When to reorder: the purchasing brain for your inventory

Short answer. You reorder when your inventory drops below the reorder point. That reorder point is your sell-through rate per day times the lead time, plus a safety stock. How much you order depends on price breaks, minimum order quantity and cash. Calculate this per product and per supplier, not on gut feeling.

I ordered on instinct for years. A product looked empty, I clicked to reorder, and two weeks later I was stuck with half a container of something that no longer sold. That is the real problem with reordering. Buying too much hits your cash, selling no hits your revenue, and both happen at the same time because you are looking at the wrong number.

The reorder point is a number, not a feeling

The moment to reorder can be pinned down with arithmetic. The formula that shows up everywhere is simple: reorder point = sell-through rate per day times the lead time, plus safety stock (Slimstock explains the reorder point well). If you sell five units a day and your supplier takes seven days, you land on 35 units plus a buffer. Drop below that, and you order.

Sounds easy, and for one product it is. The problem is that you have to do this for hundreds of SKUs at once, with sell-through rates that shift week to week and lead times that differ per supplier. At that point a number you estimate in your head is no longer reorder advice, but a gamble.

How much you order: price breaks against cash

When is half the story. How much is the other half. Two forces collide here. The price break pushes you up: order 500 instead of 100, and your purchase price drops while your margin rises. Your cash pushes you down: every euro in inventory is a euro you cannot put to work elsewhere.

The classic way to find that balance is the Camp formula, also known as the EOQ (explanation at Nevi). It looks for the order quantity at which ordering costs and holding costs are jointly lowest. Nice model, but it assumes fixed demand and ignores the price break, the minimum order quantity your supplier imposes and the season. For the reseller it is a starting point, not an answer.

What I want calculated in practice: at this price break and this sell-through rate I will have cleared the extra inventory in this many weeks, and that costs me this much cash in the meantime. That is a trade-off you can make. A gut feeling that says "let's just grab the price break while we can" is not a trade-off.

From which supplier: the reseller layer on top

For the reseller there is a dimension that standard advice skips. You buy the same product from multiple sources. One supplier is cheaper but slow, the other more expensive but delivers the day after tomorrow. On B2B platforms like Orderchamp or Ankorstore you can sometimes find the same brand at multiple parties, with different price breaks and lead times.

The reorder advice is then not "order 200 units", but "order 200 units, 150 from supplier A because the margin is better there, and 50 from B because A's lead time will not get you through the weekend of the sale". That is a choice that depends on price, lead time, reliability and what is already in transit. No off-the-shelf inventory package makes that choice for you, because it does not know your supplier matrix.

What is in transit counts too

The mistake I see most often, and have made myself: reordering while something is already on its way. Your inventory system says "low", you order, and then the first shipment arrives anyway. Now you have double.

Reorder advice that holds up looks at the available inventory plus the incoming inventory, not at the shelf stock alone. I call that the purchasing brain: a layer that brings your current inventory, your open purchase orders, your sell-through rate per channel and your supplier terms together into one piece of advice per product. If you sell on your own webshop, bol and Amazon, the sell-through rate has to come from all three channels, not from the channel you happen to have open.

Goal, process, then build

Before you build anything, the goal first. You want control: a list that says what you need to order today, how much, and from whom, with numbers that hold up. Not just time saved, but the end of ordering on instinct.

Then the process. Inventory management in Excel can get you far, and there are solid guides for minimum and maximum stock in Excel. For one supplier and one channel that might be enough, and if so I will say that too. Some ordering routines have grown to the point where they can be made smarter or dropped entirely. You audit that first.

And then, where the off-the-shelf package falls short, you build. Tools like Picqer, ChannelDock or Optiply cover the happy path: one supplier, fixed lead time, predictable demand. They do not cover your exceptions. The price break trade-off against your cash, the choice between supplier A and B per order, the incoming inventory across multiple sources, the season in your niche. That is exactly the calculation that has to happen, and the one I build when the standard solution cannot reach it.