Economic order quantity
The order size that makes total inventory cost as low as possible — the point where the cost of holding stock exactly balances the cost of placing orders.
C0 cost of placing one order · D annual demand in units · Ch cost of holding one unit for one year
Check your inputs — demand, order cost and holding cost must all be above zero.
See the worked example
A company uses 40,000 units a year. Each order costs £25 to place. Holding one unit for a year costs 50p.
- Put the numbers in: (2 × 25 × 40,000) ÷ 0.50 = 4,000,000
- Square root it: √4,000,000 = 2,000 units
- Orders per year: 40,000 ÷ 2,000 = 20 orders, costing 20 × £25 = £500
- Average inventory is half the order size: 2,000 ÷ 2 = 1,000 units, costing 1,000 × £0.50 = £500
Ordering cost and holding cost come out equal. That is not a coincidence — it is what the EOQ is defined as, and it is a free check on your answer in the exam.
Holding cost is per unit per year. If the question gives it as a percentage of purchase price, you have to work it out first: 10% of a £5 unit is £0.50, not 10.
If there is a bulk discount, EOQ is only the starting point. You then have to compare total cost at the EOQ against total cost at each discount order level — the answer is often not the EOQ.