Why Nominal Isn't Actual

In the construction industry, materials are named using "nominal" sizes that don't match their actual physical dimensions. This convention dates back over a century and continues today for practical reasons.

The Lumber Story

When lumber is first cut at the mill ("rough sawn"), it actually measures close to its nominal size. However, two processes reduce the dimensions:

1. Kiln Drying: Wood shrinks as moisture content drops from ~19% to ~8%
2. Planing (S4S): All four sides are smoothed, removing material

The result is a board that's approximately 1/2" to 3/4" smaller than its nominal name in each dimension.

Pipe Sizing Conventions

Plumbing pipe sizes are even more confusing because the nominal size typically refers to the approximate inside diameter (ID), while the outside diameter (OD) is what you actually measure with calipers.

For copper tubing, the OD is always 1/8" larger than the nominal size. For PVC, the sizing is based on historical iron pipe standards and varies by schedule (wall thickness).

Why This Matters

• Drilling holes: A "1/2 inch" pipe won't fit through a 1/2" hole
• Weight calculations: Pressure treated lumber is 30-50% heavier than the same nominal size in pine
• Project planning: Incorrect dimensions lead to material waste and rework