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Bill of Materials Example (With a Free Template)
A worked bill of materials example for a small connected sensor: every line, part number and cost, rolled up at 100, 1k and 10k units. Free CSV template.
by the HardwareMap editors5 min read
Contents
A bill of materials example is easiest to learn from when it is complete, so this one is: a small USB-powered temperature and humidity sensor with Wi-Fi, every line from the radio module to the box, costed at 100, 1,000 and 10,000 units. Copy the CSV template at the end. It sits under how to start a hardware company, the founder guide this note supports.
The device
The example product is a wall-plug sensor node. A USB-C cable powers it. It reads temperature and humidity and reports over Wi-Fi. One button, one status LED, a two-piece plastic enclosure. It is deliberately boring: the BOM shape is the same for a robot controller or a medical wearable, just longer.
Real parts make the example useful, so the electronics use parts you can look up: Espressif's ESP32-C3-MINI-1 module, a pre-certified radio module (check Espressif's certificate list for your markets), and Sensirion's SHT40 sensor. Everything else is a commodity regulator, connector, ESD diode and passives.
Bill of materials example: the full BOM
Levels follow the build. Level 0 is the finished good. Level 1 is each subassembly the factory builds or kits separately. Level 2 is what goes into each subassembly. Unit costs are illustrative at 1,000 units, before tariffs and freight. Replace them with your own quotes.
Total at 1,000 units: $10.25est per unit before tooling, freight, duty and yield loss. The electronics are 64 percent of it. The box, cable and card together cost more than two-thirds as much as the radio module, which is the lesson most first BOMs miss.
Cost roll-up at 100, 1,000 and 10,000 units
The same product costs very different amounts at different volumes. At 100 units the enclosure is 3D printed, PCB assembly setup is spread over few boards, and nobody gives you reel pricing. At 1,000 you pay for molds. At 10,000 the molds are mostly paid off.
The tooling row assumes two single-cavity aluminum molds and a test fixture, $15,000est in total, written off against the first run. Spread it across a forecast instead and the 1,000-unit number drops, but only if the forecast is real. Our notes on injection molding cost and PCB assembly cost break down those two rows. The landed cost then feeds your margin model in hardware unit economics.
Rules that keep a BOM honest
- One part number, one thing. If two parts are interchangeable, give them one internal number and list both MPNs as approved alternates.
- Quantity per parent, not per product. The CM multiplies through the levels. You should not.
- State the volume behind every price. A price without a quantity break is not a price.
- Include process lines. Assembly, test and programming are costs. Put them in, flagged as process, or your margin is fiction.
- Revision control. Every BOM sent outside the company carries a revision letter and a date. Changes go through a change log, even if the log is a tab in the sheet.
- Design for the BOM. Fewer unique parts mean fewer suppliers, fewer reels and fewer things to go out of stock. The design for manufacturing checklist covers the mechanical side.
This BOM is an engineering BOM organized by build level. The factory will turn it into a manufacturing BOM with work centers and consumables: see manufacturing BOM vs engineering BOM. When the sheet outgrows one person, bill of materials software compares the tools that replace it.
Free bill of materials template (CSV)
Copy this block into a file named bom.csv and open it in any spreadsheet. The columns import cleanly into most BOM and PLM tools. Delete the example rows and keep the header.
Level,Part number,Revision,Description,Qty,UoM,RefDes,Manufacturer,MPN,Supplier,Supplier PN,Unit cost,Cost qty break,Extended cost,Lead time (wks),Lifecycle,Alternates,Notes
0,HM-100-0001,A,Sensor node finished good,1,ea,,,,,,,,,,,,
1,HM-200-0001,A,PCBA main board,1,ea,,,,CM,,,,,,,,
2,HM-210-0001,A,Bare PCB 2L FR-4 1.6mm ENIG,1,ea,,Custom,,PCB fab,,0.60,1000,0.60,3,Active,,Panelized by CM
2,HM-220-0001,A,Wi-Fi/BLE module,1,ea,U1,Espressif,ESP32-C3-MINI-1-N4,,,2.00,1000,2.00,,Active,,
2,HM-230-0001,A,Cap 1uF 0402 X5R,4,ea,"C1,C2,C3,C4",Samsung,CL05A105KA5NQNC,,,0.005,1000,0.02,,Active,,Order attrition
1,HM-300-0001,A,Enclosure top PC/ABS,1,ea,,Custom,,Molder,,0.45,1000,0.45,,Active,,Tooling separate
1,HM-900-0001,A,Final assembly and test,1,ea,,,,CM,,1.20,1000,1.20,,,,Process line
The Unit cost and Extended cost columns hold numbers only, so the sheet can total them. Keep Cost qty break filled in: it is the column that stops a 100-unit quote being mistaken for a 10,000-unit one.
Frequently asked questions
What should a bill of materials include?
At minimum: a level or parent reference, an internal part number, a description, quantity per assembly, unit of measure, manufacturer and manufacturer part number, at least one supplier, unit cost at a stated volume, and extended cost. Add a revision, a reference designator column for PCB parts, a lifecycle status and a notes field for alternates. Anything a buyer would have to ask you about belongs in a column.
What is the difference between a BOM and a parts list?
A parts list is flat: every part once, with a count. A bill of materials is structured. It shows which parts go into which subassembly, at what quantity per parent, so a contract manufacturer can kit, build and cost each level. A BOM also carries sourcing and cost data, and it is revision controlled. A parts list usually is neither.
How accurate does an early BOM cost need to be?
Accurate enough to make a pricing decision, which usually means within 20 to 30 percent at prototype stage and tighter once you have real quotes. The biggest early errors are not component prices. They are missing lines: packaging, cables, labels, screws, test labor, attrition and tooling. A complete BOM with rough prices beats a precise BOM with gaps.
Should I use a spreadsheet or BOM software?
A spreadsheet is fine for one product, one engineer and one contract manufacturer. Move to dedicated software when several people edit the BOM, when you need change orders with approvals, or when you ship more than one revision at a time. The CSV template on this page imports into most BOM and PLM tools, so starting in a spreadsheet loses nothing.
Why does unit cost fall so much between 100 and 10,000 units?
Three things move at once. Component prices drop at reel and tray quantities. Fixed setup costs for PCB assembly, stencils and line programming spread over more boards. And the enclosure moves from 3D printing to injection molding, where the part price is cents once the mold is paid for. Tooling then becomes the cost you amortize.
HardwareMap indexes the companies that got through this spreadsheet and into production. Building one? Submit it and get a part number.
Building one?
Submit it to the index and get a part number. Every entry is reviewed by hand.