Measurements, waste factors and conversions

Despite EU regulations, the building and timber trades continue to use a combination of imperial and metric measurements. This can be not only a little confusing, but can also lead to some drastic calculation errors.

To make life simple, it is best to use one or the other.

For imperial we use inches (“), feet (‘), cubic feet (ft3) and square feet (ft2).
For metric we use millimetres (mm), metres (mt), cubic metres (m3) and square metres (m2). The timber trade does not tend to use centimetres.

Wood is sold in variety of ways:

  • Rough sawn wood is normally sold by cubic volume (ft3 or m3).
  • Machined sections are normally sold by the running length (ft or mt).
  • Flooring and some cladding is normally sold by the square area (ft2 or m2).

Calculating volume

A cubic foot = width in inches x thickness in inches / 144 x length in feet.
A cubic metre = width in mm x thickness in mm / 1,000,000 x length in metres.

(The width of a waney edge board is calculated by taking the average of three measurements along the length of the board including half the wane on each edge.)

Calculating area

A square foot = length in feet x width in feet.
A square metre = length in metres x width in metres.

Waste factors

It is important to understand what a waste factor is and how it will impact your budget. A waste factor is essentially the amount of waste wood that you will have to pay for but cannot, for one reason or another, use in your project. A waste factor is normally expressed as a percentage (%) of the required amount.

A waste factor can be caused by:

  • Natural defects in the wood.
  • Distortion of the wood.
  • Non-uniform shape and size of sawn planks.
  • Cutting the bark and sap off a waney edge plank.
  • Cutting fixed lengths out of random lengths.
  • Trimming ends.

Recommended waste factors:

  • Cutting a specification from waney edge planks (planks with the bark still attached). Add 100%.
  • Cutting a specification from thick waney edge planks 2 ½” – 4”. Add 120%.
  • Buying skirting boards. Add 15%.
  • Buying flooring to go on joists or battens. Add 15%.
  • Buying flooring to go on chipboard or glue. Add10%.

Exact conversions

Feet to metres x by 3.281
Cubic feet to cubic metres x by 35.315
Square feet to square metres x by 10.76


Rough conversions

Imperial Metric
1″ 25mm
2″ 50mm
2½” 63mm
3″ 75mm
4″ 100mm
5″ 125mm
6″ 150mm
1′ 0.3mt
2′ 0.6mt
3′ 0.9mt
4′ 1.2mt
5′ 1.5mt
6′ 1.8mt
7′ 2.1mt
8′ 2.4mt
9′ 2.7mt
10′ 3.0mt
11′ 3.3mt
12′ 3.6mt
13′ 3.9mt
14′ 4.2mt
Imperial Metric
1″ 25mm
2″ 50mm
2½” 63mm
3″ 75mm
4″ 100mm
5″ 125mm
6″ 150mm
Imperial Metric
1′ 0.3mt
2′ 0.6mt
3′ 0.9mt
4′ 1.2mt
5′ 1.5mt
6′ 1.8mt
7′ 2.1mt
Imperial Metric
8′ 2.4mt
9′ 2.7mt
10′ 3.0mt
11′ 3.3mt
12′ 3.6mt
13′ 3.9mt
14′ 4.2mt