The vacancy rate assumption is one of the most overlooked inputs in a rental property analysis. Most people either copy a generic 5% figure from an old spreadsheet template or skip it entirely and assume full occupancy. New national data suggests it is worth revisiting that number, in both directions, before you run your next deal.
What Happened
Apartment List's August 2026 National Rent Report shows the national multifamily vacancy rate fell to 7.1% in August, down from a recent peak near 7.3% earlier in the year. That is the first month-over-month decline in the vacancy rate since late 2021, according to Apartment List's research team.
At the same time, the national median rent rose 0.1% in August to $1,390, the seventh consecutive monthly increase. Rents are still down 0.8% year-over-year, but that annual figure has been climbing steadily since bottoming out at -1.6% in April 2026. Apartment List also reports that vacant units are now taking an average of 32 days to lease, a sign that available inventory is absorbing faster than it was earlier in the year.
None of this means rents are about to spike. It means the multi-year softening cycle in rental demand appears to be leveling off, and a landlord's ability to keep a unit filled has modestly improved.
Why Vacancy Assumptions Move the Whole Deal
A rental property's cash flow is gross rent minus vacancy loss minus operating expenses minus debt service. Vacancy loss is usually expressed as a percentage of gross rent held in reserve for the months a unit sits empty between tenants or during turnover. Because it is a percentage applied straight to your top-line income, a change of even 2 to 3 percentage points in your vacancy assumption moves every downstream metric: net operating income, cap rate, and cash-on-cash return.
A Rental Property Calculator is the right tool for testing this, because it lets you swap the vacancy assumption without touching anything else in the model and see the isolated effect on your return.
Running the Numbers: 8% vs. 5% Vacancy
Take a $260,000 single-family rental with 20% down ($52,000), a $208,000 loan at 6.76% (the current 30-year fixed average per Freddie Mac's PMMS, week of September 10, 2026), monthly gross rent of $2,100, and combined taxes, insurance, and maintenance reserves of $580/month.
| Vacancy Assumption | Monthly Vacancy Loss | Monthly Cash Flow | Annual Cash Flow | Cash-on-Cash Return |
|---|---|---|---|---|
| 8% (conservative, softer-market default) | $168 | $6 | $72 | 0.1% |
| 5% (in line with the improving national trend) | $105 | $69 | $828 | 1.6% |
On this particular deal, tightening the vacancy assumption from 8% to 5% is the difference between a return that barely clears zero and one that is still thin but meaningfully positive. Neither scenario makes this a great deal on paper, which is itself useful information: it shows how sensitive thin-margin properties are to a single input most investors set once and never revisit.
What Should Actually Change
National data is a starting point, not a substitute for local numbers. Vacancy varies enormously by metro and by property class, and a single city's multifamily vacancy rate says nothing definitive about a single-family rental in a specific neighborhood. The right move with a data point like this is not to blindly drop your vacancy assumption to 5% because a national report improved. It is to pull actual comparable lease-up data for your target market, whether from a local property manager, MLS rental comps, or your own portfolio history, and use that as your real input, treating the improving national trend as a reason to double check an assumption you may not have revisited in a while.
Common Mistakes
- Using a single vacancy number for every property regardless of location. A high-demand urban submarket and a rural single-family rental do not carry the same lease-up risk.
- Ignoring vacancy entirely and modeling 12 months of full rent. Even strong markets have turnover; a $0 vacancy assumption overstates cash flow and understates risk.
- Confusing a national trend with a local guarantee. A national vacancy rate ticking down does not mean every metro, or every property, is tightening at the same pace.
- Not stress-testing both directions. Run your deal at a conservative vacancy rate as well as an optimistic one so you know the full range of outcomes before you commit capital.
Try It Yourself
Plug in your own purchase price, rent, and a vacancy assumption grounded in your actual local market (not a copied default) using the Rental Property Calculator to see how sensitive your specific deal really is.