Monthly Stock Reorder Point Formula
Learn how to calculate a monthly inventory reorder point from average usage, supplier lead time, and safety stock.
A reorder point estimates the available inventory level that should trigger a replenishment order. It combines expected demand while waiting for delivery with a safety-stock buffer, helping you plan stock before normal usage exhausts it.
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Recommended reorder point
Where:
Estimate how many units will be used during the supplier lead time, then add extra buffer stock. The total is the inventory level at which a new order should normally be triggered.
Variables Explained
| Variable | What It Means | Unit |
|---|---|---|
| averageMonthlyUsage - Average monthly stock usage | The representative number of units sold, used, or issued in one month. | units per month |
| leadTimeMonths - Supplier lead time | The expected number of months from placing an order until usable stock is received. | months |
| safetyStock - Safety stock | Additional buffer inventory held for normal demand, delivery, or record uncertainty. | units |
| currentStock - Current available stock | Inventory currently available for sale or use after excluding unavailable, damaged, or committed stock where appropriate. | units |
Step-by-Step Calculation
Estimate demand during lead time
Multiply typical monthly usage by the supplier lead time to estimate how many units may be needed before a new delivery is available.
leadTimeDemand = averageMonthlyUsage * leadTimeMonths
Add the safety-stock buffer
Add the chosen buffer to lead-time demand. This creates the reorder threshold.
reorderPoint = leadTimeDemand + safetyStock
Compare stock with the reorder point
If available stock is below the threshold, this shows the units needed to bring it back to the reorder point.
stockShortfall = max(0, reorderPoint - currentStock)
Calculate current stock coverage
Divide current available units by monthly usage to estimate how many months of average demand are covered.
stockCoverageMonths = currentStock / averageMonthlyUsage
Example: Reordering a regularly used item
Calculate lead-time demand
500 × 1.5
750 units
Calculate reorder point
750 + 100
850 units
Calculate stock needed now
max(0, 850 - 650)
200 units
Calculate stock coverage
650 ÷ 500
1.3 months
Final Result
The reorder point is 850 units. With 650 units available, stock is 200 units below that threshold.
Assumptions
- ✓Average monthly usage is representative of expected demand during the supplier lead time.
- ✓Lead time runs from placing an order until stock is available for sale or use.
- ✓Safety stock has been chosen separately to reflect normal uncertainty.
- ✓Current stock represents usable available inventory rather than all physical inventory.
Limitations
- !The calculation does not automatically adjust for seasonal peaks, promotions, or abrupt demand changes.
- !It does not include quantities on open purchase orders or their expected receipt dates.
- !A fixed average lead time may not reflect occasional supplier delays.
- !The stock needed result is a threshold gap, not necessarily the final purchase order quantity.
Common Mistakes to Avoid
Entering annual demand instead of average monthly usage.
Using lead time in days without converting it to months.
Treating safety stock as part of normal monthly demand instead of a separate buffer.
Including damaged, reserved, or unavailable inventory in current available stock.
Assuming the reorder point alone determines the quantity to buy.
Related Formulas
Frequently Asked Questions
What is the monthly stock reorder point formula?
The formula is average monthly usage multiplied by supplier lead time in months, plus safety stock.
Why is safety stock added to the reorder point?
Safety stock provides a buffer for normal variation in demand, delivery timing, or stock records.
How do I calculate demand during lead time?
Multiply average monthly stock usage by the supplier lead time measured in months.
Does the reorder point tell me how much to order?
No. It identifies when to reorder. Order quantity can also depend on demand forecasts, pack sizes, order minimums, capacity, and incoming stock.
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