Multi-level, single-level, indented or summarized BOM: the difference, with an example

1. One structure, four views

A bill of materials is made of parent → child links, each with a quantity: "the wheel assembly contains 4 M8 screws". Everything else is a way of reading those links. Systems (CAD, PDM, ERP) store the links and compute the views on demand.

2. The worked example: one cart, four readings

Indented multi-level BOM

Single-level BOMs (one per parent)

• Cart: welded frame × 1, wheel assembly × 4, handle × 1.
• Welded frame: 40 × 40 tube × 4, M8 screw × 8.
• Wheel assembly: Ø 100 wheel × 1, wheel plate × 1, M8 screw × 4.
• Handle: Ø 25 tube × 1, M8 screw × 2.

Summarized BOM (for 1 cart)

The summarized BOM above keeps purchased items only. You can also keep the manufactured sub-assemblies (frame × 1, wheel assembly × 4, handle × 1): it is a usage choice, to state at the top of the list. The where-used of the M8 screw answers "welded frame, wheel assembly, handle": three parents to check if the screw changes.

3. Quantity explosion

Exploding a BOM means going from quantities per parent to quantities for the product. Two rules are enough:

1. Extended quantity of a line = its quantity × its parent's extended quantity. The 4 M8 screws of the wheel assembly become 4 × 4 = 16 screws per cart.
2. Total quantity of an item = sum of the extended quantities of all its lines, at every level: 8 + 16 + 2 = 26 M8 screws.

For a batch of N products, multiply the totals by N again. The ERP's requirements planning does exactly that, then subtracts stock and open orders (net requirements).

4. BOM explosion calculator

Paste an indented BOM, one line per component: level; item; quantity per parent (semicolon or tab separated, for example copied from a spreadsheet). Level 0 or 1 is the first level under the product. The calculator gives the extended quantities and the summarized BOM, for the number of products you choose.

The calculation runs in your browser; nothing is sent. To keep a full BOM (description, material, revision, duplicate check), start from the free multi-level BOM Excel template.

5. In CAD, PDM and ERP

• CAD: a drawing's BOM table usually offers three presentations — top level only, parts only (flat) and indented. The structure comes from the assembly: a CAD sub-assembly becomes a level.
• PDM: it extracts the multi-level BOM from the checked-in assembly, version by version, and can show a file's where-used. It is the source of the eBOM, see eBOM vs mBOM.
• ERP: it stores single-level BOMs (one per parent item) and computes the other views. In SAP, for example, transactions CS11, CS12 and CS13 display the level-by-level, multi-level and summarized explosion, and CS15 the where-used list.

That is why a sub-assembly created in CAD for the designer's convenience reaches the ERP as an item in its own right, with its own BOM, unless it is handled on the way: see BOM to ERP integration.

6. How many levels?

A level is justified when the sub-assembly has a physical or logistical existence: it is built separately, stocked, bought pre-assembled or replaced as a unit in service. A grouping that only exists in CAD, to tidy the tree, adds a level that the manufacturing BOM will have to undo. Conversely, flattening a real sub-assembly loses the assembly information and the where-used.

When a sub-assembly should not be stocked but must stay visible in the structure (a kit assembled at the workstation, for example), ERPs offer the phantom item: its components are consumed directly by the parent. Numbering is free: 0, 1, 2… or dots (.1, ..2) as in the table above; what matters is that every line states its level, otherwise the structure cannot be rebuilt.

7. FAQ

What is the difference between a single-level and a multi-level BOM?

A single-level BOM lists only an item's direct components. A multi-level BOM unfolds the whole structure, from sub-assemblies down to purchased parts, each line carrying its level. The complete product is described by several single-level BOMs, or by one multi-level BOM.

What is an indented BOM?

It is the presentation of a multi-level BOM as a list, where each component is offset (indented) under its parent and carries a level number: 0 for the product, 1 for its direct components, 2 for their components, and so on.

What is a summarized BOM?

A flat list that totals the quantity of each item needed to build one product, wherever it appears in the structure. Quantities are multiplied by those of every parent: it is the list used by purchasing and requirements planning.

How do you calculate a component's total quantity in a multi-level BOM?

For each line, multiply its quantity by those of all its parents up to the product (extended quantity), then add up the extended quantities of every line of the same item. That is the BOM explosion.

How many levels should a BOM have?

As many as there are real sub-assemblies, no more. A level is justified when the sub-assembly is built, stocked, bought or replaced as a unit. A grouping that only exists in CAD adds a useless level, which the manufacturing BOM will often remove.