A mortgage payment is typically four components added together: principal and interest (P&I) from the loan itself, property tax (divided into monthly installments), homeowners insurance, and — when the down payment is under 20% — private mortgage insurance (PMI). The P&I portion is calculated by the standard amortization formula M = P × [r(1+r)^n] / [(1+r)^n − 1]. This calculator shows each component separately so you can see exactly where the monthly number comes from.
The formula
Principal and interest (P&I):
M = P × [r(1+r)^n] / [(1+r)^n − 1]
Where:
- M = monthly P&I payment
- P = loan amount = home price − down payment
- r = monthly interest rate = annual rate ÷ 12 (as a decimal)
- n = total months = loan term in years × 12
Total monthly payment:
Total = P&I + (Annual property tax / 12) + (Annual insurance / 12) + PMI + HOA
PMI applies when down payment < 20% of purchase price. The default rate used here is 0.75% of the loan amount annually, which is the midpoint of the common 0.5%–1.5% range. PMI is removed from this calculator's estimate when the down payment reaches 20%.
Source: Consumer Financial Protection Bureau — Mortgage Key Terms; Investopedia — Amortization Formula.
Practical examples
Example 1: Standard 30-year mortgage
$400,000 home, $80,000 down (20%), 30-year term, 7% annual rate:
- Loan amount: $320,000
- Monthly P&I: $2,129
- No PMI (down payment ≥ 20%)
- Total interest over 30 years: $446,349
- Total loan cost: $766,349
Example 2: With less than 20% down
$400,000 home, $40,000 down (10%), 30-year term, 7% rate, PMI at 0.75%:
- Loan amount: $360,000
- Monthly P&I: $2,395
- Monthly PMI: $225 (0.75% × $360,000 / 12)
- P&I + PMI: $2,620
Example 3: Full PITI breakdown
$400,000 home, $80,000 down, 30-year term, 7% rate, with additional costs:
| Component | Annual amount | Monthly share |
|---|---|---|
| Principal & Interest | — | $2,129 |
| Property tax (1.2% of value est.) | $4,800 | $400 |
| Homeowners insurance | $1,500 | $125 |
| PMI (n/a — down ≥ 20%) | — | $0 |
| HOA | — | $0 |
| Total | — | $2,654 |
Common mistakes
Treating the P&I estimate as a loan offer. Actual loan terms depend on your credit score, debt-to-income ratio, loan type (conventional, FHA, VA), the lender's current pricing, and prevailing market rates. This calculator applies the formula; a lender applies underwriting.
Forgetting property tax. Property tax rates vary widely by location — from under 0.5% to over 2.5% of assessed value annually. Omitting it understates the true monthly cost significantly.
Assuming PMI is permanent. For conventional loans, PMI can typically be cancelled when you reach 20% equity in the property — either through payments, appreciation, or both. FHA mortgage insurance has different cancellation rules. The PMI line shown here applies only to the calculation period where down payment is under 20%.
Ignoring the difference between purchase price and assessed value. Property tax is based on assessed value, which may differ from purchase price. Check local assessor records for an accurate estimate.
International and regional variations
| Country | Mortgage insurance equivalent | Tax / insurance treatment |
|---|---|---|
| United States | PMI (conventional) / MIP (FHA) / VA funding fee | Property tax paid via escrow; homeowners insurance required by lender |
| Canada | CMHC mortgage insurance (required for down < 20%) | Usually added to loan balance, not monthly payment; property tax paid separately |
| United Kingdom | No direct PMI equivalent; higher LTV mortgages carry higher rates | Council Tax paid separately; buildings insurance required |
| Australia | LMI (Lenders Mortgage Insurance) | Usually capitalized into the loan; council rates paid separately |
| European Union | Varies by country and lender | Property and income tax treatment varies significantly by jurisdiction |
Quick reference: loan amount by payment budget
Approximate loan amounts at 7% annual rate for different monthly P&I budgets:
| Monthly P&I budget | 15-year loan | 20-year loan | 30-year loan |
|---|---|---|---|
| $1,000 | ~$111,200 | ~$128,100 | ~$150,300 |
| $1,500 | ~$166,800 | ~$192,100 | ~$225,500 |
| $2,000 | ~$222,400 | ~$256,200 | ~$300,600 |
| $2,500 | ~$278,000 | ~$320,200 | ~$375,800 |
| $3,000 | ~$333,600 | ~$384,300 | ~$451,000 |
These are rough estimates for P&I only. Add tax, insurance, PMI, and HOA to determine total housing cost. Actual loan offers depend on lender underwriting.