The 50% rule is a rule of thumb that says, over a full year, roughly half of a rental's gross rent will go to operating expenses, before you ever make the mortgage payment. It is a screening shortcut for the back of a napkin, not a budget: you take the rent, cut it in half, and treat what is left as the money available to cover debt and produce cash flow.
The point of the rule is speed. When a listing crosses your desk and the seller swears expenses are only 20% of rent, the 50% rule lets you sanity-check that claim in your head before you spend an hour building a real model.
In practice
Say you are looking at a duplex that brings in $1,500 a unit, so $3,000 a month and $36,000 a year in gross rent. The 50% rule says to assume about $18,000 of that goes to operating costs over the year: property tax, insurance, repairs, a vacancy allowance, water, and a management reserve. That leaves $18,000 to work with.
$36,000 gross rent × 50% = $18,000 left for debt and cash flow
Now subtract the loan. If the mortgage runs $1,250 a month, that is $15,000 a year, leaving $3,000 of estimated annual cash flow, or $250 a month. The half you set aside is a stand-in for real operating expenses, which means it does not include principal and interest. That is the most common place people misread the rule.
Why it matters to a small landlord
For a 1-to-10 unit owner, the 50% rule is a tripwire, not a verdict. A newer building in a low-tax area with long tenancies might run closer to 35%, while an old fourplex with high turnover and a leaky roof can blow past 55%. The rule is most useful when it disagrees with the seller: when their pro forma shows 25% expenses, the 50% rule tells you to go dig before you believe it. Once a property is yours, the only honest number is the one your own books produce, so track every dollar against operating expenses and compare your real ratio to the estimate at year two. You can also run a full deal with real line items in the worked deal analysis.
Treat the 50% rule as a first filter and nothing more. It overlaps with the 1% rule as a quick screen, it leans on a realistic read of your operating expenses, and what it is ultimately trying to predict is your cash flow. Use it to decide what deserves a real spreadsheet, then let the real numbers decide the rest.