Kasutusaeg

Kasutusaeg is the period during which a product, device, substance, or material is considered usable and safe for its intended purpose. It encompasses the time from manufacture until the point at which performance, safety, or quality falls outside acceptable limits, or until regulatory or manufacturer guidance requires replacement. The term covers both shelf life (the duration a product can be stored before use) and service life (the time it remains functional under use). The word derives from Estonian kasutus meaning use, and aeg meaning time.

Determination of kasutusaeg is typically based on testing, standards, and reliability assessments, and may be stated on labels as "use by," "best before," or as an estimated service life. Real-world factors such as storage conditions, maintenance, exposure to heat, moisture, load, and usage patterns strongly influence the actual usable period. Different categories have different practices: foods and medicines often carry explicit expiry dates; electronics and mechanical systems have expected service lives; materials may degrade due to corrosion, fatigue, or aging.

In practice, kasutusaeg serves as guidance for consumers, manufacturers, and regulators to ensure safety, performance, and waste reduction. It is an estimate, not a fixed arbitrary boundary; some items may remain safe beyond the stated period under proper conditions, while others may fail earlier. Related concepts include shelf life, service life, and durability.