Look beyond principal and interest
Your mortgage payment estimate starts with principal and interest, then adds the property taxes, homeowners insurance, PMI and HOA amounts you enter. The extra principal field is shown separately so you can distinguish the scheduled housing estimate from your planned cash outflow.
A worked example you can check
For a $350,000 home with $70,000 down, the loan is $280,000. At 6.5% over 360 months, principal and interest are $1,769.79/month. Add $4,200/year in taxes and $1,800/year in insurance: estimated housing payment is $2,269.79/month. This example has no PMI, HOA or closing costs.
Paying an extra $100 each month reduces interest by $60,213.35 and shortens repayment by 51 months, under the same assumptions.
Hypothetical inputs, not a current rate offer. Monthly interest and payments rounded to cents; final payment adjusted. Numerical fixtures checked with a separate Python Decimal implementation. See test cases.
How does a larger down payment affect the payment?
A larger down payment reduces the loan balance. At a fixed 6.5% rate over 30 years, each additional $10,000 down lowers principal and interest by about $63.21 per month. Taxes and insurance may stay the same, and PMI may change according to the lender’s terms.
PMI is an explicit assumption
Enter your quoted monthly PMI. The default ending month of 0 keeps that amount through payoff; a positive number includes it through that payment number and removes it afterward. This is a user-chosen scenario, not a legal cancellation determination. Confirm eligibility, loan type and lender requirements using your loan documents and the CFPB’s PMI guidance.
Closing costs: upfront or financed
Upfront costs do not change the loan payment. If you select financed closing costs, they increase the balance and earn interest. The model does not check whether a lender permits financing a particular cost or loan-to-value ratio.
What this housing estimate excludes
Property taxes, insurance, HOA and entered PMI are held constant except for your chosen PMI end month. Future increases, repairs, utilities, maintenance, escrow cushions, points-specific pricing, adjustable rates, FHA mortgage insurance and VA funding rules are not modeled. Taxes and insurance can continue after the mortgage is paid off. Total loan payments exclude recurring housing costs.