Timber is sold in a unit that belongs to no other trade. You cannot buy a board foot at a hardware shop counter, you cannot weigh it, and you cannot see it. Everything about a lumber estimate depends on getting comfortable with that unit and with the gap between the size on the label and the size in your hands.
The board foot, defined
One board foot is a piece one foot wide, one foot long and one inch thick. In cubic metres that is:
1 board foot = 0.3048 × 0.3048 × 0.0254 = 0.0023597 m³
Two conversions follow, and you will use both. To go from board feet to metres, multiply by 0.0023597. To go the other way, divide by it. A quick anchor: one cubic metre of timber is about 424 board feet.
Now the piece everyone measures: one linear metre of a 2×4 at actual dressed dimensions of 38 × 89 mm is 0.038 × 0.089 × 1 = 0.003382 m³, which is 0.003382 ÷ 0.0023597 = 1.43 board feet per metre. That number does a lot of the work below.
The lumber calculator takes dimensions in either system and reports board feet, cubic metres and linear metres together, which is the fastest way to sanity-check a figure from a supplier.
The nominal-versus-actual trap
Here is the single most expensive habit in timber estimating: treating the label as the size. A "2 × 4" is not 50 × 100 mm. It is 38 × 89 mm, because lumber is dried and planed after sawing, which takes material off every face.
The consequences are large enough to build a whole section on. Take a 2 × 4 × 8 ft length and compute board feet two ways.
The wrong way, using nominal inches. A nominal 2 × 4 × 8 ft is 2 × 4 × 96 inches, and dividing by 12 gives 5.33 board feet.
The right way, using dressed inches. The same piece is 1.5 × 3.5 × 96 = 504 cubic inches, and dividing by 144 gives 3.5 board feet.
So the naive formula over-states the volume by more than 50%. A supplier who quotes in nominal board feet against nominal inches will hand you a number more than half again as large as the timber actually contains. Use actual dressed dimensions whenever the two systems meet, and check which one a quote is using before comparing two of them.
Nominal and actual do not even have a fixed relationship. A nominal 6 × 2 is 140 × 38 mm, and a nominal 2 × 8 is 184 × 38 mm — the same thickness, different width — so there is no single conversion factor that rescues you. You have to look the actual dimensions up.
Studs per metre and per square metre
Stud spacing is a structural decision, and the two common values are 400 mm and 600 mm centres. At 400 mm the count is simply:
studs per linear metre = 1 ÷ 0.400 = 2.5
That 2.5 is per linear metre of wall, not per square metre — a distinction worth keeping, because the two numbers differ by the height of the wall. On a 2.4 m high wall, 2.5 × 2.4 = 6 studs per square metre.
The practical conversion is also worth having: 400 mm centres is about 13 boards per 10 ft, and 600 mm centres is about 8.5. Suppliers price in board feet and lengths, so being able to convert a stud count into "how many 8 ft sticks" is what actually gets the order right.
Worked example: framing a 5.4 × 3.6 m room
Frame a single-storey room 5.4 × 3.6 m with a 2.4 m storey height, using 2 × 4 studs at 400 mm centres, double top plates and three rows of blocking. Every figure below can be checked with a tape.
Step 1 — stud count. The two 5.4 m walls need 5,400 ÷ 400 = 13.5 spaces, so 14 studs each. The two 3.6 m walls need 3,600 ÷ 400 = 9 spaces, so 10 studs each. That is (14 × 2) + (10 × 2) = 48 studs. The achieved spacing is 5,400 ÷ 13 = 415 mm on the long walls and exactly 400 mm on the short ones, both within tolerance.
Step 2 — stud length. 48 × 2.4 = 115.2 m of 2 × 4. In pieces this is 48 studs of 2.4 m, or 48 lengths at about 3.7 board feet each.
Step 3 — blocking. Three rows, placed clear of openings. On a 5.4 m wall there are 13 blocks per row, each cut to the stud gap of 415 − 89 = 326 mm; three rows × 13 × 2 walls = 78 blocks, or 25.4 m. On a 3.6 m wall there are 9 per row at 400 − 89 = 311 mm; three rows × 9 × 2 = 54 blocks, or 16.8 m. Blocking total is 42.2 m across 132 pieces.
Step 4 — plates. Perimeter is 2 × (5.4 + 3.6) = 18 m. A bottom plate and two top plates is three layers, so 3 × 18 = 54 m.
Step 5 — total. 115.2 + 42.2 + 54 = 211.4 m of 2 × 4. Multiply by the per-metre cross-section of 0.038 × 0.089 = 0.003382 m² and the total is 0.715 m³, which is 0.715 ÷ 0.0023597 = 303 board feet.
Step 6 — waste. At 10% for cut blocking and offcuts, 303 × 1.10 = 333 board feet. A supplier will usually round to 50 or 100 board feet, so 350 is the practical order.
Step 7 — sheathing. Both faces of an 18 × 2.4 m wall is 86.4 m². A 1.2 × 2.4 m sheet covers 2.88 m², giving 30 sheets, and at 10% waste 33 sheets.
The check that this is plausible: 48 studs over 43.2 m² of wall is 1.11 studs per m², against the 6 per m² rule for 400 mm centres. Those look far apart, and the reason is that the 6-per-m² figure is a linear-metre rate multiplied by height, while 48 studs is the actual count around a closed rectangle where corners share framing. Use the linear rule to set out a wall; use the closed-count for ordering.
Beam depth from span
Span is what governs beam depth, and the rule of thumb is a single division. Divide the span by 15 to 20, and use 18 as the default.
beam depth ≈ span ÷ 18
A 4.0 m span gives 4,000 ÷ 18 = 222 mm, so a 200 × 200 or 200 × 300 beam is the sensible starting point. The 15 divisor gives 267 mm and suits a stiffer beam with more headroom; the 20 divisor gives 200 mm and suits a deeper, slimmer section. A longer span does not just need a deeper beam, it needs more of them: a first approximation for floor joists is one joist per 400 to 600 mm across the span, so a 4 m wide floor at 450 mm centres takes about nine.
This is a sizing sketch, not a structural design. Load, span length, species, grade and code all move the answer, and a real beam needs an engineer's check. The torque calculator and force calculator cover the load arithmetic that a check would start from.
Stairs: two numbers describe everything
A stair is fully described by the rise and the going, and comfort puts a tight limit on their combination:
2R + G ≈ 630 mm, with R between 175 and 190 mm and G between 250 and 290 mm.
Any pair satisfying that equation feels right, which is why 180 and 270 is the default: 2 × 180 + 270 = 630. Pitch R 175 and G 280 also give 630, as do R 190 and G 250 — the same comfort, different proportions.
The other constraint is that the total rise must divide into equal risers. For a 2.7 m floor-to-floor, 15 risers of 180 mm land exactly on 2,700 mm and pair with a 270 mm going; 16 risers would give 169 mm each, which is uncomfortably shallow. With 14 goings for 15 risers, the run is 14 × 270 = 3,780 mm, and the headroom needed above is whatever the landing-to-landing floor allows. Total rise divided by risers is the number to check first, because a stair that does not reach the floor is the classic framing error.
Roof battens, tiles and ridge
Roofing materials are counted per square metre of roof surface, not of plan area, which is where estimates slip. Convert first: roof area = plan area ÷ cos(pitch). A 30° pitch has a cosine of 0.866, so 100 m² of plan needs 100 ÷ 0.866 = 115.5 m² of tiles — a 15% uplift that has nothing to do with waste.
Battens and counter-battens are counted by length over the slope. Ridge and hip pieces are counted by linear metre of ridge, roughly 0.45 to 0.5 m of ridge tile per metre of ridge, and hip tile by the diagonal rather than the horizontal run.
Waste, and why 10% is usually right
Timber waste is dominated by cuts, so it runs higher than for materials bought in bulk. Studs are cut to length and produce almost no offcut; blocking, headers and rafters produce a lot. Use 5% on a simple rectangular frame with few openings and 10% on anything with a staircase, dormers or irregular geometry — and remember that every 2.4 m piece bought as 2.44 m stock already carries the offcut implicitly.
Record the count, the spacing, the actual dimensions used and the waste percentage. Those four numbers let anyone rebuild the order, which is the difference between an estimate and a number that happened to work. The concrete side of the same trade is in concrete and masonry calculations, and the finishes side in tile and flooring layout.
Frequently asked questions
What is a board foot and how do I convert it to cubic metres?
One board foot is 1 ft × 1 ft × 1 in thick, which is 0.0023597 m³. To convert board feet to metres, multiply by that figure. The reverse is to divide the volume in m³ by 0.0023597, so one linear metre of a 2×4 at 38 × 89 mm gives 0.003382 m³ or 1.43 board feet.
Why is a 2×4 actually 38 × 89 mm instead of 50 × 100 mm?
The 2 × 4 label is a historic nominal name, and timber is dried and planed after sawing, which removes material from every surface. A nominal 2 × 4 is planed down to 38 × 89 mm. Use actual dressed dimensions for any volume or board foot calculation, or you will over-order by roughly half.
How many studs do I need for a wall?
At 400 mm centres you need 2.5 studs per linear metre of wall, so 6 studs per square metre on a 2.4 m high wall. For a 5.4 m wall that is 14 studs and for a 3.6 m wall 10 studs, giving 48 studs around a 5.4 × 3.6 m room before any extra corner or opening studs.
How do I work out the depth of a beam for a given span?
Divide the span by about 18 for a rough structural sizing. A 4.0 m span gives 4000 ÷ 18 = 222 mm, so a 200 × 200 or 200 × 300 beam is the usual starting point. The divisor ranges from 15 for a stiffer, shallower beam to 20 for a deeper, slimmer one.