Multi-Level Bill of Materials (BOM)

A multi-level BOM is also called an indented BOM. Level 0 is the finished product, level 1 its direct components, and so on down to bought-in parts.

The pump as a tiered parts tree

• Bottom-up construction. Build the lowest-level BOMs first, then use those items as components in the BOMs above them.
• Explosion. Expanding the tree into the total of every bought part is called a BOM explosion. Material requirements planning (MRP) then turns those quantities into purchase and build plans; see material requirements planning.
• One part, several levels. A part can sit at more than one level. The explosion must add the quantities together.
• Single-level BOM. It lists only direct components and ignores what the sub-assemblies contain.

In a model of 100 pumps, a flat list that ignores sub-assembly stock and yield says buy 260 bearings and the multi-level calculation says 238. The gap comes from stock held at sub-assembly level. Yield losses also compound through the levels. Yield is the share of units that come out good; a 98% yield means 2 in 100 units are scrapped, so you must start more units than you need.

The structure is:

• Pump (level 0): 1 motor assembly, 1 pump body assembly, 1 carton.
• Motor assembly (level 1): 1 stator, 1 rotor assembly, 2 end covers, 4 M6 screws.
• Rotor assembly (level 2): 1 shaft, 1 rotor core, 2 bearings.
• Pump body assembly (level 1): 1 casing, 1 impeller, 1 seal, 1 bearing, 6 M6 screws.

These are illustrative assumptions, not customer data. Motor assembly yield is 99%, rotor assembly yield is 98%, and pump body yield is 100%. On hand are 15 finished rotor assemblies and 40 loose bearings. Stock is netted against the requirement first, then yield is applied to the build quantity. Quantities are rounded up at each level, because you cannot build part of a unit.

The flat list's 260 ignores sub-assembly stock and yield, and its overstatement of 22 bearings (260 − 238) comes from that omission. Yield loss can push other parts up instead: M6 screws come to 4 × 102 + 6 × 100 = 1,008, against 1,000 on a flat list.

Give a sub-assembly its own stocked level only if you hold it as stock, share it across products, or plan it separately. Make it a phantom if it is built and consumed in one run. A phantom is a sub-assembly defined in the BOM but never held as stock, so its parts pass straight to the parent. Otherwise flatten it.

Use a phantom when you want the sub-assembly's parts and quantities kept as a separate, reusable BOM; flatten when the step appears in one product only.

The recommended approach is to start flat and add a level only when one of the first three answers in the table is yes. With the same yields, a phantom and a flat list give the same purchase quantities. The difference is whether a stock record and a production order exist for that step. Every stocked level means more production orders, more material moves and more WIP inventory to value.

They go wrong at revisions, at handoffs between levels and at the line between BOMs and kits.

• Revisions ripple upward. A change to a low-level part forces new versions of every parent that uses it. Before approving a change, list every parent that uses the part.
• Sub-levels must exist first. A stocked sub-assembly must be built before the parent that uses it. Each stocked level is one more build to schedule.
• Kits are not multi-level BOMs. A kit is a set of items sold together as one line. A multi-level BOM describes things you assemble.
• Costs move with the levels. A sub-assembly's full cost, including its own labour and overhead, rolls into the parent as a single material-cost line. The parent's labour and overhead lines cover only work done at the parent level. See BOM costing.

Run these with your own deepest product, or the pump above.

1. Explosion. Enter a three-level product. Pass: one view lists every bought part with its total quantity. Fail: only direct components appear.
2. Stock netting. Set motor yield to 99% and rotor yield to 98%, enter 15 rotor assemblies and 40 bearings on hand, and rerun the 100-pump requirement. Pass: bearings to buy show 238. Fail: any other figure, once you have checked the software's rounding rule.
3. Revision. Change one low-level part. Pass: the system shows every parent that uses it and lets you version them. Fail: parents silently keep the old version.
4. Cost roll-up. Pass: the sub-assembly cost shows on its own line and in the parent, and the total reconciles by hand. Fail: you cannot reconcile it.
5. Build. Build one pump. Pass: finished stock rises by 1 and nothing is deducted twice. Fail: any bought part moves twice.

Our bill of materials feature lets you create bills of materials for multi-component products. You can check material availability against each BOM, plan purchases, and track consumption and production.

Bills of Material, production and kitting are on our Advance plan. As of October 2026 it costs US$499 a month, paid yearly, with 2 user licences and extra users at US$52 a month. Our pricing is published in US dollars. Each user signs in with a Salesforce login, and Salesforce licences are bought from Salesforce and priced separately. For buying advice, see Bill of Materials Software, or ask our sales team about your product's structure.

Run the five checks above on your deepest real product in our 15-day free trial, which gives full access with no credit card.