Let r be the annual rate divided by 12 and by 100, and n the number of months. Principal and interest is loan × r(1+r)^n ÷ ((1+r)^n − 1). A zero rate divides the loan by the months. Divide yearly property tax by 12 and yearly insurance by 12, then add both to that payment.
A $300,000 loan at 6.5% for 30 years is $1,896.20 of principal and interest. $3,600 of tax and $1,200 of insurance add $400. The housing payment is $2,296.20. Fees, mortgage insurance, and HOA dues are not in the formula. A lender’s quote can be higher. The Consumer Financial Protection Bureau explains how to read a real offer.