The actual size of a 2x4 is 1-1/2 × 3-1/2 in (38 × 89 mm), not 2 × 4 in. Almost every common size of lumber is smaller than its name, and a plan that ignores that gap will not fit together. Knowing the real sizes before you shop keeps the shopping list, the cut list and your tape measure agreeing with each other.

Nominal is the name, actual is the board.

If you have ever wondered why a 2x4 is not 2 by 4, the answer is in how the board is made. A log is first sawn rough, close to the size in the name. The boards are then dried, which shrinks them, and planed smooth on all four faces, which takes off more. The smooth, dry board is what you carry out of the store. The “nominal” size, 2x4, is only the name it kept from the rough stage.

Nominal means “in name only”. The actual size is what you measure with a tape.

Lumber sizes chart.

Here are nominal vs actual lumber sizes for the boards our plans use most.

Nominal Actual (inches) Actual (millimetres)
1x6 3/4 × 5-1/2 19 × 140
2x2 1-1/2 × 1-1/2 38 × 38
2x4 1-1/2 × 3-1/2 38 × 89
2x6 1-1/2 × 5-1/2 38 × 140
2x8 1-1/2 × 7-1/4 38 × 184
4x4 3-1/2 × 3-1/2 89 × 89
Fence picket, 5/8 in about 5/8 × 5-1/2 about 16 × 140

A pattern helps you remember it. A “1” board is 3/4 in thick, a “2” board is 1-1/2 in thick, and a “4” is 3-1/2 in. Nominal widths up to 6 in lose 1/2 in, and wider boards like the 2x8 lose 3/4 in.

Fence pickets are the loose entry. A 5/8 in picket is about 5/8 × 5-1/2 in (16 × 140 mm), but pickets vary by store and by type, so treat the table as a typical figure and measure the ones on the rack.

Only the cross-section is nominal. An 8 ft board is sold as 96 in (2.4 m) long, with a few exceptions. Precut studs are shorter, and 92-5/8 in is common, so read the label.

What it means on a build.

Small differences add up quickly, so it helps to work a few out.

  • A frame of 2x4s set on edge is 3-1/2 in (89 mm) deep. Laid flat, the same frame is 1-1/2 in (38 mm) deep.
  • Two 2x4s side by side are 3 in (76 mm) wide, not 4 in. Three side by side are 4-1/2 in (114 mm).
  • A 2x4 on edge is the same height as a 4x4 is wide, 3-1/2 in (89 mm), so a rail can sit flush with a post.
  • A 2x6 and a 2x4 side by side are 9 in (229 mm) across, where the names would suggest 10.

This is also why a cut list that says 1-1/2 × 3-1/2 is telling you the real size. The shopping list uses the name you will find on the rack: 2x4. The cut list uses the size the part has once it is cut: 1-1/2 × 3-1/2 in. Neither is wrong. They answer different questions, as in our note on shopping lists and cut lists.

Do the same arithmetic for any project you are changing. If you widen a bench to fit a space, add up the actual widths of the boards, not the nominal ones.

Measure the boards you bought.

The actual sizes above are the standard ones, and boards still vary a little. The same nominal size can differ between stores and mills, and a board can shrink slightly as it dries after you get it home. Take five minutes before you mark anything.

  1. Measure the thickness and width of a few boards from your stack, in more than one place along each.
  2. Use the same tape for every measurement, including the one you use to mark cuts.
  3. Write the real sizes on your cut list, next to the plan’s sizes.
  4. Where a joint depends on a size, such as a notch or a part that fits between two boards, mark it from the actual board rather than from the number in the plan.
  5. Keep boards of matching thickness together for parts that sit side by side.

A difference of 1/16 in (about 1.5 mm) is ordinary. A larger difference is worth stopping for. A 1x6 that measures 5/8 in thick, for example, is a different board from the 3/4 in one the table lists, and screws or joints sized for 3/4 in may need to change.

Rough-sawn lumber, sold at some lumberyards, is closer to the nominal size and is a different product. If a label says surfaced or S4S, it is planed to the sizes above.

Where our plans use actual sizes.

The cut lists in our plans name the actual size of each part, so you can hold a measured board against the page. The shopping list names the lumber to buy. Both are digital build plans (PDF): nothing physical ships, and you supply the lumber and tools.

  • Mobile Workbench with Shelves is a digital build plan (PDF) for a workbench that finishes at 48 × 24 × 36 in (122 × 61 × 91 cm). It has 15 steps with drawings, and the plan’s own time estimate is 5–8 hours.
  • Simple 2x4 Outdoor Bench is a digital build plan (PDF) for a bench that finishes at 47½ × 18½ × 17 in (121 × 47 × 43 cm). It has 5 steps with drawings, and the plan’s own time estimate is 2–3 hours.

Open either page, find the cut list, and compare its part sizes with the table above. That is the quickest way to see how a nominal 2x4 becomes a 1-1/2 × 3-1/2 in part on a real project.

General planning guidance. Follow your chosen plan and tools’ instructions; ask about unclear dimensions or substitutions before cutting.