Economic Order Quantity • Reorder Point • Safety Stock
Course Duration: 70–90 minutes
Every stocked item raises the same two questions: how much should we order, and when? This course builds the answer one tool at a time — Economic Order Quantity for how much, the Reorder Point for when, and Safety Stock for the days that don't go to plan — then puts all three together into one automatic ordering rule. Calculators in every section let you try the math with your own numbers.
Finding the optimal amount to order — every time
Imagine you manage the supply room at a busy service company. You need to keep printer paper stocked. You have two choices that both cost you money:
You're constantly calling suppliers, receiving shipments, and processing invoices. All that time and effort adds up — these are your ordering costs.
You're holding mountains of paper in the warehouse. Space, insurance, and the risk of spoilage or damage all cost money — these are your holding costs.
EOQ finds the sweet spot — the order quantity where your total costs (ordering + holding) are at their lowest point.
A hardware store sells 2,400 boxes of screws per year. Each order costs $30 to process. Holding one box costs $2.00 per year.
At 268 boxes per order, the store would place about 9 orders per year (2,400 ÷ 268), roughly every 40 days.
Why does the formula work? As order quantity increases, ordering costs go down (fewer orders placed) but holding costs go up (more inventory sitting around). EOQ is the quantity where these two costs are exactly equal — the mathematical minimum of total cost.
Two attempts per question. Finish every check in this section to unlock the next one.
Knowing exactly when to place your next order
EOQ tells you HOW MUCH to order. The Reorder Point tells you WHEN.
Here's the problem: when you place an order, it doesn't arrive instantly. Your supplier needs time to process and ship it — this is called lead time. During that waiting period, your inventory keeps being used. If you wait too long to order, you'll run out before the shipment arrives.
How to think about it: ROP represents the amount of inventory you expect to consume while waiting for your order to arrive. By the time your shipment shows up, you've (ideally) used exactly the ROP quantity — arriving at zero just as the new stock lands.
A parts warehouse uses an average of 25 units per day. Their supplier consistently delivers within 6 days of an order being placed.
By the time the shipment arrives 6 days later, the warehouse will have used approximately 6 × 25 = 150 units — landing at zero right as the new stock arrives.
What if you don't hit the ROP in time? You get a stockout — you run out of product before the order arrives. This means frustrated customers, halted operations, or costly emergency orders. That's where safety stock comes in (Section 3).
Your ROP must increase. You'll consume more inventory during the longer wait, so you need to order earlier.
Your ROP must increase. You're burning through inventory faster, so the trigger point needs to come sooner.
Two attempts per question. Finish every check in this section to unlock the next one.
Your buffer against the unexpected
The world doesn't always cooperate. Demand can unexpectedly spike because of a promotion, a seasonal rush, or a competitor going out of stock. Suppliers can run late due to production issues, shipping delays, or weather. The basic ROP assumes everything goes according to plan — safety stock is your insurance policy when it doesn't.
Safety stock is extra inventory held above and beyond what's needed to meet expected demand during lead time. It sits in the background, ready to absorb the shock of unpredictable events.
A supply room normally uses 30 units per day, but on busy days it can spike to 50. The supplier takes 5 days to deliver.
The Statistical Method — For more precise safety stock calculation (used when you have historical demand data), the statistical formula uses standard deviation and a Z-score to hit a specific service level target:
| Target Service Level | Meaning | Z-Score |
|---|---|---|
| 85% | 85 out of 100 reorder cycles have no stockout | 1.04 |
| 90% | 90 out of 100 cycles — no stockout | 1.28 |
| 95% | 95 out of 100 cycles — no stockout | 1.65 |
| 98% | 98 out of 100 cycles — no stockout | 2.05 |
| 99% | 99 out of 100 cycles — no stockout | 2.33 |
Service Level Trade-Off: A higher service level means fewer stockouts — but it requires more safety stock, which means higher holding costs. Most companies balance between 90%–98% depending on how critical the item is and how severe a stockout would be.
Two attempts per question. Finish every check in this section to unlock the next one.
EOQ + ROP + Safety Stock working as one system
HOW MUCH to order each time
WHEN to place the order
PROTECTION against variability
Together, these three tools create a complete, automatic inventory management cycle. Once set up, the system tells you exactly when to order and exactly how much — while protecting you from the inevitable bumps along the way.
Here's how the cycle works in practice: Inventory starts high (just received an order). Every day, demand pulls it down. When it reaches the ROP, you immediately place an order for the EOQ quantity. The safety stock sits at the bottom as a final buffer. By the time the order arrives, you've ideally consumed down to just your safety stock level — and the new shipment refills your stock back up.
Full Worked Scenario — Metro Supply Co.
Metro Supply Co. manages industrial cable for field technicians. Here is all the data they need:
Q* = √( 2 × 3,600 × 45 / 3.00 ) = √(324,000 / 3) = √108,000
SS = (16 – 10) × 5 = 6 × 5
ROP = (10 × 5) + 30 = 50 + 30
Metro Supply Co. sets up their system with one simple rule:
That's it. The entire inventory management system for this item boils down to a single automated trigger. Review and recalculate these numbers periodically as demand patterns change, supplier performance shifts, or costs are renegotiated.
Two attempts per question. Finish every check in this section to unlock the next one.
Covering all three topics — EOQ, Reorder Point, and Safety Stock. Take your time and use the knowledge from all four sections.
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