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Inventory management system: what it decides for you

Inventory rarely goes wrong because nobody knows what is in stock. It goes wrong because nobody knows what is still free. This page covers the number an inventory management system is built around, how reorder points should actually be calculated, where a spreadsheet stops working, and when inventory software buys you nothing.

By Tom Joseph · Last updated: 18 August 2026

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In short

An inventory management system maintains four numbers per item: what is physically there, what is already committed to orders, what is inbound from suppliers, and what remains available to sell. That last number — available to promise — is what the system exists for, and it is precisely the one a spreadsheet cannot hold reliably once several people or channels sell at the same time. We start with a €2,500 audit that establishes whether your problem is stock levels or stock locations, because those are different systems and different investments.

What the system decides for you

Five places inventory control breaks

Not what is on the shelf, but what you can still promise from it — and where that number stops being trustworthy.

Core number
Available to promise
What sits there is not what you can sell. Reserved, blocked and already promised come off first.
Reorder points
Set in 2019
Most reorder points were set once and never revisited. Demand moved; the number did not.
Excel
Where it stops
A spreadsheet works until two people edit it at once, or until nobody knows which version applies.
Channels
Overselling
If you sell across several channels, overselling is a synchronisation problem — not a stock problem.
Dead stock
Stock standing still
The cost nobody books. Stock that does not move consumes space, cash and attention.

The core number

Available to promise is the number that counts

Ask how many of an item are in stock and you get a physical count. That number is almost never the one a decision may be made on.

What you actually need is available to promise: physical stock, minus what is committed to open orders, plus what is inbound on a confirmed purchase order. Of 400 physical units, 380 may be committed — so you cannot sell another 100, however reassuring the shelf looks.

This is the dividing line between stock recording and inventory management. The first tells you what is there. The second tells you what you may promise, and that is the only number sales can use.

Reorder points

Reorder points somebody set in 2019

Almost every system supports a minimum and maximum per item. The problem is not the feature but where the numbers came from: set by hand once, against the demand of that moment, and never touched since.

A reorder point that works has three parts:

Demand during lead time — what you sell in the time your supplier needs. When lead times double, the reorder point must follow, and it rarely does in time.
Safety stock — the buffer for variation. It is the spread in demand rather than the average that sets how large it should be.
Order quantity — how much you buy at once, constrained by pack sizes, price breaks and shelf life.

Spreadsheets

Where the spreadsheet stops working

A spreadsheet is a perfectly reasonable inventory system for one person with a few hundred items and one sales channel. Starting there is not naive; staying there once one of these three things happens is.

Two people edit it at once. From that moment no number exists that everyone knows is right, and the error disappears into a version nobody kept.
You sell through more than one channel. A spreadsheet cannot update fast enough to stop two channels selling the same last unit.
Someone has to track batches or expiry. Once the same item number exists in several batches with different dates, the spreadsheet is no longer a record but a reconstruction.

Channels

Overselling is a sync problem, not a stock problem

If you sell through a webshop, a marketplace and a rep with an order pad, the question is not how much you have but how quickly each channel is told when it changes.

What decides this in practice:

Does it update in real time or on a schedule? A fifteen-minute sync is fine for most businesses and disastrous during a clearance promotion.
Can it reserve stock per channel? Holding a buffer back for your largest customer is a common requirement and not universally supported.
What happens on a conflict? Two channels sell the last unit simultaneously. Which wins, and who is told, is a business rule you must be able to configure.

Dead stock

The cost nobody books: stock that stopped moving

Stockouts get attention because a customer complains. The opposite failure is silent: stock bought once, never sold through, still sitting there tying up cash and space while looking like an asset on the balance sheet.

Most businesses cannot say what share of their stock has not moved in a year, which is why a good system earns its cost here rather than on reordering:

Ageing by item. How long each batch has sat, not the average across the range — averages hide exactly the items you need to see.
Sell-through against what you bought. An item ordered in a quantity that will take four years to clear is a purchasing decision to change, not a stock level to manage.
What it is costing to hold. Space, capital and — where relevant — the expiry date it will not reach. Naming that figure is usually what makes someone finally discount the stock and recover part of it.

The boundary

Inventory system or WMS?

These two get conflated, and the distinction decides which quote you request.

An inventory management system answers how much you have and what to reorder. It can work perfectly without you knowing a single warehouse location — plenty of webshops and wholesalers run on one for years.

A WMS answers where something is and who picks it next. You need that when finding and directing work is the constraint, not the arithmetic.

If you know how much but not where, and pickers are hunting, your problem is the warehouse.
If you know where everything is but consistently order too late or too much, your problem is inventory.
Both at once happens, and the order is almost always: get the counts right first, then the locations.

Selection

The four things packages genuinely differ on

Every package books stock in and out. The differences that touch your operation are narrower:

Units and conversions. If you buy by the pallet and sell by the unit, the system must carry that hierarchy without anyone calculating alongside it.
Batches, serial numbers and expiry. Mandatory in food, pharma and parts of engineering — and adding it later is usually a migration, not a setting.
Multiple locations. Two warehouses, or a shop plus a warehouse, needs stock per location and a consolidated view. Not every entry-level system does both.
What happens on a count. A correction should be an adjustment with a reason, not a silent overwrite your accountant discovers at period end.

No system

When you do not need an inventory system

If one person buys and sells, you have one channel, and the goods do not perish, a spreadsheet with discipline is cheaper and faster than any package. That is not an interim measure; for many businesses it is the end state.

The threshold is not revenue but the number of places where something happens to the same stock at once. One place: a spreadsheet is fine. Three or more: anything that does not hold the counts centrally will cost you money in stock you could not sell or sold twice.

If you are unsure, count first. If you do not know how far your current numbers sit from reality, you cannot justify any investment — and that count costs a day, not a project.

FAQ

Frequently asked questions

What is the difference between an inventory management system and a WMS?

An inventory system manages quantities and reordering; a WMS manages locations and work.

Inventory management can work fine without warehouse locations — many webshops and wholesalers run on one for years.
You need a WMS once finding and directing work is the constraint rather than the arithmetic.
If both apply, start with the counts. See our WMS selection page for that step.

What does inventory management software cost in the Netherlands?

Entry-level packages are priced per user per month; the costs that surprise people are the integrations.

Expect integration costs to your webshop, marketplaces and accounting — these drive price more than item count does.
Migrating open orders and current quantities is work that rarely appears in the quote.
With us: a €2,500 audit establishing whether counts or locations are your problem, a €20,000 proof of concept where the approach is uncertain, production from €50,000. Package licences are paid to your supplier.

Can we keep doing inventory in Excel?

Yes, as long as there is one place where stock changes. Three signals mean it is no longer viable.

Two people edit the file at once — then no number exists that everyone knows is right.
You sell through more than one channel and have occasionally sold something you did not have.
Batches or expiry dates have to be tracked.

What is available-to-promise stock?

Physical stock minus what is committed to open orders, plus what is confirmed inbound. It is the only number sales may promise against.

Of 400 physical units, 380 may be committed — the shelf looks full and you can sell nothing.
Stock recording shows the physical count; an inventory management system shows what is available.
If you sell through several channels, this number must be right per channel, not only in total.

How do we set reorder points properly?

Calculate them from three parts rather than setting them by hand once.

Demand during lead time: what you sell in the time your supplier needs.
Safety stock based on the spread in demand, not the average.
Order quantity constrained by pack size, price breaks and shelf life.
Revisit them when lead times change — that is the variable that most often shifts quietly.

Is it your counts or your locations?

The €2,500 audit establishes whether you need an inventory management system or a WMS — and says plainly when a spreadsheet with discipline is enough.

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