What Is Economic Order Quantity?
Economic Order Quantity (EOQ) is the order size that minimizes the combined cost of placing orders and holding inventory. It was first derived by Ford W. Harris in 1913 and later popularized by R.H. Wilson, which is why it's often called the Wilson formula.
The core trade-off is straightforward: order in large batches and you place fewer orders (lower ordering costs) but hold more inventory (higher holding costs). Order in small batches and the opposite happens. EOQ finds the balance point where total cost is lowest.
The Wilson EOQ Formula
The classic formula is:
EOQ = √(2DS / H)
Where:
- D = Annual demand (units per year)
- S = Order cost (cost to place and receive one order)
- H = Holding cost per unit per year (storage, insurance, opportunity cost)
At the EOQ, total ordering cost and total holding cost are exactly equal. This is a property of the math, not a coincidence — you can verify it with the calculator above.
Assumptions and Limitations
EOQ makes several simplifying assumptions that are important to understand before relying on it:
- Constant demand — The formula assumes you sell the same number of units every day. Real e-commerce demand is spiky, seasonal, and affected by promotions.
- Fixed order cost — Each order costs the same regardless of size. In practice, larger orders may qualify for quantity discounts or different shipping rates.
- Fixed holding cost — Warehousing costs per unit stay flat. But if you run out of warehouse space, marginal storage cost jumps significantly.
- Instantaneous replenishment — Inventory arrives all at once. Real lead times mean you need to factor in when to order, not just how much.
- No stockouts — The model assumes you never run out. It doesn't account for safety stock or demand uncertainty.
When EOQ Works Well
EOQ is most useful for stable, predictable products with consistent demand. Think consumables with steady reorder rates, commodity items where demand doesn't swing much week to week, or raw materials in a manufacturing context.
It's also a good starting point when you have no ordering system at all. Even with its simplifications, EOQ is better than guessing or always ordering the same arbitrary amount.
When EOQ Breaks Down for E-Commerce
For most e-commerce sellers, strict EOQ has real gaps:
- Seasonality — Demand during Q4 may be 3-5x your off-season. A fixed EOQ will leave you overstocked in spring and understocked in November.
- Product launches — New SKUs have no demand history. EOQ requires a known annual demand figure, which you don't have yet.
- Promotions and ads — Running a sale or increasing ad spend changes demand overnight. EOQ can't react to that.
- Supplier minimums — Your supplier may require a minimum order of 500 units. If EOQ says 320, you still have to order 500.
- Multi-channel complexity — Selling on both Shopify and Amazon means demand, fulfillment costs, and lead times differ by channel.
Beyond Static Formulas: Demand-Driven Ordering
EOQ gives you a single number based on annual averages. That's useful as a benchmark, but modern inventory management needs to respond to what's actually happening with your sales.
ReplenishRadar takes a different approach. Instead of static formulas, it uses your actual sales velocity across all channels, accounts for trend and seasonality in demand forecasts, and factors in real supplier lead times and constraints. The result is purchase order recommendations that adapt as your business changes — not a fixed number you calculated once.
That said, understanding EOQ is valuable. It teaches the fundamental trade-off between ordering frequency and inventory holding costs, and that trade-off doesn't go away just because you're using more advanced tools.