How are Investment Property Mortgage Loan Amounts Calculated?

[Kelly Terry]

I am researching financing for my first investment property, and want to know how the maximum loan amount is calculated for mortgage loans for investment properties by banks? 

My only point of reference is going through the loan process when I purchased my primary residence through a mortgage broker 5 years ago. I understand credit, income, and debt are used to determine loan worthiness and interest rate, but I was given a maximum amount of money I could borrow, and I want to know how that number is calculated. 

Is there an industry standard calculation, or does it differ from institution to institution? If so, an example from actual banking institution would still be helpful.