Multilevel Bill of Material
Definition
A multilevel bill of material is the complete product structure showing every level of assembly (from the finished item through subassemblies down to purchased components and raw materials) with quantities linking each parent to its children.
In Practice
The multilevel BOM is the master map MRP explodes: demand for the end item cascades level by level, multiplying quantities and offsetting lead times, until every purchased part has a time-phased requirement. A finished pump at level 0 might contain a motor assembly at level 1, wound stators at level 2, and copper wire at level 3, four levels the planning run walks in sequence.
Depth carries cost: each level typically adds a stocking point, a lead-time increment, and transaction overhead, which is why lean redesigns often flatten BOMs by merging levels or converting subassemblies to phantoms. Displayed with indentation it becomes the indented BOM; queried bottom-up it becomes the where-used report.
Frequently Asked Questions
How does MRP use a multilevel BOM?
It explodes demand top-down: the master schedule quantity for the end item generates gross requirements for level 1 components, netted against their stock and open orders; planned orders for those generate requirements one level deeper, offset by lead time; and so on until purchased materials are reached.
How many levels should a BOM have?
As few as the process genuinely requires. Levels should mirror real stocking and build points. If a subassembly is built and immediately consumed without ever being stored, making it a phantom or flattening it removes planning noise, transactions, and lead-time stacking without losing anything real.
Related Terms
An indented bill of material is a multilevel BOM display that shows the product structure hierarchically, indenting each level so parents, subassemblies, and components appear in their true structural relationships.
Single-Level Bill of MaterialA single-level bill of material lists only an item's immediate components (one level down) with quantities per parent, serving as the building block from which multilevel product structures are assembled.
Phantom Bill of MaterialA phantom bill of material defines a subassembly that exists in the product structure but is never stocked. It is built and immediately consumed in the parent, so MRP plans through it directly to its components.
Low-Level CodeA low-level code is a number assigned to each item identifying the lowest level at which it appears in any bill of material, used by MRP to ensure an item's requirements are fully accumulated before its needs are calculated.
Vertical DependencyVertical dependency is the parent-child relationship between levels of a bill of material: demand for a parent item directly creates demand for the components beneath it, cascading down through every BOM level.