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336x280 · Finance/Insurance · Est. CPM $22-$48
How the mortgage payment formula works
The calculator uses the standard loan amortisation formula to compute your monthly principal and interest payment:
Formula: M = P × [r(1+r)ⁿ] / [(1+r)ⁿ − 1] — where P = loan principal, r = monthly interest rate (annual rate ÷ 12), n = total number of payments (years × 12)
Property tax and home insurance are divided by 12 and added to the P&I figure to give your full estimated monthly outgoing.
Worked example with real numbers
Home price: $450,000 | Down payment: $90,000 (20%) | Interest rate: 6.8% | Term: 30 years | Property tax: $5,400/yr | Insurance: $1,800/yr
Input
Value
Monthly Impact
Loan amount
$360,000
—
Monthly rate (r)
6.8% ÷ 12 = 0.5667%
—
Payments (n)
30 × 12 = 360
—
Principal & interest
Formula result
$2,344
Property tax
$5,400 ÷ 12
$450
Home insurance
$1,800 ÷ 12
$150
Total monthly
All combined
$2,944
Common mistakes to avoid
Ignoring PMI. If your down payment is under 20%, add 0.5–1% of the loan annually (roughly $150–$300/month on a $360,000 loan). The calculator does not include PMI automatically.
Using gross income for the 28% rule. The rule says housing costs should not exceed 28% of gross monthly income — but lenders also look at your debt-to-income ratio. Include all monthly debt payments in your planning.
Forgetting HOA fees. In many US communities, monthly HOA fees of $200–$800 can significantly change affordability. Add these to your monthly total manually.
Comparing only the monthly payment. A 30-year mortgage at $2,344/month looks cheaper than a 15-year at $3,190/month — but the 30-year costs nearly $200,000 more in total interest.
Locking in the calculator rate. The default rate is an average. Your actual rate depends on your credit score, down payment, and loan type. A 720 vs 760 credit score can easily differ by 0.3–0.5%, which changes monthly payments by $60–$100 on a $400,000 loan.
What is the monthly payment on a $400,000 mortgage?
At 6.46% with 20% down on a $400,000 home, expect approximately $2,019/month in principal and interest on a 30-year loan, plus property tax and insurance.
How much income do I need for a $400,000 mortgage?
Using the 28% rule, you need at least $86,500/year gross income. Your total monthly housing cost should not exceed 28% of gross monthly income.
What is PMI and when do I pay it?
Private Mortgage Insurance is required when you put less than 20% down. It typically costs 0.5-1% of the loan annually and cancels automatically when you reach 20% equity.
Is it better to get a 15 or 30 year mortgage?
A 15-year mortgage saves significantly in total interest but has 40-50% higher monthly payments. A 30-year is more affordable monthly. Choose based on your cash flow and how long you plan to stay.