Confusion over Metric and Imperial Sizes in Timber Measurement

Timber sizes – confusion over metric and imperial measurements

Occasionally I encounter confusion over timber sizes with some of our customers. It is not without reason that customers not familiar with timber sizes can be confused as even though the transition from imperial to metric occurred many years ago the building trade still “speaks” in imperial. Also from a customer’s perspective what is delivered is not actually ordered when you order a piece of timber in imperial and this may seem like there is a massive rip off by the suppliers.


The transition from imperial to metric happened way back in the 70s and metric sizes were introduced into buildings materials like every other sector. All imported timber and timber processed by Irish sawmills from that time on is cut to metric sizes. The measurements introduced tried to mirror what was already supplied on the market and what resulted was a near metric equivalent of the existing imperial sizes. 10ft became 3.0m, 12ft became 3.6m and so on (Timber lengths table shows most of the sizes converted) but this conversion rate isn’t very accurate and results in a discrepancy of almost three inched in a 16ft (4.8m) length of timber.


Metric timber widths are pretty close to that of imperial, for example, 9 inch converts to 225mm (should be 228.6mm). All the widths of timber are a multiple of 25mm and the discrepancy is minimal as the overall widths only go up as far as 9 inch in construction sizes.

The confusion continues when we look at the thickness section dimensions of timber.

The thickness of timber sizes have no resemblance in one inch (22mm) and two inch (44mm) thickness (Timber Thickness table shows full range). The three inch is pretty close to 75mm in metric.

So why do we still “speak” imperial sizes and actually deliver metric sizes. I guess it has to do with tradition and probably more so to do with the convenience of communicating imperial sizes in timber over metric.

Which is easier and quickest to communicate:

Can I have a sixteen foot length of nine by two OR

Can I have a four point eight metre length of two hundred and twenty five millimetres by forty four millimetres

The first, in my opinion, is the quickest and more intelligible way of communicating what is actually required and when dealing with long lists of timber this becomes more evident.

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