What kanban replenishment is
Kanban (from the Japanese for "signboard" or "visual card") is a signal-based system for triggering replenishment: instead of a calculated number determining when to reorder, a physical or visual cue does, an empty bin, a card moved to a designated spot, a marked line on a shelf reached.
The core idea is that the physical act of running low is itself the trigger, with no separate calculation step required between "we are getting low" and "reorder now." This makes it fast to run on the floor, since anyone can see the signal without checking a screen or a report.
It originated in lean manufacturing (most associated with the Toyota Production System) as a way to pull material through a process only as it is actually consumed, rather than pushing material in based on a forecast.
The two-bin kanban system
The most common practical form is a two-bin system: two identical bins of the same item, both full to start. One bin is in active use; when it empties, that is the reorder signal, and work shifts to the second, still-full bin while a replenishment order goes out for the first.
By the time the second bin empties, the reorder for the first should have arrived, so it gets refilled and put back into rotation, and the cycle repeats. The bin size itself functions as the buffer, sized to comfortably cover the expected replenishment lead time.
A card-based variant works the same way without a second physical bin: a card is placed at a reorder trigger point inside a single bin, and reaching that card is the signal, functionally identical to the two-bin version but using less physical space.
Kanban vs. a calculated reorder point
A calculated reorder point (lead-time demand plus safety stock) adapts precisely to changing demand and lead time, because it is recalculated from real numbers. A kanban bin size is fixed once set — it does not automatically adjust if demand or lead time shifts, someone has to notice and resize the bin.
What kanban trades away in precision, it gains in simplicity and speed of execution: no software lookup, no calculation, just a visual state anyone on the floor can recognize instantly. For fast-moving, low-value, stable-demand items, that tradeoff is usually worth it, since the cost of a slightly oversized buffer is small.
For high-value or highly variable-demand items, a calculated reorder point generally wins, since the cost of an imprecise fixed buffer (either tying up too much cash or risking a stockout when demand spikes past what the bin was sized for) is larger relative to the item's value.
Setting up kanban replenishment in practice
Size each bin (or set the card trigger point) to cover expected demand during the replenishment lead time, plus a margin for variability, the same underlying logic as a reorder point calculation, just expressed as a physical quantity rather than a recalculated number.
Review bin sizes periodically, especially after a meaningful shift in demand or a supplier lead-time change — a kanban system set correctly a year ago can silently under- or over-buffer today if nobody revisits it.
In a digital system, a kanban trigger can still be logged as a reorder event (a scan on the empty bin's barcode, for instance) so the physical simplicity of kanban is kept while still producing a record and a purchase order automatically, rather than relying purely on someone remembering to place a manual order.
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