The direct answer
There is no single U.S.-wide financial definition that determines whether a rental is legally a short-term rental. An underwriting model may group nightly or vacation stays as STR activity, while cities, counties, states, HOAs, leases and tax authorities can use different day thresholds, permit categories and definitions.
Separate the model label from the legal definition
A financial model can use nightly rate, available nights and occupancy to estimate revenue. That modeling choice does not establish whether the activity is permitted or how a jurisdiction classifies the stay. Legal definitions must be checked for the property’s exact location.
To apply the approach to an editable scenario, use the Airbnb / Short-Term Rental Calculator. For the neighboring question, explore Midterm Rental Calculator.
Check every relevant rule layer
Review municipal and county zoning or STR ordinances, state requirements, lodging or occupancy tax rules, HOA or condominium restrictions, lease terms and applicable licensing. A platform listing does not by itself prove that the use is lawful.
Record the rule date and source
Local STR rules can change. Save the ordinance, government page, permit guidance or written confirmation used in diligence and note the date reviewed. Re-check before acquisition and before beginning operations.
Worked example
An investor may model stays averaging four nights as short-term rental revenue. A local ordinance might define regulated rentals using a different maximum stay period or a specific permit category. The spreadsheet label therefore cannot answer the compliance question.
Open Airbnb / Short-Term Rental Calculator and test your assumptions →
Common mistakes and limitations
Do not publish or rely on a universal number of days as the legal STR definition for every U.S. jurisdiction. Local rules and tax definitions can differ.
Uses user-entered assumptions and does not access Airbnb/AirDNA or guarantee local STR legality, demand or pricing.
Frequently asked questions
Does Airbnb approval mean a property is legally permitted as an STR?
No. Platform availability does not replace local zoning, licensing, tax, HOA or other applicable requirements.
Can an HOA restrict an STR even if the city allows it?
Potentially, depending on enforceable governing documents and applicable law. Review the actual documents and local legal requirements.
Sources and further reading
The article’s examples are illustrative arithmetic. The references below provide context for operating, valuation or transaction concepts; they do not endorse the assumptions or the calculator.
- Fannie Mae — Selling Guide B3-3.8-03, Short-Term Rental (09/02/2026)Current short-term-rental income context and vacancy/maintenance treatment.
Read our editorial policy for the scope of these educational methods.