The design life of a component or product is the period of time during which the item is expected by its designers to work within its specified parameters; in other words, the life expectancy of the item. Engineers follow a theory to calculate the life expectancy from expected conditions, uses and physical properties. It is not always the actual length of time between placement into service of a single item and that item's onset of wearout.

Another use of the term design life deals with consumer products.