Bill of Materials for Small Manufacturers: A Plain English Guide

17 April 2026 · 9 min read

If you make physical products to sell, you have a bill of materials. You might not call it that. You might call it a recipe, a formula, a components list, or just “the stuff that goes into it.” The concept is the same: a structured list of everything needed to make one unit of a product.

A properly structured bill of materials - often abbreviated to BOM - is one of the most useful documents a small manufacturer can have. It’s the foundation of accurate costing, consistent production, and sensible stock management. And yet most small makers either don’t have one in writing, or have something partial that lives in their head.

This guide covers what a BOM is, why it matters, how to build one, and the common mistakes that make them less useful than they should be.

What a bill of materials actually is

A bill of materials is a list of all the inputs required to produce one unit of a product. For a batch-produced product (like a food recipe that makes 20 jars), it’s all the inputs for the batch.

Each line on the BOM includes:

The material: the name or SKU of an ingredient, component, or raw material
The quantity: how much of it you need per unit (or per batch)
The unit of measure: grams, millilitres, pieces, metres - whatever makes sense for that material

That’s the core. Some BOMs include additional information: the cost of each material, the supplier, notes on specification or substitution, or lead time. We’ll come to those.

A very simple example, for a soy wax candle:

Material	Quantity	Unit
Soy wax flakes	200	g
Fragrance oil (vanilla)	20	ml
Cotton wick	1	piece
Glass jar (200ml)	1	piece
Lid	1	piece
Label	1	piece

This is a BOM. It tells you exactly what you need to make one candle.

Why it matters

Many small makers have an approximate version of this in their head. They know roughly what goes into each product; they just haven’t written it down precisely.

The rough mental version causes predictable problems:

Inconsistent products. When quantities aren’t documented, they drift over time. A product you’ve been making for two years probably isn’t made the same way it was on day one - small adjustments accumulate, and you may not even notice until a customer mentions it.

Inaccurate costs. If you don’t know exactly what goes in, you can’t accurately calculate what a product costs to make. You’re guessing, and guesses tend to underestimate. (See: the ribbon you forgot to count.)

Unreliable stock forecasting. “Can I make 50 units this week?” is a question you can only answer confidently if you know exactly what goes into each unit and you can check whether you have enough of each.

Slower production. Without a BOM to hand, you’re relying on memory during production - fine when everything’s going smoothly, less good when you’re tired, under pressure, or training someone else.

A written, accurate BOM fixes all of these.

Building a BOM for your product

Start with one product. Pick your most important or most complex one.

Step 1: List every physical input

Write down everything that physically goes into the product. Don’t filter yet - just list. Include:

Raw materials (fabric, metal, wood, chemicals, food ingredients)
Packaging (boxes, bags, tissue paper, labels, stickers)
Consumables used in production (thread, solder, oil used for cooking)
Sub-components you make yourself (a charm that goes onto a bracelet, a sauce that goes into a meal kit)

Common omissions to watch for:

Postage materials (if you’re calculating fulfilment cost)
Labels and stickers (often forgotten because they seem trivial)
Inner packaging (tissue paper, crinkle fill)
Small quantities of “background” ingredients (seasonings, preservatives, binders)

Step 2: Nail the quantities

For each material, establish exactly how much goes in per unit.

Work from a recipe or specification if you have one. If you don’t, make the product and weigh/measure everything as you go. Be precise - “roughly a teaspoon” is not a BOM quantity.

For batch-produced items, decide whether your BOM is per-unit or per-batch. Per-unit is often more useful (easier to scale, easier to cost), but some recipes naturally express in batches. Pick one and be consistent.

Waste and yield. Real production doesn’t have zero waste. If you cut fabric, there’s off-cut waste. If you pour resin, some sticks to the mixing pot. If you brew kombucha, there’s loss during filtering. Your BOM should reflect the actual quantity of material consumed, not the ideal quantity.

The practical way to handle this: calculate the theoretical quantity (what you’d need with zero waste) and apply a waste factor. If you lose roughly 5% of your resin to the pot, multiply the theoretical quantity by 1.05. Check this against reality periodically - waste rates change as your process improves.

Step 3: Set units consistently

Use the same unit of measure that you buy the material in, or a consistent sub-unit. This makes cost calculation much simpler.

If you buy flour in 25kg bags and measure in grams, use grams for your BOM - you can calculate cost per gram from the purchase price. If you buy ribbon by the metre and measure in centimetres, use centimetres.

Avoid mixing units within the same BOM (some items in grams, some in “pieces” of unstated size). Define what a piece is if piece is your unit.

Step 4: Add sub-BOMs if needed

If your product includes a component that is itself assembled from materials, that component has its own BOM. This is called a multi-level BOM.

Example: a gin-making kit might include a bottle of botanicals blend. The botanicals blend is itself a product - it has its own recipe (juniper, coriander seed, orange peel, etc.). The kit BOM references the botanicals blend; the botanicals blend has its own BOM.

For simple products, this doesn’t apply. For products with assemblies or prepared sub-components, it’s important to represent this accurately. Otherwise your costs and material requirements will be wrong.

Adding cost to your BOM

Once you have quantities, you can calculate material cost. For each line:

Cost per unit of material = Purchase price ÷ quantity purchased

For example: you buy vanilla extract in 100ml bottles at £8. Cost per ml = £0.08.

If your product uses 15ml of vanilla extract, the material cost for that line = 15 × £0.08 = £1.20.

Do this for every line and sum them to get your total material cost per unit.

This is different from your total cost of goods (which includes labour and overheads), but material cost is the foundation.

A few things to keep accurate:

Update costs when supplier prices change. A £5 increase in a key input at scale can wipe out your margin without you noticing.
Use the price you actually pay, not the list price. If you have a trade discount, use the discounted price.
If you buy in bulk at a lower price, decide whether to use the bulk price or the standard price. The bulk price gives you a better sense of current production cost; the standard price is more conservative for pricing decisions.

Common BOM mistakes

Incomplete lists. The most common error is simply missing lines. The fixes: make the product while you’re documenting the BOM; check your supplier invoices for items you regularly buy; look at your production notes or labels for anything the BOM doesn’t account for.

Incorrect quantities. Quantities that haven’t been verified against actual production, or that use “target” quantities without waste. Test your BOM quantities against real production runs and reconcile.

No sub-BOMs. Products with assemblies or prepared components where the sub-component is just listed as a single-line item, without its own BOM. Fine if you buy the sub-component finished; a problem if you make it.

Stale costs. A BOM with costs that were correct 18 months ago. Set a reminder to review costs at least annually, and whenever you get a supplier price increase notification.

One BOM for multiple variants. If you make the same candle in three fragrances and two containers, you might be tempted to have one BOM. Usually it’s cleaner to have separate BOMs for each variant - they may share most lines, but the differences (fragrance, container, label) will diverge in cost.

BOMs and stock management

The power of a BOM really shows when you connect it to your stock records.

With accurate BOMs and current stock levels, you can answer questions like:

“Can I fulfil this order?” - check whether you have enough of every material to make the required quantity
“How many can I make right now?” - the answer is determined by your most constrained material (the one you have the least of, relative to the recipe requirement)
“What do I need to buy to make 100 units next week?” - multiply each BOM line by the required quantity, subtract what you already have, get your shopping list

This is sometimes called “material requirements planning,” but the underlying logic is simple maths. The difficulty is just maintaining the accurate inputs: current stock levels and accurate BOMs.

If your BOMs are wrong or incomplete, your stock calculations will be wrong. If your stock records are inaccurate, the calculations still won’t help. Both need to be maintained.

Starting simple and building up

You don’t need to document every product’s BOM on day one. Start with your best-seller, or the product with the most materials/complexity (usually the one where costing errors hurt most).

Get one BOM accurate - quantities verified against real production, costs up to date - and see how it feels to use it. Once you’ve seen the value, the process of documenting the rest becomes less daunting.

The goal isn’t a perfect system on the first attempt. It’s a written, accurate record that replaces the rough mental version - something you can hand to someone else, audit against reality, and calculate from with confidence.

If you make things, you already know what goes in them. A BOM is just writing that down properly.