The lower of cost or market rule is associated with the accounting guideline or constraint known as conservatism. (Conservatism means that if doubt exists between two alternatives, the accountant should choose the alternative that will result in a lesser asset amount and less profit.)
When the lower of cost or market is applied to inventory, the accountant will value the inventory items at the lower of (1) the cost, or (2) the replacement cost within a ceiling and a floor.
The ceiling is the highest amount for the market amount. The ceiling is the net realizable value (NRV) and it is calculated as follows: the selling price in the ordinary course of business minus any costs of completion and disposal.
The floor is the lowest amount for the market amount and it is the net realizable value minus the normal profit.
Here is one example. Assume an inventory item had a cost of $10. Its replacement cost is now $8. Its net realizable value is $9, and the normal profit is $2. The cost of $10 is compared to the market amount. Market is the replacement cost of $8 constrained by a ceiling of $9 (NRV) and a floor of $7 (NRV of $9 minus $2 profit). Since the $8 replacement cost is within the ceiling and the floor, the accountant compares the cost of $10 to the market of $8. The lower of cost or market is $8.
Here's another example. An item in inventory had a cost of $10. Its replacement cost is $5. The net realizable value is $7 and the normal profit is $1. The market amount is the replacement cost of $5, but it is constrained by a ceiling of $7 and a floor of $6 ($7 minus the normal profit of $1). Because the replacement cost is below the floor amount, the floor amount is used. This means the accountant chooses between the lower of the cost of $10 or the market of $6. Hence, the item should be valued at $6 under the lower of cost or market rule.