How to Use This Calculator
Enter three numbers and the calculator returns the inventory level at which you should reorder. Average daily sales is your typical unit volume per day for the SKU. Lead time is the total days from placing an order to having it on your shelf (or checked in at FBA). Safety stock is the buffer that covers demand spikes or late shipments. Need the theory behind these inputs? Here's how the reorder point formula works.
Where to Get Each Input
Garbage in, garbage out. I've seen sellers plug in a gut-feel number for daily sales and wonder why they keep stocking out. Pull real data instead.
Average daily sales: In Shopify, go to Analytics → Sales by product and divide the total units sold by the date range. On Amazon, the Business Reports detail page gives you unit session percentages you can convert. Use the last 30-90 days unless demand has shifted recently - a new ad campaign or a seasonal ramp makes older data misleading.
Lead time: Ask your supplier what their current turnaround is, not what it was six months ago. Add transit days. If you ship ocean freight, add port delays. The number you want is order-placed to stock-available, not just manufacturing time.
Safety stock:If you don't have a number, use our safety stock calculatorfirst. It accounts for demand variability and lead time uncertainty so you're not just guessing a buffer.
Interpreting Your Result
The number the calculator returns is a trigger, not a schedule. When on-hand inventory for this SKU hits that level, place your next order. Set it as a low-stock alert in whatever system you use - Shopify notifications, an FBA restock alert, or a dedicated inventory tool. Recalculate at least monthly, and bump it up before peak seasons when daily sales climb and supplier lead times stretch.
When This Calculator Fits (and When It Doesn't)
This works well for SKUs with relatively steady demand and a supplier you order from on an as-needed basis. I use it for bread-and-butter products where last month's sales are a reasonable proxy for next month.
It's the wrong tool for highly seasonal SKUs where daily sales swing 5x between peak and off-peak - you need a demand forecast that shifts the inputs over time, not a static snapshot. It also won't help with one-time buys like limited editions or liquidation inventory. And if you consolidate orders on a fixed schedule (every Tuesday, first of each month), you're operating under a periodic review model where the order timing is fixed and the quantity varies - a different calculation entirely.
Related Calculators
Reorder points don't exist in isolation. Your safety stock number feeds directly into this calculation - run the safety stock calculator if you haven't already. Once you know when to reorder, the next question is how much: the EOQ calculator finds the order quantity that keeps ordering and holding costs balanced. And it helps to know what a stockout actually costs you - the stockout cost calculator puts a dollar figure on that risk.