Nominal vs. Actual lumber
Whether starting a woodworking or construction project, it is essential to understand the difference between nominal and actual lumber. These terms may sound confusing, but they are vital in choosing the right material for your project. Put simply, nominal refers to the rough size of the wood when it is first cut and milled-the familiar “2×4, for instance. But its dimensions are reduced as the lumber is dried, planned, and smoothed. You end up with the actual size, which is always smaller than the nominal measurement. Example: A 2×4 measures 1.5 inches by 3.5 inches.
This can catch you off guard if you’re not expecting it and will result in incorrect measurements or miscalculated materials. Builders, carpenters, and even DIY have to make that adjustment in size to get an accurate measurement or result. Understanding these sizes is essential, whether framing a house, building a deck, or making furniture. The following article examines why these discrepancies occur, how they may affect your projects, and tips for working effectively with nominal and actual dimensions. By nailing these details, you won’t make expensive mistakes and ensure that your project will last.
What is Nominal Lumber?
Nominal lumber refers to the size of the wood when it is rough cut before it goes through the drying, milling, and planning process. “Nominal” refers to the dimensions given to the wood before it undergoes processing. These nominal sizes are what we typically see when buying wood at a hardware store or lumberyard. Nominal size, in simple terms, is the size of the lumber when it is first cut from the tree or log. In most instances, though, the actual dimensions of the wood can be considerably smaller than the nominal size listed on the label because of shrinkage and smoothing.
Measurement Standard
These rough pieces were tagged with nominal dimensions, which, for convenience, were often rounded up. Even though milling processes today produce far more refined wood, the nominal size system is still widely used within the industry.
Common Nominal Size
2×4: a nominal 2×4 piece of umber is one of the most common and frequently used sizes in construction. It measures only 1.5 inches by 3.5 inches after drying and milling.
2×6: Similarly, a nominal 2×6 measures 1.5 inches by 5.5 inches once processed.
4×4: A nominal 4×4 measures 3.5 inches by 3.5 inches in actuality.
Characteristics
Rough vs. Dressed lumber
When you purchase nominal lumber, you can get rough-cut and dressed lumber. Rough-cut is in its natural, raw state, having irregular edges and a rough surface. In contrast, dressed lumber is smoothly planned and much easier to work with in specific applications.
Variability in actual dimensions
One of the significant issues with nominal lumber is that it isn’t the same size. A nominal 2×4 should be 2 inches by 4 inches, but are only 1.5 inches by 3.5 inches. The drying process causes the wood to shrink, and the planing process reduces the size even further.
What is Actual Lumber?
Actual lumber is the exact size of the finished wood after it has gone through processing, which includes drying, milling, and planning, to result in a smaller, smoother piece of wood ready for use in many areas. Unlike nominal lumber, actual lumber is the correct measurement of the world and is often more helpful in tasks that require precision.
Measurements Standard
Lumber size is determined after the wood has dried and lost moisture, which causes shrinkage. The wood is then planed down and smoothed to a consistent size. The measurements are usually rounded to the nearest quarter inch.
- A nominal 2×4 is 1.5 inches by 3.5 inches.
- A nominal 1×6 is generally 0.75 inches by 5.5 inches after processing.
Characteristics
The actual size of the lumber depends a lot on drying and milling. As the wood dries, it loses moisture and shrinks in thickness and width. This is why, in most cases, the nominal is not the size. In addition, the milling of the wood smoothes out the wood and further reduces its dimensions.
Impact on Project Planning and Execution
In a construction project, actual sizes are essential to correctly cut, fit, and assemble. The builder or contractor must take these exact measurements when a piece of lumber has to go into a particular space. For example, while framing, the dimensions of 2×4 would be 1.5 inches by 3.5 inches, affecting the way the wood interfaces with other materials, such as drywall and insulation.
Lumber Dimensions – Nominal vs. Actual
The table below highlights various lumber types, their nominal and actual sizes, and each finds common uses in construction, furniture making, decking, etc. This should guide your decision-making when choosing lumber for your next project.
| Type of Lumber | Nominal Size | Actual Size (Inches | Actual Size (millimeters) |
| Softwood Lumber | 1×2 | 0.75″ x 1.5″ | 19mm x 38mm |
| 1×3 | 0.75″ x 2.5″ | 19mm x 64mm | |
| 1×4 | 0.75″ x 3.5″ | 19mm x 89mm | |
| 2×4 | 1.5″ x 3.5″ | 38mm x 140mm | |
| 4×4 | 3.5″ x 3.5″ | 89mm x 89mm | |
| Hardwood Lumber | 4/4 (1″) | 13/16″ x 0.8125″ | 20mm x 21mm |
| 5/4 (1.25″) | 1″ x 1.0″ | 25mm x 25mm | |
| 6/4 (1.5″) | 1-5/16″ x 1.3125″ | 33mm x 34mm | |
| Engineered Lumber | |||
| LVL | – | 1.75″ x 9.5″ | 44mm x 241mm |
| I-Joists | – | 11.875″ Depth | 302mm x 44mm |
| OSB | 4×8 Sheet | 1220mm x 2440mm | |
| Pressure Treated Lumber | 5/4×6 (Decking) | 1.0″ x 5.5″ | 25mm x 140mm |
| 6×6 | 5.5″ x 5.5″ | 140mm x 140mm | |
| Specialty Lumber | |||
| Balsa Wood | 1/8″ Sheet | 0.125″ Thick | |
| Baltic Birch | 4×8 Sheet | 1220mm x 2440mm | |
| Glulam Lumber Dimensions | |||
| 3×5 | 2-5/8″ x 4-1/2″ | ||
| 4×6 | 3-1/2″ x 5-1/2″ | ||
| 5×8 | 4-1/2″ x 7-1/4″ |
The key difference Between Nominal and Actual Lumber
The key difference between nominal and actual lumber is size. Nominal sizes reflect the wood’s dimensions as it come off the log mill, whereas actual sizes are the final measurement after the wood is dried and planed. For instance, a 2×4 nominal board is 1.5 inches by 3.5 inches.
This difference matters to construction projects. Framing, spacing, and material estimate would need actual dimensions for proper calculation. Otherwise, there would be wrong estimates, waste of material, or improper structural setting.
Wood shrinks as it dries, and planning reduces even further the size from nominal dimensions. Yet, the industry uses nominal measurements in classifying and pricing the lumber.
To avoid mistakes, always measure the actual dimensions before cutting or ordering lumber. Understanding this difference ensures precision in woodworking and construction, leading to better project alignment, cost control, and minimal waste.
How to Handle Nominal vs. Actual Lumber?
To avoid miscalculations and wasted material, ensure you approach your project with the right planning. One of the best ways to do this is by incorporating the professionals in a lumber takeoff before starting, to ensure that you have accurate calculations and ensure precision and efficiency in your construction or woodworking project.
Check Actual Dimensions Before Cutting
Always measure the actual size before cutting. A nominal 2×4, for example, is 1.5 inches by 3.5 inches. Relying on nominal measurements can lead to errors, so double-checking ensures precise cuts.
Estimate Material Based on Actual Size
When estimating materials, use actual dimensions. If you base estimates on nominal sizes, you may order too much or too little, leading to waste or shortages. For accurate calculations, consider lumber takeoff services to streamline the process.
Account for Shrinkage
Lumber shrinks as it dries, making actual dimensions smaller. Always factor in this reduction when purchasing materials, and order slightly more than needed to compensate. It allows you to follow the steps on how to accurately cut, properly estimate, and efficiently use the materials in a project.
Conclusion
Knowing the difference between nominal and actual will help you better plan your projects more accurately. You will be able to avoid expensive mistakes and miscalculations by double-checking the activity of your materials before cutting, adjusting your estimates, and accounting for shrinkage. Always use the exact dimensions when basing your needs for materials to ensure success, efficiency, and minimum waste in a project.
Tony Cummins began his career as a carpenter in a sawmill, where he learned the ins and outs of cutting and processing wood. Over the years, he honed his skills and gained vast knowledge about different types of lumber and their uses. After many years of working directly in the industry, he decided to start his company for preparing lumber takeoffs to help contractors.
