Cash Flow Calculator
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Sam's Take
Cash flow per door is the metric that scales. A property cash-flowing $400/month sounds great until you realize it's a 4-unit and that's only $100/door. I won't touch a multifamily under $200/door these days. The reserves are non-negotiable: skip them and you'll think you're cash-flowing right up until the boiler dies.
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What "cash flow" actually means on a rental
Cash flow sounds simple: money in minus money out. The trap is that most people forget to count the money going out that doesn't show up as a monthly bill. Vacancy, maintenance, and capital expenditures don't appear on a credit card statement, but they're real, and they're the three things that turn a deal that "looks like it cash flows" into a deal that loses money over a hold period.
The three reserves you have to budget for
- Vacancy reserve. Even great units turn over. A unit empty for 1 month a year is already 8% vacancy. Budget 5-10% of gross rent. National average runs 5-7%. New England has tighter markets where you can pencil 5%; secondary markets need 8-10%.
- Maintenance reserve. Toilets break, drains clog, garbage disposals die, locks need rekey when a tenant moves out, paint touch-ups, snow plowing, the lawn. Budget 5-10% of gross rent. Older buildings need more.
- CapEx (capital expenditure) reserve. The big stuff that breaks on long timelines. Roof every ~25 years. HVAC every ~15. Water heater every ~10. Full kitchen or bathroom rehab every ~25. None of these hit on a monthly schedule, but if you don't reserve for them you're going to get hit with a $12,000 bill the year the boiler dies. Budget 5-10% of gross rent.
Add it up: 15-30% of gross rent goes to reserves before you've paid the mortgage, taxes, insurance, or management. That's why deals that look like 25% margins on paper come in at 8-12% in real life.
Why "per door" is the number that matters
Cash flow on one property can mislead you. A property with $300 of cash flow looks fine until you realize it has 3 units, which is $100 a door — basically zero margin. As you scale, the metric that actually matters is cash flow per door per month, normalized across the whole portfolio.
Across my New England portfolio, the bands I use: $200/door is healthy in 2026 across most US markets. $400/door is excellent. Below $100/door means a single bad year — one extended vacancy, one big repair, one late tenant — wipes out the whole year's profit. That's not investing, that's holding your breath.