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Monthly P&I = L × r × (1 + r)^n ÷ [(1 + r)^n − 1] where L = loan amount, r = monthly rate (annual rate ÷ 12 ÷ 100), n = number of months.
Total interest = (Monthly payment × n) − Loan amount
Worked example: $380,000 loan ($400,000 price minus $20,000 down) at 7.00% for 30 years. r = 7 ÷ 12 ÷ 100 = 0.005833. Monthly P&I = $380,000 × 0.005833 × (1.005833)^360 ÷ [(1.005833)^360 − 1] = $2,529/month. Total interest = $530,442. Add PMI at 0.8% ($253/month until 20% equity), property tax $400/month, insurance $150/month: total PITI + PMI = approximately $3,332/month until PMI cancels at 80% LTV.