Where-used in a BOM: find every assembly and product that uses a part

1. The BOM read upwards

A bill of materials is made of parent → child links, each with a quantity. Read top-down, it answers "what does this item contain?": that is the explosion, covered in multi-level, indented and summarized BOM. Read bottom-up, it answers "where is this item used?": that is the where-used, also called BOM implosion.

Key point: a BOM is stored on the parent. The assembly knows its components; the part does not know where it is used. A where-used is therefore always a computation over all the BOMs (an inverted index). That is why a standalone Excel file or a CAD system without a database cannot answer it, while a PDM, PLM or ERP system does it in one query.

2. The worked example: one screw, two products

We take the cart from the multi-level BOM article and add a second product, a hand truck, which shares the wheel assembly and the M8 screw. The single-level BOMs are:

Single-level where-used of the M8 screw

Multi-level (indented) where-used

Each level goes one step up. The extended quantity is the number of M8 screws contained in one unit of the line's item: multiply the quantities along the path.

End-item where-used

Reading: changing the M8 screw affects 4 BOMs and 2 products. For 100 carts and 50 hand trucks, you need 100 × 26 + 50 × 14 = 3,300. That is exactly the calculation to make before replacing a fastener reference or using up a stock.

3. Where-used calculator

Paste your BOMs as links, one line per component: parent; child; quantity (semicolon or tab separated, for example copied from a spreadsheet). Enter the item you are looking for: the calculator gives its direct parents, the inverted tree up to the finished products and the quantity in each.

The calculation runs in your browser; nothing is sent. A loop (an item that contains itself, directly or not) is reported instead of followed. For the explosion the other way round, use the BOM explosion calculator.

4. What where-used is for

The first use is by far the most important: a change request without a where-used is a change whose impact is unknown. It is the analysis step of an ECR, and it is also what decides between a new revision and a new part number: if the changed part is no longer interchangeable in all its where-used, it gets a new number.

Beware of a common confusion: the where-used tells where a part is planned in the BOMs. Knowing on which serial number it was actually fitted is traceability (as-built BOM, lots and serial numbers), which needs other data.

5. In Excel, CAD, PDM and ERP

• Excel: keep the BOM as links, with Parent, Child and Quantity columns (A, B, C). The single-level where-used is a filter on the Child column, or a formula: =FILTER(A2:C100,B2:B100="M8 screw"). Multi-level means repeating the filter on each parent found: that is where the calculator above saves time. A BOM entered as an indented list, like the free BOM Excel template, can also be filtered by item to find its lines, but each line's parent is then read above it.
• CAD: an open assembly knows which files it uses, but a part does not know which assemblies use it. Without a database, every assembly has to be opened or indexed to answer. CAD software sometimes offers a reference search in a folder, limited to the files you point it at.
• PDM: each check-in records the references between files; a part's where-used (a "Where Used" tab) lists the assemblies and drawings that reference it, version by version.
• PLM: the where-used is on items rather than files, across all products, with revision, effectivity and configuration filters. It is the reference view for impact analysis.
• ERP: it computes the where-used on the manufacturing BOMs, at a given date. In SAP, for example, transaction CS15 displays it single-level or multi-level; other ERP systems have an equivalent function, usually called "where used".

The PDM where-used and the ERP where-used do not necessarily give the same result: the first reads the engineering BOM, the second the manufacturing BOM, and the two differ by design. See eBOM vs mBOM and BOM to ERP integration.

6. Reading pitfalls

1. Versions: an "all versions" where-used also returns the old assembly revisions that used the part. For an impact analysis, filter on the latest versions, or on released versions.
2. Effectivity dates: in the ERP, a BOM line can be valid up to a date or a serial number. A where-used without a date mixes past and future.
3. Configurations and variants: a part used in only one configuration of an assembly shows up in its where-used; check which one, or the impact is overestimated.
4. References that are not BOM lines: in PDM, a drawing, a derived part or a component excluded from the BOM also reference the file. They count for the impact (the drawing must be updated) but not for the quantities.
5. Phantom items: a phantom sub-assembly shows up as a parent in the where-used although it is never stocked; go one level up to find the item actually built.
6. Duplicate part numbers: if the same screw exists under two references, each where-used shows only part of the picture. Where-used quality depends on part numbering quality.

7. FAQ

What is a where-used in a bill of materials?

It is the list of all the parent items that use a given component, with the quantity used in each. It is the BOM read backwards: instead of starting from a product to find its components, you start from a component to find the assemblies and products that contain it.

What is the difference between where-used and BOM explosion?

The explosion goes down the structure: what does this item contain? The where-used (implosion) goes up: where is this item used? Both read the same parent → child links, in opposite directions.

What is the difference between single-level and multi-level where-used?

A single-level where-used gives only the component's direct parents. A multi-level where-used climbs each branch up to the finished products and can multiply the quantities: it answers "how many of this part in each product?".

How do you do a where-used in Excel?

Keep the BOM as links, one line per parent; child; quantity. The single-level where-used is then a simple filter on the child column, or the FILTER function. Multi-level means repeating the filter on each parent found, which is what this page's calculator does.

What does an item with no where-used mean?

That no BOM uses it. That is normal for a finished product (the top of the structure), but for a part it often points to an orphan item: old, duplicate or created by mistake. Before archiving it, check that it is not sold on its own, as a spare part for example.