These two get mixed up constantly, and it’s easy to see why — both are inventory formulas, both produce a number, and both show up in the same conversation about “how do we manage this SKU better.” But they answer completely different questions, and mixing them up leads to a specific, predictable failure: knowing how much to order but never checking when, or knowing when to order but ordering the wrong amount every time.
EOQ: How Much to Order
Economic Order Quantity answers exactly one question: given your demand, ordering cost, and holding cost, what order size minimizes your total annual inventory cost? It assumes demand is roughly steady and gives you a single, fixed order quantity to use every time you place an order for that SKU.
What EOQ doesn’t do: it says nothing about when to place that order. It also doesn’t protect you against uncertainty — if demand spikes unexpectedly or a shipment arrives late, EOQ has no mechanism for absorbing that. It’s a cost-optimization tool, not a risk-management tool.
Reorder Point: When to Order
Reorder Point answers a different question entirely: at what inventory level should you trigger the next order, so that it arrives before you run out? It’s calculated from your average demand during the supplier’s lead time, plus a safety buffer for demand or lead-time variability:
ROP = (Average Daily Demand × Lead Time) + Safety StockWhat Reorder Point doesn’t do: it says nothing about how much to order once you hit that trigger point. You could reach your reorder point and then place an order for 10 units or 10,000 — ROP is silent on that question. It’s a timing and risk-buffer tool, not a cost-optimization tool.
Side-by-Side
| EOQ | Reorder Point | |
|---|---|---|
| Question it answers | How much should I order? | When should I place the order? |
| Optimizes for | Minimizing total ordering + holding cost | Avoiding a stockout before the next delivery arrives |
| Built from | Annual demand, ordering cost, holding cost | Average daily demand, lead time, safety stock |
| Assumes | Demand is roughly steady | Lead time and demand can vary, so a buffer is needed |
| Doesn’t address | Protection against variability or a late shipment | Whether the order size itself is cost-efficient |
How They Work Together
The two aren’t competing methods — they’re complementary answers to different halves of the same purchasing decision. A complete reorder policy needs both: Reorder Point tells you when to pull the trigger, and EOQ tells you how big the order should be once you do.
Put together, the policy reads like this: “When inventory drops to [Reorder Point] units, place an order for [EOQ] units.” Two numbers, one sentence, and a purchasing team that can execute it without recalculating anything on the fly.
A Practical Example
Take a distributor stocking a moderately fast-moving product with the following figures: EOQ works out to 600 units, and separately, the Reorder Point — based on average daily demand and the supplier’s lead time, plus a safety buffer for variability — works out to 300 units.
The resulting policy: when stock on hand drops to 300 units, place an order for 600 units.
Now consider what happens if either half is missing. Without EOQ, the purchasing team might reorder in small, arbitrary batches — say 150 units at a time — because that’s the amount that “feels right,” racking up far more in ordering costs over the year than necessary. Without Reorder Point, the team might know the right order size but have no discipline around when to place it, either ordering too early (tying up cash sooner than necessary) or, worse, not noticing stock has run low until a customer order can’t be filled.
Why People Mix Them Up
Part of the confusion is that both formulas draw on overlapping inputs — both need some version of a demand rate, for instance — and both get discussed in the same “inventory optimization” conversations. But the fastest way to keep them straight is the one-line test: if the question is “how much,” it’s EOQ; if the question is “when,” it’s Reorder Point. Neither formula can answer the other’s question, no matter how the inputs are massaged.
It’s also worth noting that Reorder Point depends on Safety Stock as a direct input — the buffer that protects against demand spikes or a late delivery during the lead-time window. EOQ has no equivalent concept; it’s built entirely around steady-state cost minimization, with no buffer for uncertainty built in at all. That’s the clearest structural difference between the two: EOQ is a pure cost formula, Reorder Point (through its safety stock component) is partly a risk formula.
Decision Rule
Start with EOQ to get your cost-efficient order size. Add Reorder Point on top to protect against running out before the next order arrives. Use one without the other and you’re solving half the problem — a cost-efficient order size with no discipline about timing, or a disciplined reorder trigger with no thought given to whether the order size itself makes financial sense.
Once you have both a reorder point and an order quantity, the next question worth asking is whether either one still applies to this SKU at all — some inventory, like fast-moving or highly seasonal products, doesn’t fit either formula cleanly. EOQ Reality Check walks through how to tell.