Why It Matters
Cash flow per door tells you how much money each individual unit contributes to your bottom line once the mortgage, taxes, insurance, maintenance, and management are all paid. It is primarily useful as a comparison metric — it lets you evaluate a 4-unit property against a 40-unit property on an equal-footing, per-unit basis rather than comparing raw totals that reflect completely different scales.
At a Glance
- Measures: net monthly (or annual) cash flow divided by total unit count
- Used for: cross-property comparison, portfolio benchmarking
- Applies to: multi-unit rentals (duplex through large apartment)
- Strong benchmark: $200–$400/door/month in most U.S. markets
- Calculated after: debt service, taxes, insurance, maintenance, management
- Does not capture: total portfolio income or property value
Cash Flow Per Door = (Total Net Cash Flow) / Number of Units
How It Works
Step 1 — Calculate total net cash flow. Start with gross rental income, then subtract every operating expense: property taxes, insurance, repairs, property management fees, vacancy allowance, and capital expenditure reserves. Then subtract your monthly debt service (principal + interest). What remains is your net cash flow.
Step 2 — Divide by unit count. Take that net figure and divide it by the number of rentable units in the property. Each unit is one "door."
Step 3 — Decide monthly or annual. Most investors use the monthly figure for quick gut checks during deal screening. The annual figure (multiply by 12) fits better in spreadsheets and year-end portfolio reviews. Use one consistently and label clearly.
Applying the formula: A 6-unit building brings in $7,200/month in rent. After $2,400 in operating expenses and a $2,100 mortgage payment, net cash flow is $2,700. Divide by 6 units: $450 per door per month.
What counts as a "door"? Only rentable residential units. A storage room that earns revenue is counted in gross income but is not a door. A vacant unit still counts as a door — it is dragging your per-door number down, which is exactly the signal you want to see.
Using it for comparison. Suppose you are choosing between a 4-unit property at $600/door and a 20-unit property at $280/door. The 4-unit looks better per door, but the 20-unit may carry lower per-unit operating risk through diversification. Cash flow per door frames the conversation — it does not end it.
Portfolio-level use. Tracking per-door cash flow across your entire portfolio over time surfaces trends that raw totals mask. If your overall portfolio cash flow grows 10% but your unit count grows 15%, per-door cash flow is declining — a sign that newer acquisitions are underperforming.
Real-World Example
Hiro owns two rental properties and is trying to decide which deserves his next capital improvement budget.
Property A is a duplex. After all expenses and debt service, it nets $900/month total. Per door: $450.
Property B is a 10-unit apartment building. It nets $3,200/month total — more than three times Property A in absolute terms. Per door: $320.
On a per-unit basis, the duplex is performing better. Hiro traces the gap to an annual lease on Property B's largest unit that locked in rent two years ago with no lease escalation clause. Without a built-in CPI adjustment, that unit's rent has fallen 18% behind market. Meanwhile, a recent tenant turnover on a second unit cost $1,400 in cleaning, painting, and lost rent — further compressing per-door returns.
Hiro also discovers that one tenant in Property B has been subletting without authorization, creating an unrecognized income stream that flows to the tenant rather than the landlord.
Armed with per-door data, Hiro prioritizes Property B for lease renegotiation and operational tightening rather than capital improvements. The lower per-door number pointed him directly to where the real work needed to happen.
Pros & Cons
- Normalizes properties of different sizes so comparisons are apples-to-apples
- Quickly surfaces underperforming assets in a multi-property portfolio
- Simple to calculate and communicate to partners or lenders
- Tracks operational efficiency over time, independent of scale
- Useful as a screening filter before committing to deep underwriting
- Ignores property value and appreciation potential entirely
- A high per-door number at a low-value property may offer less total return than a lower per-door number at a higher-value property
- Averages hide unit-level variation — one exceptional unit can mask three poor ones
- Does not account for capital expenditure timing; a new roof next year is not visible in today's per-door figure
- Less useful for single-family rentals where the "per door" is simply total cash flow
Watch Out
Don't mix monthly and annual figures. If one property's data is monthly and another's is annual, the comparison is meaningless. Standardize before comparing.
Vacancy distorts the denominator. A building with three vacant units still has those doors in the denominator, pulling per-door figures down. Note the occupancy rate alongside per-door cash flow so the context is clear.
Gross rent vs. net cash flow confusion. Some investors accidentally use gross rent per unit rather than net cash flow per door. Gross rent per door is a different metric — useful, but it ignores all expenses and debt. Always confirm which figure you are working with.
Don't use it as the only filter. A $500/door property in a declining market may be a worse investment than a $200/door property in a growing one. Per-door cash flow describes current performance — it says nothing about where the market or the property is heading.
The Takeaway
Cash flow per door is a fast, reliable way to compare rental properties of different sizes and to spot underperformers inside a growing portfolio. It works best alongside occupancy rate, cap rate, and cash-on-cash return — not as a standalone decision-maker. Any time you are screening deals or reviewing your portfolio's health, cash flow per door should be one of the first numbers you calculate.