Every warehouse manager faces the same tension. Order too little and you run out, disappoint customers and pay for rush shipping. Order too much and cash sits on a shelf, taking up space and risking obsolescence. Two simple concepts — safety stock and the reorder point — turn that guesswork into a decision you can defend with numbers.
Start with lead time
Lead time is the gap between placing an order and having the goods available to sell or use. It is rarely a single number. A realistic view breaks it into stages:
- Time to approve and issue the purchase order internally
- Supplier production or picking time
- Transit time, including customs where relevant
- Receiving, inspection and put-away at your own site
Teams often measure only the supplier’s promised time and are then surprised by shortages. If approval takes three days and put-away takes two, those five days are part of your real exposure.
What safety stock is for
Safety stock is the buffer you hold to absorb uncertainty. It exists because two things refuse to behave predictably:
- Demand varies. Some weeks sell twice the average.
- Lead time varies. A shipment that usually takes ten days occasionally takes eighteen.
If both demand and lead time were perfectly constant, safety stock would be unnecessary — you could time deliveries to arrive exactly as the last unit is sold. In the real world, the buffer is what stands between normal variation and a stockout.
A simple way to size the buffer
A widely used starting point compares the worst realistic case with the average case:
Safety stock = (maximum daily demand × maximum lead time) − (average daily demand × average lead time)
Suppose a product sells an average of 40 units a day, peaks at 65, and the supplier takes 10 days on average but has occasionally taken 14:
| Maximum scenario | 65 × 14 = 910 units |
| Average scenario | 40 × 10 = 400 units |
| Safety stock | 910 − 400 = 510 units |
More advanced methods use statistical measures of variability and a chosen service level — the probability of not running out during a replenishment cycle. Those formulas are more precise, but they demand clean historical data. The simple version above is a reasonable first step for most small and mid-sized operations.
Calculating the reorder point
The reorder point is the inventory level that triggers a new purchase order:
Reorder point = (average daily demand × average lead time) + safety stock
Using the same figures: (40 × 10) + 510 = 910 units. When stock on hand drops to 910, it is time to order — not when the shelf looks empty.
One detail matters here: the trigger should consider stock on hand plus stock already on order, otherwise you will place duplicate orders while a shipment is still in transit.
How much to order
The reorder point answers when. Order quantity answers how much, and it balances two opposing costs:
- Ordering costs — administration, freight, inspection. These favour fewer, larger orders.
- Holding costs — capital tied up, storage, insurance, spoilage, obsolescence. These favour smaller, frequent orders.
The Economic Order Quantity model formalises this trade-off. In practice, supplier minimums, pallet sizes, container capacity and volume discounts often constrain the answer more than the formula does.
Not every item deserves the same treatment
Applying detailed calculations to thousands of items is unrealistic. Classification helps you focus:
| Class | Typical profile | Suggested control |
|---|---|---|
| A | Few items, large share of value | Tight monitoring, frequent review |
| B | Intermediate value and volume | Periodic review |
| C | Many items, small share of value | Simple rules, larger buffers |
Counter-intuitively, class C items often justify generous safety stock: they are cheap to hold, and running out of a low-value component can still halt an order.
Keeping the numbers honest
Formulas only work if the inputs reflect reality. A few habits protect the system:
- Cycle counting. Count a portion of items regularly instead of relying on one annual inventory. Recorded stock that differs from physical stock breaks every calculation downstream.
- Track actual lead times per supplier, not the promised ones.
- Separate unusual demand. A one-off bulk order should not permanently raise your “maximum daily demand”.
- Review parameters seasonally. Values set in a quiet month will fail in a peak month.
- Watch slow movers. Stock that has not moved in months is a cash problem disguised as an asset.
Signs your parameters need adjusting
- Frequent emergency orders and expedited freight
- Recurring stockouts on the same handful of items
- Storage areas overflowing while service levels stay poor
- Write-offs of expired or obsolete goods
- Buyers overriding the system “by feel” most of the time
That last point is worth taking seriously. If experienced buyers constantly ignore the reorder point, the parameters — not the buyers — are usually wrong.
Putting it together
Good inventory control is not about holding as little stock as possible. It is about holding the right stock: enough to protect service levels for the items that matter, and no more than necessary for the rest. Lead time, demand variability, safety stock and reorder points are the four levers that make that balance visible instead of intuitive.
If you want to build these skills further, the free logistics and supply chain courses on Cursa cover inventory planning, warehousing and purchasing in more depth — useful whether you manage a small stockroom or a distribution centre.
