- and 30-year terms; then adjust the down payment and interest rate independently. A lender’s loan estimate is authoritative for actual fees and payment obligations.
Assumptions and limitations
This payment covers principal and interest only. Property tax, homeowner’s insurance, mortgage insurance, HOA fees, closing costs, and adjustable-rate changes can make actual housing costs higher.
Additional reference: Consumer Financial Protection Bureau — Buying a house
SiCalcs tools provide estimates and educational calculations; they do not replace professional financial, medical, tax, or legal advice.