Hi Khang,
The 50% rule of thumb is based on acquiring a buy and hold (rental) property:
Say you purchase a house for $100,000 dollars and requires $5,000 dollars to repair (in a perfect world) with a conventional mortgage that will loan you $75,000 dollars for 20 years at 4% interest.
(I cheated and used my Deal or No Deal Real Estate Calculator and e-mailed myself the results but this should explain it by numbers:)
Purchase Price:$100,000.00
Estimated Repair Costs: $5,000.00
Total Investment: $105,000.00
Mortgage Amount: $75,000.00
Cash Required: $30,000.00
(Out of Pocket Money Required)
Estimated Gross Operating Income: $1,200.00
(Estimated Rent per month)
Monthly Expenses:
Vacancy Expense (%): 5.00%
Management Expense (%): 10.00%
Repairs & Upkeep (%): 10.00%
Adjusted Rent per month: $900.00
Property Taxes (Year): 1,100.00
Insurance (Year): 900.00
Additional Expenses: 0.00
Cash flow before Debt Service (Month): $733.33
Debt Service (Mortgage)
Loan Term (Years): 20.00
Interest Rate: 4.000%
Mortgage Payment (Month): $454.49
Monthly Cash Flow: $278.85
Yearly Cash Flow: $3,346.18
Annual Return on Investment: 11.15%
Now, the 50% rule of thumb is a conservative approach to evaluate whether or not a buy and hold deal will cash flow. The bold figures above is all you need to apply this rule of thumb to:
Estimated Gross Operating Income (Rent/Month): $1,200.00
Now take 1,200 and multiply it by 50% (0.5) which will yield you 600.
1,200*(0.5)= 600
This will factor all of your expenses tied into the property such as management, vacancy, depreciation, taxes, insurance (as noted above with my calculator).
Next you will take 600 and subtract debt service, which is your monthly payments towards your loan:
Mortgage Payment (Month): $454.49
600-454.49= 145.51 Cash flow/month
This last figure is the conservative figure you will collect after everything is all said and done, which is your cash flow.
(Also, please note its a bit less than what my calculator has calculated..better safe than sorry!)
Hope this helped you out.