US-FAA14 CFR 121.187
Airplanes: Reciprocating engine-powered: Landing limitations: Alternate airport
Read the official ruleOfficial FAA regulation text (public domain)
(a) No person may list an airport as an alternate airport in a dispatch or flight release unless the airplane (at the weight anticipated at the time of arrival at the airport), based on the assumptions in § 121.185, can be brought to a full stop landing, within 70 percent of the effective length of the runway. (b) This section does not apply to large nontransport category airplanes operated under § 121.173(c).
Verbatim text of a US Federal Aviation Regulation (public domain), reproduced for reference — not legal advice and not for navigation. A plain-English explainer may be added later; always rely on the current official rule linked above.