Housing Estimate
Principal and Interest payment $
Insurance $
Taxes $
Mortgage Insurance $
(if over 80 percent of loan total value)
Total PITI $ (b)

Monthly Payments
(on revolving and installment debt) $ (c)

Total of (b) and (c): $ (d)

(b) divided by (a) equals housing ratio = %

(d) divided by (a) equals total debt ratio = %

Common standard is 28 percent housing ratio, 36 percent total debt ratio.
Exceptions to the rule depending on the type of loan and credit scores.