Why is 2×4 lumber not actually 2 inches by 4 inches?

The answer: Because of the lumber manufacturing process. A 2×4 starts as a rough-sawn board that's actually 2" × 4". Then it goes through:

• Drying: The wood shrinks as it loses moisture
• Planing: The surface is smoothed down with machinery, removing an additional 1/4"-1/2"
• Result: Final actual size is roughly 1.5" × 3.5"

The "2×4" is the nominal size (what you call it), not the actual size (what you measure).

Are these dimensions accurate for all lumber types?

Mostly yes: Standard dimensional lumber (the stuff at your hardware store) follows consistent sizing across all US mills.

Exception: Pressure-treated lumber is sometimes slightly thicker in the nominal 2× thickness category due to the treating process.

Check: Specialty lumber (reclaimed, exotic, rough-sawn) may vary—measure before you buy.

What's the difference between nominal and actual size?

Nominal size: What you call it at the store ("I need a 2×4")

Actual size: What you measure when you get home (1.5" × 3.5")

This site shows actual sizes because that's what matters for your project.

Do I need to account for the difference in my project?

Yes, absolutely. If you're building a deck frame, fitting boards in a space, or doing precision joinery, use the actual dimensions.

Example: You're building a box that needs to be 20" wide. If you use two nominal 2×10 pieces, you'll get 19" (two × 9.5" actual width), not 20".

What about hardwood lumber?

Hardwood (oak, maple, walnut, etc.) is sold by actual thickness, not nominal. A "1-inch" hardwood board is actually close to 1" thick. This is very different from dimensional softwood lumber.

How are nail sizes measured?

Nails are measured by the "penny" system (abbreviated "d"). This is the length of the nail:

• 10d nail = 3" long
• 16d nail = 3.5" long
• 20d nail = 4" long

The thickness varies with length, but the penny number tells you the approximate length.

What does "gauge" mean for screws and bolts?

Gauge is the diameter of the fastener. Higher gauge numbers = thinner fasteners (counterintuitive, I know).

Examples:

• #6 wood screw = 0.138" diameter
• #8 wood screw = 0.164" diameter
• #10 wood screw = 0.190" diameter

Why are bolt sizes written as "1/4-20"?

That's diameter-by-pitch notation:

• 1/4 = diameter of the bolt shaft (1/4 inch)
• 20 = threads per inch (coarse or fine pitch)

This matters because the threads on the bolt must match the threads in the nut or hole.

Can I substitute fasteners?

No: Use the exact fastener type specified for your project. Different materials and sizes are engineered for different loads and purposes.

Exception: You can sometimes upsize (use a slightly larger fastener), but never downsize or change type without knowing what you're doing.

Why does 1/2" PVC pipe have a different actual size than 1/2" metal pipe?

Different materials, different standards:

• PVC (plastic): 1/2" nominal = ~0.840" OD
• Steel pipe: 1/2" nominal = ~0.84" OD
• Copper tubing: 1/2" nominal = ~0.625" OD

Each material has its own sizing standard. Always specify the material when buying pipe.

What's the difference between OD and ID?

OD (Outside Diameter): The outer measurement of the pipe

ID (Inside Diameter): The inner measurement (what actually flows through)

The difference is the wall thickness. Thicker walls = smaller ID, better for high pressure.

What does "Schedule 40" mean?

Schedule is a thickness rating. Higher schedule = thicker walls.

• Schedule 40: Standard residential/light commercial (most common)
• Schedule 80: Thicker walls, higher pressure rating
• Schedule 120: Very thick, heavy-duty industrial

For home projects, Schedule 40 is the default. Check your local code for specific requirements.

Can I mix pipe materials?

Not directly: You need transition fittings. PVC to metal, copper to PVC, etc. all require special adapters.

Check local code: Some materials aren't allowed for certain applications (e.g., some areas don't allow PVC for main water lines).

Is this reference accurate?

Yes. All dimensions are based on official industry standards:

• Lumber: USDA Forest Products Laboratory standards
• Fasteners: ANSI (American National Standards Institute)
• Pipe: ASME (American Society of Mechanical Engineers)

But always verify before you cut or install—especially for critical fits.

Why should I bookmark this?

Because you'll forget these numbers. Next time you're at the hardware store or planning a project, you'll want quick access to actual dimensions without googling for 10 minutes.

Can I use this for international projects?

Partially: These are US standard sizes. We include metric conversions for lumber, but international lumber and pipe standards may vary significantly. Check local standards for your country.

Is there a mobile app?

Not yet, but this site works great on mobile. Bookmark it and you've got a reference tool in your pocket.

Can I download this as a PDF?

Use your browser's print function (Ctrl+P or Cmd+P) and select "Save as PDF". You'll get a printable reference guide for your workshop.