Reference formulas and worked examples below keep their stated units. Monetary examples use USD; dimensional examples use US customary units unless labeled otherwise. Metric calculator inputs are converted to these reference units before calculation. Choosing another currency does not convert example amounts or exchange money.
How to use this mortgage calculator
Estimate principal and interest plus your entered housing costs. Add a monthly extra or a one-time payment to see loan interest saved and an earlier payoff. The schedule covers the loan only.
How the calculation works
Down payment dollars = home price × down payment percent ÷ 100 when percentage mode is selected, rounded to cents. Principal = home price − down payment dollars.
Monthly rate r = annual interest rate ÷ 1,200. With financed principal P and n months, the scheduled payment is P × r ÷ [1 − (1 + r)^(−n)]. At 0% interest, it is P ÷ n. A term in years is converted to whole months.
Each month, interest is the opening balance × r, rounded to cents. The scheduled payment and any monthly extra reduce interest first and then principal. A one-time extra reduces the remaining principal in the selected payment month.
Payments are capped at the balance plus interest. The final contractual payment adjusts for any cent-rounding remainder. Extra-payment savings compare the same loan with no voluntary extra payments; a one-time payment after the loan is already repaid has no effect.
Base monthly housing payment = scheduled principal and interest + annual tax ÷ 12 + annual insurance ÷ 12 + monthly HOA + monthly PMI. Monthly tax and insurance are each rounded to cents. Planned recurring outlay adds the voluntary monthly extra; a one-time extra is separate.
Worked example
A $350,000 home with 20% ($70,000) down finances $280,000. At 6.5% for 30 years (360 months), principal and interest are $1,769.79. Add $350 monthly tax and $150 insurance for $2,269.79 before HOA, PMI, or extra principal. Adding a $200 monthly extra makes the planned recurring outlay $2,469.79 while the base payment stays unchanged.
Limits and assumptions
Models a fixed-rate, fully amortizing loan with monthly interest and the first payment one month after funding. Daily accrual, irregular dates, variable rates, balloon payments, refinancing, prepayment penalties, and fees not included in principal are excluded. This does not compute APR. Tax, insurance, HOA, and PMI are constant entered estimates. Closing costs, maintenance, utilities, tax deductions, adjustable rates, lender recasts, and automatic PMI cancellation are excluded. Extra-payment savings cover loan interest only, not ownership costs.
Frequently asked questions
Can I enter a 20% down payment?
Yes. Choose Percent of home price and enter 20. The calculator derives the dollar down payment and amount financed. Changing modes converts the current value using the entered home price.
Can I combine monthly and one-time extra payments?
Yes. The monthly extra begins with the first payment. The one-time amount is applied after the regular payment in the selected loan month. Extras are capped at the amount still due and shorten payoff rather than recasting the regular payment.
Does the payment include taxes and insurance?
The base housing payment includes your monthly equivalents for tax and insurance plus entered HOA and PMI. The amortization schedule and interest-savings comparison cover principal and interest only.
Does PMI stop when I reach a certain balance?
No. Enter your applicable monthly PMI; cancellation rules depend on the loan and are not modeled.
What changes after the loan is paid off?
Principal and interest payments stop. Property taxes, insurance, maintenance, and other ownership costs may continue; the loan’s payoff date does not mean housing becomes cost-free.
Is this a current mortgage offer or an APR calculation?
No. Use your own fixed nominal interest rate and expense assumptions. APR may include other financing costs and is not calculated here.