A trailing-12 (T12) is the property's actual income and expenses over the last twelve months. It is the closest thing to the truth you will get: a full year smooths out seasonality, one-off repairs, and lucky months that make shorter windows misleading.
Why twelve months
Rentals breathe in annual cycles — property taxes hit once or twice a year, insurance renews annually, summer turns cluster, heating bills spike in winter. A 3-month window can show a property at double or half its true run rate. Twelve months captures one full cycle of everything, which is why lenders and serious buyers ask for a T12 rather than last quarter.
Reading a T12 like an operator
Scan month by month, not just the totals. Look for: rent steps (a renewal or new lease mid-year means the current rent runs higher than the T12 average), gaps in rental income (vacancies — count the months), expense spikes (one-off repair or a recurring problem?), and missing categories (no maintenance expense for 12 months means deferred maintenance, not a perfect building). Normalize one-time items out before computing NOI from it.
Worked example
A duplex T12 shows $30,600 rent collected, but months 1–3 show $2,400/month and months 4–12 show $2,600 — the current run rate is $31,200/year, not the T12 average. Expenses total $12,900, but $2,800 was a one-time sewer repair. Normalized NOI ≈ $31,200 − $10,100 ≈ $21,100 — materially different from the naive $17,700 you'd get from raw totals in a valuation.
When this rule of thumb breaks
A T12 is history, and history breaks at inflection points: a tax reassessment after sale, an insurance repricing, a lease signed last month, or a market where rents moved 10% this year. For a property you're buying, the seller's T12 may also omit self-performed labor and management. Use the T12 as the floor of your understanding — then adjust forward for what you know is changing. On brand-new or repositioned properties, there is no meaningful T12; underwrite from market data instead.