Min/Max Inventory: How the System Works and How to Set Levels

A min/max inventory system sets two numbers for each product: a minimum level that triggers a reorder, and a maximum level you top back up to. When stock falls to the min, you order enough to reach the max, so the reorder quantity is simply max minus current stock. It is the simplest reliable way to keep the right amount of each SKU on hand without watching every item by hand.

Min/max is popular because it is easy to explain and easy to run: set two thresholds per item and let them drive purchasing. The trouble is that most businesses set the numbers once, guess them, and never revisit them, which is how you end up overstocked on dead lines and short on your best sellers. Here is how the system works, how to calculate the levels properly, and where it starts to break.

How a min/max inventory system works

Every SKU gets a minimum and a maximum. The minimum is your reorder point, the level at which a new order has to go out to avoid running dry before it arrives. The maximum is the ceiling you replenish back up to, sized so you are not carrying more than you need. When on-hand stock drops to or below the min, the system flags a reorder, and the quantity you order brings you back up to the max. That gap between the two is the buffer that keeps you from ordering every day while still never sitting on months of excess.

How to calculate the minimum level

The minimum is the reorder point, and it has to cover demand across your full lead time plus a safety buffer. The formula is:

Minimum = (Average Daily Demand × Lead Time in Days) + Safety Stock

Say you sell 5 units of an item a day, the supplier takes 4 days to deliver, and you hold 20 units of safety stock. Your minimum is (5 × 4) + 20 = 40 units. When stock hits 40, you reorder. The safety stock term is what absorbs a demand spike or a slow delivery, so do not set the min to bare lead-time demand unless supply and sales are both very steady.

How to calculate the maximum level

The maximum sets how much you top up to, which controls your carrying cost. A common approach is to add a target amount of coverage on top of the minimum:

Maximum = Minimum + Reorder Quantity

The reorder quantity is how much you want to bring in each cycle, often set to a sensible number of days of supply or an economic order quantity that balances ordering and holding costs. If you want, say, 20 days of coverage at 5 units a day, that is 100 units, so the max would be 40 + 100 = 140. When stock falls to 40, you order 100 to return to 140.

Min/max vs a plain reorder point

A plain reorder point only tells you when to order; you still have to decide how much. Min/max answers both at once: the min is the when, and the max fixes the how much. That makes it a good fit for items you want to keep on a steady shelf level, and for warehouses running two-bin or shelf-tag systems where a visual trigger says "reorder to full." The downside is that a fixed max can overstock an item whose demand has quietly fallen.

Where min/max breaks

The weakness is not the formula, it is the maintenance. Min and max are usually set once and left, but demand, lead times and seasonality all move. A max that made sense last spring can be double what you need this autumn. Set static levels across a big catalog and you drift into the worst of both worlds: overstocked on the fading lines, understocked on the ones that took off. Min/max also treats every SKU the same unless you do the work per item, which nobody does by hand past a few hundred products.

The fix is to treat the min and max as living numbers that update as demand changes. That only holds up if the demand data feeding them is clean and current, so it pays to trust the numbers underneath before you automate decisions on top of them. Recalculating levels from real, recent per-SKU sales, rather than a figure someone typed a year ago, is what keeps min/max honest.

Making min/max dynamic

The modern version of min/max is the same two-threshold idea, but with the thresholds recalculated continuously from each SKU's own demand and lead time. Instead of a person revisiting hundreds of items, the software reforecasts demand, resets the min and max, and flags what to reorder. That is the difference between a spreadsheet that stores your levels and a system that keeps them right. It is the core of what Storekeeper is being built to do: live inventory control that moves the levels for you.

Want to see whether your current min and max levels still fit your demand? Paste your stock and recent sales into the live stock scan at the top of the site. It shows which SKUs are sitting above a sensible max and which are already below a safe min, so you can fix the worst offenders first.