What Is Inventory Replenishment? Methods and How to Do It

What inventory replenishment means

Inventory replenishment is the process of restocking products in the right quantity at the right time.Also called stock replenishment, it exists to keep goods available for sale while tying up as little cash as possible in stock you don't yet need.

Every replenishment decision boils down to two questions, asked for each SKU:

• When do I reorder? The trigger — the stock level or the date at which you place a new order.
• How much do I reorder? The quantity — enough to last comfortably, not so much that it turns into dead stock.

Get both right and stock quietly refills itself just ahead of demand. Get either wrong and you swing between empty shelves and a warehouse full of cash you can't use.

The four main replenishment methods

There is no single correct method — the right one depends on how predictable your demand is and how much you want to automate. These are the four you'll meet most often:

1. Reorder point

You set a trigger level for each SKU. When on-hand stock falls to that level, you reorder. It reacts to actual sales rather than the calendar, so fast movers get reordered sooner and slow movers later — automatically. This is the workhorse method for most ecommerce sellers, and the one we calculate below.

2. Periodic review

You review stock on a fixed schedule — say every Monday — and top each SKU back up to a target level. Simple to run and easy to batch orders with one supplier, but because you only look on review day, a fast seller can run out mid-week. It suits businesses with a regular ordering rhythm and steady demand.

3. Min/max

A close cousin of the reorder point. You set a minimum (the trigger) and a maximum (the ceiling). When stock hits the minimum, you order enough to return to the maximum. It keeps order sizes consistent and is easy to reason about, which is why many warehouse systems default to it.

4. Demand-driven replenishment

The most sophisticated: orders are driven by live sales signals and forecasts rather than a static trigger. Reorder points flex with recent velocity, seasonality and promotions. It gives the tightest stock levels but needs good data and software — it's where automated inventory tools earn their keep.

How to calculate when to reorder

The core of any replenishment policy is the reorder point — the stock level that triggers a new order. The formula:

Reorder point = (average daily sales × lead time in days) + safety stock

Say you sell 20 units a day, your supplier's lead time is 10 days, and you keep 50 units of safety stock as a buffer:

• Lead-time demand = 20 × 10 = 200 units
• Reorder point = 200 + 50 = 250 units

So when on-hand stock drops to 250, you place the order. It arrives around day 10, just as you're dipping into the safety buffer — which is exactly what the buffer is there for, absorbing a late delivery or a demand spike. For the full method, including how to size the safety buffer, see how to calculate your reorder point.

How to decide how much to reorder

Knowing when to order is only half the job; you also need the order size. Ordering tiny amounts constantly racks up shipping and admin; ordering huge amounts rarely ties up cash and risks obsolescence. The balance point is the Economic Order Quantity (EOQ) — the order size that minimises the combined cost of ordering and holding stock.

In practice most sellers set a sensible reorder quantity per SKU — often rounded to a full carton or a supplier's minimum — and revisit it quarterly. The pairing is what makes a complete policy: when stock hits the reorder point, order the reorder quantity. Set once per SKU, it runs on autopilot.

Common replenishment mistakes

Most stockouts and most dead stock trace back to a short list of avoidable errors:

• Reordering on gut feel. Guessing when to reorder instead of using a data-backed trigger. Fine with five SKUs; a disaster at fifty.
• Ignoring lead time. Reordering only when you're nearly out forgets that stock takes days or weeks to arrive. The reorder point exists precisely to account for lead time.
• No safety stock. Running with zero buffer means any late delivery or demand spike is an instant stockout.
• A stock figure you can't trust. If you sell across eBay, Shopify and Amazon but your stock counts aren't synced, your reorder trigger fires on the wrong number — you reorder too late, or oversell what you don't have.
• Treating every SKU the same. Fast movers need tighter, more frequent replenishment than slow ones. One blanket rule over-stocks the slow lines and under-stocks the stars.

The last two are really the same problem: replenishment is only as good as the sales and stock data feeding it. If the numbers are stale or scattered across channels, even a perfect reorder-point formula fires at the wrong moment.

That's the piece software handles well. Salync keeps one accurate stock figure synced across eBay, Shopify, Amazon and the UK marketplaces, watches live velocity per SKU, and fires a replenishment signal the moment stock hits its reorder point — so raising the next order is a prompt you act on, not a spreadsheet you have to remember to check. Free for up to 50 SKUs.