Add the monthly payments you want in the ratio, and divide by the monthly income. Multiply by 100. $1,500 of payments and $5,000 of income is 30%. The remainder is $3,500.
Include a payment only if you want it in this ratio. Rent, a car loan, and a minimum credit-card payment are common pieces. A lender may ignore some of them and add others. This page divides the two numbers you type.