TL;DR
- The lean inventory management does not seek to eliminate stock: it seeks to eliminate only the stock that is unnecessary, the stock that protects against no real risk.
- Three methods structure the approach: just-in-time, kanban (pull-flow system) and reorder points kept dynamic.
- The central issue is the balance between holding cost and stockout risk – not reducing stock at all costs.
- A well-managed industrial SME can reduce its stock by 15 to 30 % without increasing its stockout rate, provided it reviews its thresholds regularly.
Many managers confuse lean inventory management with zero stock. It is a costly mistake: stock that is too low generates stockouts, emergency overtime and unhappy customers. Lean does not aim for the absolute minimum stock. It aims for the stock that is just necessary, no more, no less.
The lean principle applied to stock: hunt waste, not stock
In lean philosophy, excess stock is one of the seven classic wastes (muda). But note the nuance: it is not stock as such that is a waste, it is stock that serves no purpose.
Three types of stock, three different treatments:
- Useful stock : it covers an identified risk (demand variability, uncertain supplier lead time). You keep it and you size it.
- Hidden stock : it masks a process problem – changeover times that are too long, unstable supplier quality. You do not remove it by emptying it, you treat the cause.
- Useless stock : accumulated out of habit, out of fear, or through a lack of visibility on real consumption. This is the one to eliminate first.
The classic mistake in SMEs: cutting all stock abruptly without distinguishing between these three categories. The result is that useful stock is removed along with useless stock, and stockouts follow.
Just-in-time: produce what will be sold, when it will be sold
Just-in-time (JIT) means manufacturing or sourcing only what will be consumed, at the moment it is needed – neither earlier, nor in greater quantity.
What this changes in practice:
- On ne lance pas un ordre de fabrication “parce qu’il reste de la capacité machine disponible”.
- You synchronise supplies with real downstream consumption, not with an approximate rolling forecast.
- You reduce production and delivery batch sizes, even if that means increasing frequency – which presupposes fast changeovers (SMED).
JIT works on condition that two prerequisites are in place: sufficient supplier reliability and changeover times that are compressed. Without these two building blocks, applying JIT amounts to removing the safety cushion with nothing to put in its place.
Kanban: the visual signal that triggers replenishment
Kanban is the most concrete tool for managing lean stock day to day. It is a system based on pull flow : replenishment is triggered only when real consumption requires it.
How it works:
- Two-bin system : one bin is consumed first, a second serves as a reserve. As soon as the first bin is empty, a kanban card goes to procurement.
- Colour codes : a green zone (sufficient stock), an orange zone (alert threshold, launch possible), a red zone (critical threshold, launch mandatory).
- Calculating the number of cards : the number of kanbans in circulation determines the maximum stock authorised in the loop. Reducing the number of cards mechanically reduces stock – but only if the replenishment lead time allows it.
The formula for calculating the number of kanban cards incorporates three variables: average demand, the replenishment lead time and a safety coefficient (generally between 0.1 and 0.3). That safety coefficient is precisely the dial you adjust to balance cost and stockout risk.
Reorder points: the variable nobody remembers to review
The reorder point (order point) is often set once, when a part number is launched, and then never reviewed. This is a frequent mistake in SMEs.
What should make a reorder point move:
- A change in average demand for the part number – seasonality, a new customer, a product being discontinued.
- A change in the real supplier lead time, often different from the contractual lead time after a few months of the relationship.
- A change in criticality of the part number – a component that has become single-source must have its threshold raised, even if its consumption has not changed.
Good practice: review the reorder points of critical part numbers every quarter, and standard part numbers once a year. An ERP or a simple, well-kept spreadsheet is enough to automate this review if the consumption data is reliable.
Finding the balance between cost and stockout risk
This is the heart of the matter: every euro less in stock reduces the holding cost, but potentially increases the stockout risk. Good lean inventory management means finding the balance point for each part number – not applying a single rule to the whole catalogue.
A simple method for prioritising:
- Classify your part numbers using the method ABC : A items (high impact, high value) deserve a detailed, individual analysis. C items (low impact) can follow a simplified generic rule.
- Cross this classification with supply criticality : a part number that is C in value but single-source with a 12-week lead time deserves A treatment in terms of securing supply.
- Measure the real cost of a stockout (line stoppage, customer penalty, overtime) and compare it systematically with the storage cost before reducing a stock level.
Le résultat d’une gestion des stocks lean bien menée n’est pas “moins de stock partout”. C’est “le bon stock, au bon endroit, pour la bonne raison” – avec des références en tension quasi nulle, et d’autres protégées par un stock tampon assumé.
FAQ – Lean inventory management
Does lean inventory management mean zero stock? No. It aims for just the stock needed to cover demand and real contingencies, eliminating only the stock that serves no purpose.
What is the difference between kanban and just-in-time? Just-in-time is the general principle – produce what is asked for, when it is asked for. Kanban is the operational tool that drives this pull flow day to day.
How do you calculate a reorder point? It is based on average demand during the replenishment lead time, increased by a safety stock that depends on demand variability and supplier lead time.
What stock reduction can you expect from a lean approach? A reduction of between 15 and 30 % in the value of stock tied up is common in industrial SMEs, without any deterioration in the service level, provided the thresholds are reviewed regularly.
Does lean work with unreliable suppliers? Just-in-time presupposes a minimum level of supplier reliability. If that is not the case, supply must be secured before stock is reduced.
Sources
- Manutan – Inventory management using the method Kanban
- FFB – Better managing stock and supplies with Lean
- Institut Lean France – Do not confuse Lean with inventory management
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Xavier Schuster
Consultant at SXE Consulting. Industrial consulting firm based in Luxembourg, 25 years of experience in operational excellence.
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