Understanding timber size charts: a complete guide for builders and contractors

Timber is one of the most widely used materials in construction, spreading from joinery and structural framing right through to landscaping and interior design. However, many professionals encounter confusion when interpreting timber dimensions because the labelled timber size is not always the actual size. 

Understanding timber size charts helps everyone from builders and contractors to procurement teams choose the right timber for structural performance, cost efficiency, and regulatory compliance.

As knowing how timber sizes are defined and measured ensures accurate planning and installation, this guide explains timber size charts in detail, including nominal vs actual sizes, common timber dimensions, manufacturing process, and practical selection tips. 

What is a timber size chart?

A timber size chart is a reference table that shows standard timber dimensions used in a multitude of construction areas, including: 

• Woodworking 
• Structural framing 
• Stud walls 
• Flooring systems 
• Decking and outdoor structures 
• Joinery and carpentry

These charts help professionals to identify the dimensions of lumber and understand how the labelled size compares to the finished size. They are quick and simple to use once you understand them, and ensure compatibility with building designs and engineering specifications. 

Why size charts are important in construction

Using the correct timber dimensions is essential for ensuring: 

• Structural integrity 
• Accurate material estimation 
• Compliance with building standards 
• Preventing costly and time-consuming size adjustments 

Without timber size charts, the industry could be plagued by structural weaknesses and failures, installation delays, and material waste. 

How timber sizes are standardised

Timber lengths, widths, and depths are governed by national and international standards that ensure consistency across manufacturers and suppliers. 

In the UK, organisations like Timber Development UK offer guidance on timber that is to be used in construction, ensuring it meets standards relating to strength, grading, and dimensional tolerance.

Timber sizes in the UK are standardised using metric measurements (mm) based on nominal timber sizes and dimensions that reflect older imperial, or the actual, finished dimensions after planing. Key aspects of UK timber standardisation include: 

• Nominal vs. actual (finished) sizes
• Metric standards 
• Common structural sizes for common timber types, including CLS and PAR timber
• Tolerances

Structural timber grades 

This grading is based on its strength. Two widely used grades include C16 and C24 timber, used for general applications and higher-strength, structural applications, respectively. 

Nominal vs actual timber sizes 

To have success with timber size charts, contractors and other trades need to understand the difference between nominal and actual size. 

• Nominal size refers to the rough-sawn dimensions of timber when cut from the log and before processing.
• Actual size refers to the final dimensions after drying and planing - when it is ready for use.
• The difference is the loss in size that timber goes through as a result of processing, including drying, planing, and machining.

It’s also worth being aware that in framing lumber, these dimensions are still given in imperial sizes, despite the general switch to metric. For example: 

Metric timber sizes 

In the UK and Europe, standardisation also meant swapping to metric dimensions to better align with modern construction standards and engineering calculations. Typical metric sizes for well-used timber include: 

As such, many now need to convert between the two systems, especially if they are working on international or mixed-standard projects. So, why not use this imperial to metric conversion chart to help you? 

Types of timber and their common structural sizes 

The different types of timber products are manufactured in specific dimensions depending on their intended use. 

Softwood structural timber

Used widely in structural framing thanks to its balance between strength, weight, and cost, common sizes for floor joists, roof rafters, and stud walls are:

• 38mm  × 89 mm
• 47mm  × 100 mm
• 47mm × 150 mm

CLS timber 

Canadian Lumber Standard (CLS) timber is useful for stud wall construction and has rounded edges and smooth surfaces. Common CLS sizes include: 

• 38mm × 63 mm
• 38mm × 89 mm
• 38mm × 140 mm

PAR (Planed All Round) timber 

As it is planed on all 4 sides, for a smooth finish, PAR timber is slightly smaller than sawn timber. Common uses include: 

• Furniture 
• Interior joinery 
• Decorative woodwork

Timber tolerances 

Timber manufacturing standards allow small dimensional variations to account for natural variations in wood and processing conditions. 

As a natural product, this is appropriate despite the aim of standardisation. 

Practical tips for contractors ordering timber

1. Always specify the finished size, timber grade, and moisture content required. 
2. Ensure you understand timber labels and check them – this will assist with remaining compliant. 
3. Use size charts to make estimates about the quantity of timber needed for framing, flooring, decking, or other tasks. Doing so will save waste and improve project cost control. 

Timber size charts are the keys to success 

Timber size charts are an essential tool for builders, contractors, and construction professionals across the country. Taking the time to become familiar with them and understand them brings a wide range of benefits, including accurate material selection, reliable timber performance, compliance with industry standards, and completed projects that don’t run over. 

Whether you are designing framing systems, building decks, or working on joinery projects, timber sizing charts can help. 

For more advice and to ask for advice before a trade order, contact NWTT on 01942 720 777. We can help you unlock benefits and give multiple options for direct delivery or Click & Collect.