Imperial timber sizes.

Sawn and finished timber sizes.

 Imperial timber sizes.

The chart below is a conversion chart between standard imperial timber sizes and their metric equivalent, these sizes were the standard sizes of sawn and prepared timber before the change to metric. the conversion has changed some of the standard sizes and to match up to the original size there will be times when you have to source a larger metric size and plane it down to match the imperial size you are trying to match.

This is useful when matching up to heritage joinery, many traditional joiners had component sizes that everyone worked too. such as tread sizes of 1 1/8″ or 1 3/8″. Scotia would be either 5/8″ or 3/4″ tall etc.

This was not only in the staircase and handrail side of joinery but carried throughout the whole industry including doors and windows.

On timber sizes 1″ and under 1/16″ was removed from each face to clean the face meaning a  1/4″ reduction from the nominal size. On material over an inch 1/4″ was the amount removed from each face to clean the face or 1/2″ from the over all nominal size.

The list below gives the sizes that can be achieved from the sawn timber size, those marked in Blue are the original off the shelf nominal sizes and their corresponding finished sizes.

Glossary terms.

Sawn – Nominal.

PAR – Planed All Round or Prepared All Round.