Good inventory forecasting comes down to a few simple ideas. Master these and you'll stop selling out of bestsellers and stop tying up cash in slow movers.
Velocity is how many units a product sells per day. Take units sold over the last 30 days and divide by 30. A product that sold 150 units last month has a velocity of 5/day.
Divide current stock by velocity. 40 units at 5/day means 8 days of stock. This single number tells you how urgent a reorder is.
Lead time is how long your supplier takes from order to shelf. If your bestseller has 8 days of stock but a 14-day lead time, you're already too late — you needed to order a week ago.
Your reorder point is the stock level at which you must place an order to avoid a stockout: velocity × lead time, plus a safety buffer (commonly 20–30%) to cover demand spikes. When stock drops to this level, reorder.
A practical rule: order enough to cover your lead time plus your next coverage window (say 30 days), minus what you already have. The goal is to arrive just as you'd otherwise run out.
Forecasting isn't only about stockouts. Products with 120+ days of stock are tying up cash you could put into winners. Track those and slow your reorders.
Calculating this by hand across dozens of SKUs is where it breaks down. Restockly does all of the above from your Shopify export in seconds — and ranks your products by the revenue at risk so you act on the costliest first.
Restockly tells you exactly what to reorder and when. Flat $19/mo, free to start.
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