Quick Summary
Cost per unit is the price you pay for one usable item, once pack sizes, bulk discounts and hidden charges are stripped out. Calculate it by dividing the total cost by the usable quantity, adding delivery and any non-recoverable GST. This lets you compare quotes from different suppliers fairly.
- Divide total cost by usable quantity to get the figure for each item.
- Include delivery charges, because they can change which supplier is cheapest.
- Add a waste allowance, typically 5 to 15%, depending on the material.
- Convert every quote to the same unit, such as square metres, before comparing.
- Use a reusable spreadsheet so future quotes can be compared quickly.
Table of Contents
- What Cost Per Unit Means and Why It Matters
- Gathering the Figures You Need Before You Start
- Working Out Cost Per Unit Step by Step
- Converting Units and Calculating Cost per Square Metre
- Allowing for Waste and Usable Yield
- Building a Reusable Cost Comparison Spreadsheet
- Using Your Figures to Compare Quotes and Order with Confidence
- Frequently Asked Questions
What Cost Per Unit Means and Why It Matters
Cost per unit is the price you pay for one usable item, once you strip out pack sizes, bulk discounts and hidden charges.
Most quotes show a price per box, pack or roll, which rarely matches how you use the material. You can compare current insulation batt prices per square metre to see how these figures stack up.
Dividing the total cost by the usable quantity gives you a figure you can compare across every supplier. Here is the core formula:
Cost per unit = Total cost ÷ Usable quantity
This matters for three reasons:
- It shows the real price of each square metre, metre or item
- It makes quotes from different suppliers comparable
- It exposes hidden costs such as delivery, waste and minimum order charges
Many small builders find their biggest budget gaps come from materials they never costed properly. Getting cost per unit right at the start saves money at the end.
Gathering the Figures You Need Before You Start
Before you calculate anything, collect the numbers your maths will depend on. Missing a single input can throw off the whole result.
!A tradesperson with a tape measure and a notepad beside a stack of timber packs on a building site, reviewing supplier invoices
Gather these details for each material:
- Pack price: the price on the quote or invoice, before GST
- Quantity per pack: how many units the pack contains (boards, metres, litres, items)
- Unit of measure: square metres, linear metres, each, kilograms or litres
- Delivery charge: any freight cost tied to the order
- Waste allowance: the extra percentage you expect to lose on cuts and damage
- Area or length needed: the size of the job, taken from measured plans
Check the product description too: some items are sold by the box but described per square metre, so confirm which unit the supplier is quoting.
Working Out Cost Per Unit Step by Step
Follow these steps for any material. The method stays the same whether you are buying plasterboard or floor tiles.
- Find the total pack cost. Use the price before GST, unless you are costing for a GST-registered business.
- Find the usable units in the pack. If a box holds 1000 screws, that is 1000 units. If a roll covers 15 metres, that is 15 units.
- Divide the pack cost by the usable units.
- Repeat for each supplier. Use the same unit each time.
Here is a worked example. Suppose a supplier sells a pack of 20 plasterboard sheets for $360 before GST. Each sheet covers 2.88 square metres.
- Usable area per pack: 20 × 2.88 = 57.6 square metres
- Cost per square metre: $360 ÷ 57.6 = $6.25
A second supplier charges $385 for the same 20-sheet pack: $385 ÷ 57.6 = $6.68 per square metre. The first supplier is cheaper per square metre. In other cases the cheaper pack is more expensive per unit, so always check the maths.
Keep your figures in a simple table so comparisons are easy to read:
| Supplier | Pack price (ex GST) | Usable units per pack | Cost per unit |
|---|---|---|---|
| Supplier A | $360 | 57.6 m² | $6.25 per m² |
| Supplier B | $385 | 57.6 m² | $6.68 per m² |
Adding Delivery and GST to the Landed Cost
The cost per unit above does not include delivery or tax. A complete figure, called the landed cost, adds those extras before you divide.
Landed cost = Pack price + Delivery + GST (if applicable) ÷ Usable units
Continuing the example, if Supplier A charges $45 delivery and you are not GST-registered, the figure looks like this:
- Total cost: $360 + $45 = $405
- Cost per square metre: $405 ÷ 57.6 = $7.03
Delivery can change the ranking. A slightly dearer product with free delivery may end up cheaper overall. Always compare landed costs, not sticker prices.
If you are GST-registered, you can usually claim back the GST on business purchases.
Converting Units and Calculating Cost per Square Metre
Suppliers quote in whatever unit suits their stock: boxes, packs, rolls, linear metres, square metres, cubic metres or bulk loads.
Write down the unit the supplier sells in, then the unit you consume on site, they are often different.
Use these common conversions as your reference set, and always convert to the unit you will measure the finished work in:
- 1 square metre = 10,000 square centimetres = 1.1959 square yards (useful when a spec sheet is in imperial)
- 1 linear metre = 1000 millimetres = 3.2808 feet
- 1 cubic metre = 1000 litres = 1,000,000 cubic centimetres
- 1 litre = 1000 millilitres
- 1 kilogram = 1000 grams
- 1 tonne = 1000 kilograms
Worked example: reconciling three quotes for the same floor
Suppose a floor measures 4.2 metres by 3.5 metres, giving 14.7 square metres. Three suppliers quote differently:
- Supplier A sells boxes covering 2.2 square metres each at $58.90 per box ex GST.
- Supplier B sells planks priced at $27.40 per lineal metre, with each plank 0.19 metres wide and 1.8 metres long.
- Supplier C sells a bulk pack covering 18 square metres at $410 ex GST.
Convert each to a cost per square metre before comparing:
- Supplier A: $58.90 ÷ 2.2 = $26.77 per square metre
- Supplier B: each plank covers 0.19 × 1.8 = 0.342 square metres. Cost per square metre = $27.40 ÷ 0.342 = $80.12 per square metre. That looks expensive, but check whether the quote is per plank or per lineal metre of a wider board, a common source of error.
- Supplier C: $410 ÷ 18 = $22.78 per square metre
On headline cost per square metre, Supplier C wins.
Watch out: a quote described as "per square metre" may be based on nominal coverage of the pack, not usable coverage after laps, overlaps or offcuts.
For volume-based materials such as concrete, sand or gravel, convert the job volume to the supplier's order unit. Accurate calculations of this nature prevent significant budget overruns when you estimate building materials for larger construction projects.
Keep at least two decimal places during conversion and round only the final answer. Rounding early, especially on small units such as screws or fixings, can shift a comparison by several percent.
Allowing for Waste and Usable Yield
Waste is not a single percentage you bolt on at the end.
Separate the two ideas before you calculate:
- Order quantity is what you buy, including any extra to cover waste.
- Usable yield is the portion of that order that finishes up in the job. Offcuts, breakage, defects, laps and overlaps reduce yield.
The effective cost per usable unit is therefore:
Effective cost per usable unit = Total landed cost ÷ (Order quantity × Usable yield)
Material-specific waste and yield patterns
The figures below are common starting points, not rules.
| Material | Typical waste allowance | Yield notes |
|---|---|---|
| Plasterboard | 5 to 10% | Higher on rooms with many openings or angled walls; sheets are large so offcuts are often unusable |
| Floor tiles | 10 to 15% | Higher for diagonal or herringbone patterns; lower for straight lay in a simple rectangular room |
| Timber framing | 5 to 10% | Higher where lengths must be cut to suit; defects and bows reduce yield further |
| Fixings and screws | 2 to 5% | Loss is mostly dropped or damaged fixings; pack rounding usually dominates |
| Concrete | 5 to 10% | Over-order is common because part-loads are expensive; spillage and uneven subgrade add to waste |
| Paint | 5 to 10% | Coverage figures assume a smooth, sealed surface; porous or textured surfaces reduce yield |
| Insulation batts | 5 to 10% | Offcuts around services and noggings reduce usable coverage; some brands have tighter nominal coverage than others |
Applying waste and yield to a worked example
Return to the 14.7-square-metre floor.
Now apply yield.
That is the number to compare against other suppliers, not the sticker price per box.
Watch out: waste and yield are not the same as pack rounding.
Adjusting waste for the shape of the job
For sheet goods such as plasterboard or plywood, sketch the layout before ordering.
Recording yield for future jobs
After each job, note the actual waste and yield for each material.
Building a Reusable Cost Comparison Spreadsheet
Once you understand the method, a spreadsheet saves time on every future job. A simple layout is enough.
Set up columns for:
- Material name
- Supplier
- Pack price (ex GST)
- Delivery charge
- Usable units per pack
- Waste allowance (%)
- Units needed for the job
- Packs to order
- Total cost
- Cost per unit
Useful formulas:
- Landed cost per pack:
=C2+D2 - Cost per usable unit:
=(C2+D2)/E2 - Units needed with waste:
=G2*(1+F2) - Packs to order:
=ROUNDUP(H2/E2,0) - Total cost:
=I2*(C2+D2)
Enter your own figures in the blue cells and let the formulas do the rest. Save one sheet per project type, such as bathroom, kitchen or deck, and copy it for each new quote.
Building cost templates are widely available, and you can also use a construction material cost spreadsheet as a starting point. Adjust the columns to match the units you use most.
Using Your Figures to Compare Quotes and Order with Confidence
With your cost per unit calculated, the comparison becomes clear. Put every supplier's landed cost per unit side by side, then check three things before you order:
- Are the units identical? Square metres against square metres, not boxes against sheets.
- Is the waste allowance included? Order the quantity you need, not the quantity on the plan.
- Are delivery and minimum order rules clear? A low unit price can hide a high freight charge.
For smaller fixings, cost per unit is especially useful. You can also compare them with other decking fixings in the same way, such as Prolnx self-drilling wing tip decking screws, box of 1000, various sizes and materials when you want an alternative supplier figure.
Work through your spreadsheet, choose the lowest landed cost per usable unit that meets your specification, and order the correct number of packs. Keep your figures on file. They make the next quote faster and help you spot when a supplier's prices change.
Prices move, so check current quotes before you commit. Pricing depends on quantity, delivery location and order date. For current prices, visit the Trade Warehouse Direct website or request a quote.
Frequently Asked Questions
How do you calculate the cost per unit of a building material?
Divide the total landed cost of a purchase by the number of usable units you receive. Landed cost is the supplier price plus delivery, and GST if you cannot claim it back as a registered business. For example, if 20 boards cost $300 with $25 delivery, the cost per board is $325 divided by 20, or $16.25. Use the same unit of measure for every supplier so the comparison is fair.
How do you account for waste when estimating material costs?
Add a wastage allowance to your measured quantity before you multiply by the unit price. Common allowances depend on the material and the cut pattern, and tiling, flooring and timber often need extra. Divide the measured area by the coverage of one unit, round up to whole packs, then add the allowance. Recording the allowance separately makes it easier to adjust the figure on future jobs.
Should delivery and GST be included in a material unit cost?
For an accurate comparison, include delivery in the unit cost, because it is a real cost of getting the material to site. GST should be included only if you cannot claim it back as input tax credits. If you are GST-registered, exclude GST from your calculations, but make sure you compare quotes on the same basis. Always confirm with your accountant how GST applies to your business.
How do you compare building material prices when pack sizes differ?
Convert every price to the same unit, such as price per square metre, per linear metre or per each. Then compare the converted figures rather than the sticker prices. A larger pack can look cheaper but cost more per usable unit once off-cuts and waste are included. Check current prices and pack details on the Trade Warehouse Direct website, as pricing depends on quantity, dates and delivery.
