Inventory Reorder Calculator
Find your reorder point and quantity so you never run out or overstock.
Running out of stock costs you sales and search ranking; overstocking ties up cash and accumulates storage fees. The reorder point sits between them: the stock level at which you must order so that new inventory arrives just before the old runs out. This calculator computes it, including a safety buffer sized to your actual variability.
Sales, stock and supplier
Result
Fill in the fields above and your result will appear here.
What the Inventory Reorder Calculator does
The reorder point is demand during the lead time plus safety stock. Safety stock exists because two things go wrong together: demand spikes and suppliers run late. Sizing it from the worst case of both — peak daily sales over the longest realistic lead time — is a simple and robust approach.
Order quantity is a separate decision, driven by how often you want to place orders. Ordering more frequently reduces stock held and frees up cash, at the cost of more administration and sometimes worse per-unit pricing.
How to use this tool
- Enter your average daily sales from the last 30–60 days, excluding any sale event that distorted the figure.
- Enter peak daily sales — your busiest normal day, not a festival spike.
- Enter the supplier lead time from order to sellable stock, and the worst case you have actually experienced.
- Enter stock on hand and anything already ordered but not yet received.
- Set your preferred order cycle and any supplier minimum order quantity, then read the reorder point and order size.
Formula and method
Lead time demand = Average daily sales × Lead time
Worst case demand = Peak daily sales × Worst-case lead time
Safety stock = Worst case demand − Lead time demand
Reorder point = Lead time demand + Safety stock
Order quantity = max(Average daily sales × Order cycle,
Supplier minimum order quantity)
Days of cover = Available stock ÷ Average daily sales
Worked example
Example: 12 units/day, 12-day lead time
| Lead time demand (12 × 12) | 144 units |
| Worst case (20 × 18) | 360 units |
| Safety stock | 216 units |
| Reorder point | 360 units |
| Order quantity for a 30-day cycle | 360 units |
With 240 units on hand you are already below the reorder point and should order immediately — 20 days of cover against an 18-day worst-case lead time leaves no margin.
What your result means
The reorder point is a trigger, not a target. When available stock touches it, place the order that day.
Days of cover is the quick health check. It should always exceed your worst-case lead time.
Large safety stock is a symptom, usually of an unreliable supplier or volatile demand. Fixing either releases working capital more effectively than any pricing change.
Important considerations
- Stockouts on marketplaces damage search ranking, and recovering position after restocking takes weeks. The cost is larger than the lost sales alone.
- Festival seasons need a separate calculation with a higher daily rate and often a longer lead time, since suppliers are busiest at the same moment.
- Storage fees at fulfilment centres rise sharply for slow-moving stock. Overstocking is not a free insurance policy.
- If your supplier minimum order quantity far exceeds your cycle requirement, either negotiate it down or accept a longer order cycle deliberately.
- Track lead time actuals rather than what the supplier promises. The gap between the two is what safety stock exists to absorb.
Limitations of this tool
- It uses a worst-case method for safety stock rather than a statistical service-level model. This is simpler and usually more conservative.
- It assumes steady demand across the period. Products with strong seasonality need seasonal parameters.
- It does not optimise order quantity for cost — economic order quantity models weigh ordering cost against holding cost and would give a different answer.
Frequently asked questions
How do I calculate safety stock?
The simple method used here is worst-case demand during the longest lead time, minus expected demand during the normal lead time. It covers the case where a demand spike and a supplier delay happen together.
How often should I reorder?
Frequently enough to keep working capital free, infrequently enough to avoid excessive administration and shipping cost. Thirty days is a common cycle for fast-moving products; slow movers can run at 60–90 days.
What happens if I run out of stock on a marketplace?
Beyond the lost sales, your listing loses search ranking and any velocity-based badges. Recovering position typically takes several weeks, which is why a stockout costs more than the immediate revenue.
Should I include stock in transit?
Yes — it is committed inventory that will arrive. Excluding it leads to double ordering, which is a common cause of overstocking.