Lumber calculator

Nominal vs actual lumber dimensions

A 2x4 is not 2 by 4 inches: dry dressed dimension lumber is 1-1/2 in thick and 1/2 in narrower than nominal up to 6 in, 3/4 in narrower from 8 to 12 in. These are the minimum dressed sizes in the American Softwood Lumber Standard, PS 20^[1]. Dry means 19% moisture content or less; green lumber is cut slightly larger.

Timbers (5 in and up) are 1/2 in under nominal for 5 and 6 in sizes, so a 6x6 is 5-1/2 in square^[1]. Canada's NLGA rules list the same dressed sizes, 38.1 × 88.9 mm for a 2x4^[2].

How much lumber do I need?

Start from a framing take-off rather than a guess. The tools on this site produce the pieces for each part of a job; this calculator adds them up:

• Walls: studs, plates and headers from the framing calculator or the stud calculator.
• Floors and decks: joists from the joist span calculator and decking from the deck calculator.
• Roofs: rafters and their board length from the rafter calculator.

Board feet are worked out from nominal sizes, the way softwood lumber is tallied and often priced in bulk. For hardwood or a cut list of odd sizes, the board foot calculator accepts any thickness and width.

Lumber sizes outside the US

Metric countries sell structural softwood under their own size lists. In the UK, CLS studs are 38 × 63 and 38 × 89 mm, and sawn sizes such as 47 × 100 and 47 × 150 mm are common^[3]. Australian MGP10 framing is supplied in sizes like 70 × 35, 90 × 35 and 90 × 45 mm^[4]. Pick the nearest nominal size in the list, or enter the real section in the board foot calculator. To know what a load weighs, use the lumber weight calculator.

Lengths and waste

Lumber comes in standard lengths, usually in 2 ft steps (8, 10, 12, 14, 16 ft); the standard allows lengths in multiples of 1 or 2 ft under the grading rules^[1]. Order pieces that cut cleanly into your parts. Under the IRC, double top plates need end joints offset at least 24 in^[5], so plate stock longer than the wall saves splices.