How Much Timber Do I Need – Hawksbury Timber

The finished parts in a project are not the same as the timber you need to buy. Stock lengths may not divide neatly into the parts, saw cuts remove material, defects can interrupt a cutting pattern and some boards need selecting for visible faces or matching.

The arithmetic is straightforward once those decisions are separated. The mistake is trying to solve all of them by adding one waste percentage to the net total.

This guide turns a set of finished components into a purchase list. It assumes you already know the required supplied section; if you do not, start with 3. How to Order Timber When You Don’t Know the Correct Size. Units and pricing methods are covered in 1. How to Buy Timber: A Beginner’s Guide.

1. Keep three different lists

Do not ask one total to do three jobs.

List	What it records	Why it matters
Component list	Every finished part in the project	Defines what the finished work requires
Cutting plan	Which parts will be cut from each board, length or sheet	Accounts for kerf, trimming, defects and orientation
Purchase list	The stock sizes, lengths, sheets, boards or packs to buy	Matches what the supplier actually sells

The total finished length is only useful within one section and product. Ten metres of 20 × 45 mm and five metres of 45 × 95 mm are not one meaningful fifteen-metre order. Keep different sections, species, grades, treatments and finishes on separate lines.

The same applies to sheets and boards. Square metres may help compare coverage or price, but they do not tell you whether the required shapes fit on the available panel.

2. Build the component list from the drawing

Start with a sketch, drawing or measured schedule. Reconcile it with the existing work rather than automatically trusting either one. An old frame may have been altered or packed; a drawing may be out of date.

Use one row per distinct component:

Part	Finished section	Finished length	Qty	Visible or matched?	Notes
Screen slat	20 × 45 mm	1,750 mm	20	Visible as a set	Same orientation
Top rail	45 × 45 mm	2,200 mm	1	Visible	Clean face
Bottom rail	45 × 45 mm	2,200 mm	1	Less visible	—

Record dimensions in millimetres and label whether the section is sawn or finished. Also note:

grain or profile direction;
faces that will remain visible;
components that need colour or figure matching;
rebates, mouldings or veneered faces that prevent a part being rotated;
structural, treatment or moisture requirements inherited from the specification.

Do not combine the slat and rail lengths into one grand total. They require different stock sections and therefore separate cutting plans.

3. Find out what stock can actually be bought

A cutting plan is only useful when it uses available stock.

Ask the supplier for:

actual supplied thickness and width;
available fixed lengths or length ranges;
whether random-length or random-width boards must be bought whole;
sheet dimensions and whether the edges need trimming;
minimum quantity, pack quantity or machining setup charge;
whether individual boards can be selected;
whether the supplier can optimise or cut from your list.

Common UK structural-softwood lengths often run in 300 mm increments from 2.4 m to 4.8 m, with 5.4 m and 6.0 m sometimes available to order. That pattern does not govern every product. CLS, planed timber, mouldings, cladding and hardwood boards may have different ranges, so use the supplier’s current offer rather than a memorised list.

If more than one stock length is available, price more than one cutting plan. The longest board is not automatically the most economical, and the cheapest rate per metre may not give the best usable yield.

4. Account for kerf and trimming before rounding

Every saw cut removes the width of the kerf. There is no universal 3 mm figure: current circular and plunge-saw blades, for example, are sold with cutting widths from about 1.8 mm to 3 mm and beyond. Use the blade or cutting service that will actually make the cuts.

For each proposed stock length, test this condition:

Total finished part lengths + required saw kerfs + deliberate end trimming must fit within the usable stock length.

The number of kerfs depends on the cutting sequence and whether stock ends are usable. Rather than relying on division alone, draw each stock length as a line and place the parts and cuts on it.

For example, two parts at 1,200 mm do not fit into an exact 2,400 mm usable length once a separating saw cut is required. Dividing 2,400 by 1,200 and rounding down incorrectly gives two because the calculation has ignored the blade.

Only add end trimming where it is genuinely required. A supplier may provide clean, square stock; rough hardwood or weathered stock may need appreciably more preparation. Inspect the material or agree an allowance instead of assuming every end loses the same amount.

Machining allowance is different again. If rough timber must finish at a specified section, confirm that the starting board has enough thickness and width to clean up. That is a dimension and yield decision, not merely extra length.

5. Plan mixed parts instead of using one division sum

The simple calculation

stock length ÷ part length

works only as an early check for identical parts, and only after allowing for kerfs and trimming. A real project usually contains mixed lengths. One offcut may take a shorter component that would otherwise require another board.

A practical cutting plan can be made with:

a scale sketch of each stock length;
a spreadsheet with one row per stock piece;
a one-dimensional cutting optimiser for linear stock;
a panel optimiser for sheets.

Start with the longest or least flexible components, then fit shorter parts into the remaining lengths. Keep pieces requiring matched grain, continuous figure or a particular face together even when a different arrangement looks better numerically.

For sheets, this becomes a two-dimensional layout. Include:

actual panel length and width;
saw kerf in both directions;
required edge trimming;
grain, face or decorative direction;
whether parts may be rotated;
support needed during cutting;
a safe and practical cutting sequence.

Dividing total part area by sheet area cannot prove that the shapes fit. It is only a lower-bound estimate.

6. Treat defects and appearance as yield decisions

A knot, split, wane or colour change is not automatically waste. Whether it matters depends on the grade and the part being cut.

For each component, decide:

which faces and edges will be visible;
what characteristics the applicable grade permits;
whether a defect can fall in an offcut or hidden face;
whether adjacent parts need matching;
whether a required clear cutting can be moved elsewhere in the board.

This is why usable yield is more useful than gross length or volume. A board with more character may be ideal for one job and poor value for another.

Rough and random-width hardwood needs particular care. The purchase may be based on the whole board or sawn volume, while the useful output depends on defects, board shape, colour selection and the finished sections that can be machined from it. Where possible, select boards against the cutting list or send the list to a specialist supplier.

Do not assume that a fixed proportion of every delivery will be defective. Yield depends on the agreed grade and the actual stock.

7. Add contingency for named risks

There is no universal timber-waste percentage. A flat allowance can be useful for an early budget, but it should not replace the cutting plan used to place the order.

After optimising the stock, ask what could still make the project short:

a visible component is damaged during machining;
a board contains an allowed characteristic in the wrong place for this cutting list;
several parts need closer colour or grain matching than expected;
the supplier cannot guarantee individual selection;
a replacement from a later batch may not match;
the lead time or delivery charge makes a second order disproportionately expensive.

Then choose a specific response: one spare component, one additional stock length, one extra sheet, or a clearly stated selection requirement. Keep that contingency separate from the base quantity so you can see what risk you are paying for.

An additional length is not automatically cheap insurance. It may be sensible for repeated visible components and poor value for an expensive board that can be selected in person. The decision belongs to the job, the stock and the consequences of being short.

8. Worked example: a slatted screen

The screen needs:

Part	Finished section	Finished length	Qty
Slat	20 × 45 mm	1,750 mm	20
Rail	45 × 45 mm	2,200 mm	2

The slats and rails remain separate because they use different sections.

Assume for this example:

3,600 mm and 2,400 mm stock are available;
the stock ends are usable without trimming;
the actual cross-cut kerf is 2.2 mm;
the quoted sections are the required supplied sizes.

Slats from 3,600 mm stock

Two slats require:

1,750 + 1,750 + two 2.2 mm cuts = 3,504.4 mm

They fit, leaving about 95.6 mm. Ten lengths produce the twenty required slats.

Slat calculation	Result
Finished slat length required	35,000 mm
Base stock purchase	10 × 3,600 mm = 36,000 mm
Gross length above finished parts	1,000 mm, before any contingency

If only 2,400 mm stock were used, each length would yield one slat and twenty lengths would be required: 48,000 mm of stock. The 3,600 mm option therefore uses 12,000 mm less stock for the same slats, before comparing prices or delivery constraints.

Rails from 2,400 mm stock

Each 2,200 mm rail needs its own 2,400 mm length. Two lengths buy 4,800 mm for 4,400 mm of finished rails, leaving roughly 197.8 mm after one 2.2 mm cut from each length.

Contingency decision

The base purchase is ten slat lengths and two rail lengths. Whether to add an eleventh slat length depends on the grade, selection policy, required matching, likelihood of damaging a part and cost of returning for another board.

If one extra 3,600 mm slat length is deliberately added, record it as contingency rather than hiding it inside a percentage:

Base: 10 lengths of 20 × 45 mm at 3.6 m

Contingency: 1 additional length of 20 × 45 mm at 3.6 m

Rails: 2 lengths of 45 × 45 mm at 2.4 m

That makes the judgement visible and easy to change after speaking to the supplier.

9. Turn the plan into a purchase list

Write one line for each distinct stock product:

Supplied section	Stock length or board	Base quantity	Contingency	Specification and notes
20 × 45 mm finished	3,600 mm	10	1 optional	Visible slats; confirm tolerance and selection
45 × 45 mm finished	2,400 mm	2	None	Rails; confirm supplied section

Before accepting the quote, check:

supplied section and tolerance;
stock lengths and quantities;
species or product description;
grade, treatment and moisture requirements;
whether machining or cutting is included;
whether boards may be selected;
unit price, VAT, delivery and minimum quantity;
lead time for a replacement if anything is damaged or unsuitable.

Ask the supplier whether another available length gives a better cutting yield. A knowledgeable merchant may spot a simpler pattern, but give them the component lengths and constraints rather than only the net total.

10. Quick method

List every finished component by section, length and quantity.
Keep each different stock product on its own line.
Confirm the stock lengths, board sizes or sheet dimensions actually available.
Draw a cutting pattern that includes the real kerf and any necessary trimming.
Account for defects, appearance, grain direction and matching.
Round to whole purchasable lengths, boards, sheets or packs.
Add a separate contingency only for risks you can name.
Send the component list and proposed purchase list to the supplier for confirmation.

The useful question is not simply “What percentage should I add?” It is:

Which parts fit into each stock piece, what could prevent that plan working, and what is the cheapest sensible way to cover that risk?

Technical references

No standard percentage determines how much timber every project should buy. The reliable quantity comes from the cutting pattern, the agreed material specification and a separate decision about contingency.