It refers to monthly rents as 1% of the purchase price or total cost to get the unit rent ready. If you buy turn key or currently occupied, it's just the purchase price. If you buy and put in some cash to get the place up to par, go with the all-in amount.
And to clarify, the downpayment has nothing to do with the 1% rule, sorry if I wasn't clear. What I meant was that the 1% rule may not give you enough gross revenue to cashflow if you have a hefty interest payment every month.
As a conservative estimate, I like to do the quick math like this when evaluating a property:
1
Gross Revenue minus 10% for vacancy, minus 10% for repairs, minus 2% for credit loss gives you a rough estimate of gross operating income. Subtract your monthly expenses and mortgage payments and if there is $$ left over, the property deserves further attention. If (Gross Revenue X 78%) - operating costs - mortgage payments is > $0, it looks good.
Again, it's quick math (my accountant would not approve) but it gives me a good idea of what we are looking at and if it makes sense to invest the time, energy, and money into writing a contract and going through the due dilligence.
I haven't used the calculators on BP (I joined after my last acquisition) but I hear they are quite good. Just don't get caught in analysis paralysis and spreadsheet yourself to death!
Cheers!