Gross monthly income is simply the total amount one is paid per month without any deductions for taxes and benefits. To calculate, simply multiply the hours worked per month by the hourly wage. If paid a salary, the monthly amount is the gross monthly income.
Gross monthly income can be calculated from an end-of-year pay stub by looking at the gross annual income and dividing that by the total number of months worked. If self-employed, gross monthly income is figured by subtracting total expenses from total revenues to arrive at total gains for the month. Then cost of goods sold is subtracted from total gains to arrive at gross monthly income.