Inventory replenishment: when and how much to reorder
Inventory replenishment is the routine of putting stock back before you run out, in the right quantity, at the right time. Get it right and you rarely stock out and rarely overbuy. Get it wrong in one direction and you lose sales to empty shelves; wrong in the other and cash sits frozen in stock you cannot sell fast enough. This guide covers the three numbers that decide every replenishment decision, how to calculate each one, and how to run the whole thing without doing the math by hand every week.
What replenishment actually decides
Every reorder comes down to two questions: when do I order, and how much do I order. Three numbers answer them.
- Safety stock is the buffer that covers surprises: a busy week, a late shipment, or both at once.
- The reorder point is the stock level that triggers a new order. Hit it, and you order.
- The order quantity is how much to buy each time.
Safety stock and the reorder point answer when. The order quantity answers how much. Miss any one and the routine breaks: too little safety stock and normal variation sells you out; a reorder point set too low and stock lands after you have already run dry; an order quantity chosen by habit and you either reorder too often or sit on months of excess.
The three numbers, in order
1. Safety stock
Safety stock sizes the cushion for uncertainty. A practical formula derives it from your own peaks:
Safety stock = (peak daily sales x maximum lead time) minus (average daily sales x average lead time)
The idea is to hold enough to cover the gap between your worst realistic case and the normal case you already plan for. Steady sales from a reliable supplier need little; spiky demand or unpredictable shipping needs more. Work out your number with the safety stock calculator.
2. The reorder point
The reorder point is the trigger. It is the stock you expect to sell while waiting for the new order to arrive, plus the safety stock from step one:
Reorder point = (average daily sales x lead time) + safety stock
When a variant drops to this level, it is time to order, and the new stock should arrive just as you would have run out. The reorder point calculator works it out and tells you whether you should already be ordering based on what is on hand.
3. The order quantity
Once you know it is time to order, decide how much. Ordering in big batches means fewer orders but more stock sitting around; ordering little and often flips the trade. The economic order quantity is the batch size where ordering cost and holding cost balance and the total is lowest:
EOQ = square root of (2 x annual demand x cost per order / holding cost per unit per year)
Use it as a baseline with the economic order quantity calculator, then adjust for supplier minimums, bulk discounts, and seasonal spikes, which the textbook formula does not account for.
Putting it together: a replenishment routine
With the three numbers in hand, the weekly routine is short:
- Check each variant against its reorder point. Anything at or below it goes on the order list.
- Order the economic order quantity for each, adjusted for any supplier minimum or price break.
- Keep safety stock under review. If a supplier's lead times drift or a product's demand gets spikier, recalculate, because a stale buffer is how surprises turn into stockouts.
Done consistently, replenishment stops being a scramble and becomes a few minutes of checking a list.
Doing it for a whole catalog
The math above is simple for one product. The problem is scale: a store with hundreds of variants cannot recompute three numbers for each, every week, by hand, especially when sales velocity and lead times keep changing. That is where it breaks down in practice, and where stores either overstock everything to be safe or lurch from one stockout to the next.
Restockly runs this routine across your entire Shopify catalog automatically. It watches real sales velocity, forecasts demand with transparent math you can check, and for every variant shows how much to order and by when, with safety stock and reorder points already worked in. It flags what crossed its reorder point today and exports a purchase order in one click. The calculations are the same ones on this page, so you can always verify any number yourself, and it was built as a clean replacement for Stocky ahead of its August 31, 2026 shutdown.
Start by working out the three numbers for a few key products with the calculators above. Once you see how they fit together, letting an app run them across the whole catalog is the natural next step.