About the EOQ Calculator
The Economic Order Quantity calculator finds the order size that minimizes the combined cost of placing orders and holding inventory, using the classic EOQ inventory model. It's meant for anyone managing physical stock who wants to stop guessing at reorder sizes and instead balance ordering frequency against storage cost mathematically.
How It Works
You enter annual demand in units, the fixed cost to place a single order, and the annual holding cost per unit (storage, insurance, and tied-up capital). The calculator applies the standard EOQ square-root formula to find the optimal order quantity, then derives how many orders per year that implies and the total annual cost of ordering plus holding inventory at that quantity.
Formula & Methodology
The formula sets ordering cost equal to holding cost at the optimum: total ordering cost is (demand/Q) x cost per order, which falls as order size Q grows, while total holding cost is (Q/2) x holding cost per unit, which rises as Q grows because average inventory on hand is half the order quantity. Solving for the Q that minimizes their sum produces the square root expression. Plugging that Q back into both cost terms and summing them gives the total annual cost at the optimal order size.
Examples
Moderate-volume retailer
With annual demand of 12,000 units, a $75 cost per order, and a $3 annual holding cost per unit, EOQ = sqrt((2 x 12,000 x 75) / 3) = sqrt(600,000) which is about 775 units, requiring roughly 15.5 orders per year.
High holding cost, lower order frequency
If holding cost rises to $12 per unit annually (bulkier or higher-value inventory) with the same 12,000 unit demand and $75 order cost, EOQ = sqrt((2 x 12,000 x 75) / 12) = sqrt(150,000), about 387 units, roughly doubling the number of orders needed per year to about 31.
Advantages
- Turns two competing cost pressures (frequent small orders vs. infrequent large ones) into a single optimal number instead of a guess
- Reports total annual cost at the optimum, so you can see the dollar impact of following the recommended order size
- Requires only three inputs that most inventory or purchasing systems already track, making it fast to apply to real SKUs
Common Mistakes
- Applying EOQ to products with highly seasonal or volatile demand, where the model's constant-demand assumption doesn't hold
- Leaving out capital cost (the return you're giving up by having cash tied up in inventory) when estimating holding cost per unit
- Treating the EOQ output as a hard rule rather than a starting point, ignoring supplier minimum order quantities or bulk discount breakpoints
Edge Cases to Watch For
- If annual demand, order cost, or holding cost is zero or negative, the calculator returns an error, since the square root formula requires all three to be positive.
- The model assumes constant, known annual demand with no seasonality and instantaneous replenishment - it does not account for lead time, safety stock, or demand variability.
- Holding cost is treated as a flat per-unit annual figure; it does not separately break out storage, insurance, and capital cost components even though the field description references all three.
Common Use Cases
- A small retailer or wholesaler deciding how much stock to reorder each cycle for a steady-selling SKU
- An operations or supply chain analyst comparing how order cost or holding cost changes would shift optimal order size
- A student or professional working through inventory management coursework that covers the classic EOQ model