The Vacancy Assumption That Quietly Decides Your Cash Flow

The Line Nobody Argues About

When investors fight over a deal, they fight about the price, the rent, and the rehab number. Almost nobody argues about the vacancy assumption. It sits on the pro forma as a tidy 5% and everyone nods. That's exactly why it's dangerous. A number nobody scrutinizes is a number that can quietly decide whether your deal actually cash flows.

Vacancy is a percentage haircut applied to your gross rent. Set it too low and every downstream number — net operating income, cash flow, cash-on-cash return — looks better than reality. The trap is that the error doesn't show up on closing day. It shows up eight months later when a tenant moves out and you eat two months of turnover you never budgeted for.

What Vacancy Is Actually Paying For

Vacancy isn't just "days the unit sits empty." It's a catch-all reserve for the full cost of turnover. When a tenant leaves, you typically lose rent during the make-ready period, again during marketing and showings, and sometimes during lease-up if the market is soft. On top of that, you often have turn costs — paint, cleaning, small repairs — that some investors bury in maintenance and others fold into vacancy.

The point is to be consistent. If your vacancy line only covers lost rent, make sure your maintenance and capex lines cover the turn work. What you cannot do is assume near-zero vacancy AND a skinny maintenance budget. That's double-counting your optimism.

Here's the mechanic that trips people up. A single 30-day vacancy on a unit that stays occupied the rest of the year isn't a 30/365 event — it's roughly 8% for that year. If tenants turn over every two years and each turn costs you about six weeks of downtime and prep, your true long-run vacancy allowance is closer to 5-6% just from that one pattern, before you account for a soft rental market.

The Math That Should Worry You

Let's use round, illustrative numbers. Say a property brings in $2,000/month, or $24,000/year. Your operating expenses and debt service leave you with what looks like $250/month in cash flow at a 5% vacancy assumption.

Now bump vacancy to 8%. That's an extra 3% of $24,000, or $720/year — $60/month. Your $250 becomes $190. Push it to 10% because the unit is in a slower-leasing submarket, and you've knocked roughly $100/month off the number you underwrote. On a thin deal, that's the difference between a keeper and a liability.

This is why I run vacancy as a range, not a point estimate, and why the calculators I trust let you flex the assumption instead of hard-coding a friendly default. If a deal only works at 4% vacancy, it doesn't work. It's a bet on flawless operations you don't yet have.

How to Set the Number Honestly

Start from the property, not the spreadsheet. A stabilized single-family rental in a strong school district with a tenant likely to stay for years carries less turnover risk than a C-class multifamily unit with annual churn. Setting both at the same 5% is lazy.

Work through the actual pattern you expect. How long do tenants stay? How many weeks between move-out and rent-ready? How fast do comparable units lease in that area? Local rent velocity matters, and it's the kind of thing worth checking against market reports before you commit to a number. A hot metro that leases in a week justifies a lower assumption than a market where units sit for a month.

Then add a buffer for the things you can't predict — an eviction, a unit that needs more turn work than expected, a rent you priced too high and had to drop. I'd rather be pleasantly surprised than caught short on a mortgage payment.

Stress-Test Before You Sign

The habit that saves deals is running the numbers at a worse vacancy than you expect. If you think 6% is realistic, model it at 9% or 10% and see if you can still cover debt service and reserves. When I'm building a deal in the analyzer, I look at that stressed number as the real floor. If the property survives a bad year on paper, I can sleep. If it only survives a perfect one, I pass or renegotiate the price until the math works.

Remember: you can't control the vacancy rate the market hands you, but you completely control the assumption you underwrite to. Pick the honest one.

The Takeaway

Before you buy your next property, run it twice — once at your realistic vacancy and once three points higher. If the higher number still cash flows after debt service and reserves, the deal has a margin of safety. If it doesn't, either lower your price or walk. Don't let a default percentage decide your cash flow for you.