A pro forma is a forward-looking projection of a rental property's income and expenses, built to estimate what the building should earn under normal operation. It is a model, not a record: every line is an assumption about future rent, vacancy, and costs rather than a dollar that has actually changed hands. When a listing comes with a pro forma, treat it as the seller's best case until you have checked each number against something real.
In practice
Say you are looking at a fourplex listed at $480,000, and the broker hands you a pro forma showing four units at $1,400 each. That is $5,600 a month, or $67,200 a year in gross potential rent. The pro forma then applies a 5% vacancy factor, which trims it to $63,840. That trimmed figure is the effective gross income, the rent you would actually expect to collect.
From there the pro forma subtracts operating costs: property taxes of $6,200, insurance of $2,400, water and trash of $3,600, repairs of $3,200, and a management allowance of $5,100. That is $20,500 in expenses, leaving a projected net operating income of $43,340. Divide that by the $480,000 price and the pro forma claims a 9% cap rate. The trouble is the assumptions underneath it. If the units actually rent for $1,250 today, not $1,400, gross potential rent falls to $60,000, effective gross income to $57,000, and NOI to roughly $36,500, which is a 7.6% cap rate. Same building, two very different deals, and the only thing that changed was who picked the rent number.
Why it matters to a small landlord
A pro forma is where a deal is won or lost on paper, so the work is verifying every input before you trust the output. Pull the current rent roll and the leases to confirm what tenants pay now, not what they could pay. Ask for the last two years of tax returns or bank statements to ground the expense lines, because seller pro formas tend to show a management line of zero and a repair line that no real building ever hits. The gap between the projection and the verified numbers is the whole exercise: see pro forma vs actuals for how to rebuild a listing's figures from the ground up, and walk a full deal through how to analyze a rental property before you write an offer. You can also run your own corrected numbers through the cap rate calculator so the cap rate reflects rents you can actually defend.
A pro forma is only as honest as the assumptions you feed it, and most of those assumptions live in numbers you can check. Confirm the rent against real leases, test the expenses during due diligence, and replace the seller's guesses with verified figures so that the projected net operating income and the effective gross income describe the building you are about to own rather than the one the listing wishes it were.