How mortgage payments are calculated
Your monthly P&I uses the standard amortization formula. The true PITI payment adds monthly property tax (annual ÷ 12) and homeowner's insurance (annual ÷ 12). If your down payment is below 20%, PMI typically adds $50–$200/month on top.
M = P × [r(1+r)^n] ÷ [(1+r)^n − 1]
P = loan · r = monthly rate · n = total payments
What affects your payment most?
- Loan amount — a $50k difference changes payments by ~$300–$350/month
- Interest rate — 0.5% on $300k = ~$90/month, ~$32,000 total interest
- Term — 15-year costs ~40% more monthly but saves dramatically on interest
How is a monthly mortgage payment calculated?
Using the amortization formula M = P×[r(1+r)^n]/[(1+r)^n-1], where P is loan amount, r is monthly rate (APR÷12), and n is total payments.
What is PITI?
PITI = Principal + Interest + Taxes + Insurance — your true monthly housing cost. Lenders often quote only P&I, which looks lower.
What percentage of income should my mortgage be?
The 28% rule: housing costs should not exceed 28% of gross monthly income. On $80,000/year that is a maximum of $1,867/month.