Bill of Materials and Cost

One-sentence summary

A bill of materials (BOM) collects every part a project needs into a single table, with quantities and estimated prices, so we can see ahead of time what we already have, what we need to buy and roughly how much the whole thing might cost.

Why it matters

After finishing the draft and the flowchart, the first thought is usually "let's start building right away." But if we begin without knowing which parts we have, we often stop in the middle because a single wire or sensor is missing.

A bill of materials prevents that pause. Before we begin, it makes us think about every part, decide how many we need and be realistic about cost.

Another benefit is budget awareness. In a student project money is usually limited. Seeing what each part costs helps us ask, "Do I really need this part?" That question is asked often in engineering.

BOM is short for "Bill of Materials." It also goes by "parts list." Factories, workshops and school projects all use the same idea.

What goes into a bill of materials?

A good bill of materials is more than a list of part names. Each row holds a few pieces of information together.

What should one row include?

• Part name: State clearly what it is. "Male-to-male jumper wire" is better than just "wire."
• Quantity: How many are needed. One LED, or four?
• Already have it?: Is this part already in our kit, in class or at home?
• Estimated unit price: Roughly how much is one? This is a sample figure, not an exact price.
• Estimated total: Quantity multiplied by unit price.
• Note: Extra details such as colour, size or an alternative.

Why do we say "estimated"?

Prices change over time and from seller to seller. So the numbers we write in a bill of materials are not exact claims; they are estimates. The goal is not to get every cent right, but to see roughly how much a project might cost. When you actually shop, current prices should be checked together with an adult.

Example project: An automatic night light

Let's work through an example. Suppose we want to design a small night light that turns on by itself when it gets dark and switches off when the room brightens. Using the sensor and LED ideas from earlier modules, we sketched this as a draft. Now let's turn that draft into a bill of materials.

Step 1: Pull the parts from the draft

Looking at the flowchart and the draft, we write down each part we need:

• A controller board (for example a microcontroller board)
• A light sensor (LDR)
• An LED
• A resistor
• A breadboard
• Jumper wires
• A USB cable

Step 2: Decide quantity and status

For each part we think about how many are needed and whether we already have it. Some parts may still be in our kit from earlier lessons.

Step 3: Fill in the table

Below is a sample bill of materials for this project. The prices are examples only and are not a claim about real prices.

Note: Parts marked "Yes" in the "Already have it?" column show 0 in the total column, because we do not need to buy them again.

Step 4: Work out the total

We add up only the parts we need to buy:

To buy:
  Light sensor (LDR)      $1
  Breadboard              $3
  Jumper wires (pack)     $2
  ----------------------------
  Estimated total:        $6

The parts we already own have value too, but the extra spending for this project comes to about $6. This way we know whether our budget is enough before we even start.

Budget awareness and choices

Once the bill of materials is ready, a common question appears: "The total is more than I expected, what can I do?" There are a few honest options here.

Use what you already have

The cheapest part is one you already own. An LED or resistor sitting in your kit, added to the list as "already have," brings the total down.

Think of alternatives

Sometimes a more suitable option exists in place of a part. For example, instead of buying a breadboard, you might borrow shared classroom equipment for a temporary connection. Decisions like this are made by talking with an adult.

A second everyday example: A class poster

A bill of materials is not just for robotics projects. Suppose your class is making a display poster. You can build a list with the same logic:

This example shows that a bill of materials is a simple but powerful tool, useful for anything from planning a birthday to setting up a science-fair stand.

Mini practice

Build a bill of materials for a small project of your own. If you like, use a line-following robot idea from earlier modules.

1. Write your project in one sentence.
2. List the parts you need, one by one.
3. Enter a quantity for each part.
4. Fill in the "Already have it?" column.
5. Write sample unit prices as estimates (you do not need to find exact prices).
6. Add up only the parts you need to buy.

You can use the blank template below:

Project: ______________________________

| Part | Qty | Already have it? | Unit (estimate) | Total |
|------|-----|------------------|------------------|-------|
|      |     |                  |                  |       |
|      |     |                  |                  |       |
|      |     |                  |                  |       |

Estimated total to buy: __________

When your list is done, ask yourself: "Are all of these parts really needed, or is there one I could remove?"

Common mistakes

Leaving parts out

Many people forget small parts such as wires, resistors or screws. When one of these is missing, the project stops in the middle. Reviewing the draft line by line prevents this.

Not giving a quantity

Writing "LED" without a quantity creates confusion. One, or three? Always fill in the quantity column.

Not counting what you already have

Showing parts you already own as "to buy" makes the total look more expensive than it is. That is why the "Already have it?" column matters.

Mistaking an estimate for an exact price

The numbers in the list are there to give you an idea. Real prices may turn out different when you shop. That is why we always say "estimated."

Safety note

Making a bill of materials is safe desk work. But the shopping part should be done together with an adult. Do not order online by yourself or enter any payment details; a parent or teacher always handles that. When you list parts such as batteries, motors or sharp tools, adding a short safety note beside them will make the later steps easier.

Lesson summary

• A bill of materials (BOM) shows all the parts of a project together with quantities and estimated prices.
• Each row holds the part name, quantity, whether you already have it and an estimated price.
• Parts you already own are not added to the total, which keeps the real spending accurate.
• Prices are estimates; exact prices need a current check and adult support.
• A bill of materials builds budget awareness and helps us question parts we may not need.

Check questions

1. What does the abbreviation BOM stand for, and what is it also called?
2. What basic information should a row in a bill of materials include?
3. How does the "Already have it?" column affect the total cost?
4. Why do we write prices as "estimates"?
5. Name two common mistakes people make when building a bill of materials.

Answers

1. BOM stands for "Bill of Materials"; it is also called a "parts list."
2. At least the part name, quantity, whether you already have it and an estimated unit price; a total and a note column can also be added.
3. Parts you already own are not bought again, so they are not added to the total; this keeps the real spending lower and accurate.
4. Prices change over time and from seller to seller, so we make no exact claim; the goal is to see roughly what a project will cost.
5. For example, forgetting small parts (wires, resistors) and leaving the quantity column blank. Not counting what you already have is another common mistake.

Source and verification note

For “Bill of Materials and Cost”, verification focuses on whether the relationship between What goes into a bill of materials? and Why do we say "estimated"? remains consistent across examples. A project page should make a result claim only when it is supported by a real prototype, test record or observation. Numbers such as cost, duration and success rate must be labelled clearly when they are estimates.

Next lesson

Prototyping: Once the materials are ready, turning the design into its first working example for the very first time.