Inside vs outside: why a 4 x 8 raised bed holds less soil than you think

Plans, kits and store tags name a bed by the outside of its frame. The soil only fills what is left between the boards. Under the federal lumber standard, PS 20, every 2x board is 1 1/2 inches thick once dried. There is a board at both ends of the length and both sides of the width, so each direction loses 3 inches.

Source: NIST, PS 20-20 American Softwood Lumber Standard, checked October 10, 2026.

• 1 1/2 inreal thickness of any 2x board (PS 20)NIST, PS 20-20 American Softwood Lumber Standard, October 10, 2026
• 93 x 45 ininside a 4 x 8 ft frameArithmetic
• 9%less soil area than 32 sq ftArithmetic
• 99 inlong sides for a bed that is 4 x 8 ft insideArithmetic

Small beds lose a bigger share. The boards always take 3 inches each way, whatever the bed size, so the smaller the bed, the more those inches count.

Frames of 2x lumber, 1 1/2 inches thick (PS 20). Inside area = inside length x inside width ÷ 144.

Source: NIST, PS 20-20 American Softwood Lumber Standard, checked October 10, 2026.

It is easy to make two errors at once: using the outside size, and calling a bed of two 2x6 boards a foot tall. A 2x6 is 5 1/2 inches wide, so two of them stack to 11 inches. Fix both and the soil drops from 32 cubic feet to 26.6, the difference between 16 bags of 2 cu ft and 14.

Source: NIST, PS 20-20 American Softwood Lumber Standard, checked October 10, 2026.

1. 1

   Hook the tape on an inside face

   Run the tape from the inside of one board to the inside of the opposite board. Do the length, then the width.

2. 2

   Measure at both ends

   Frames bow and corners drift. Measure each direction near both ends and in the middle, and use the smallest reading.

3. 3

   Measure the depth you'll fill

   Stand a ruler on the ground or old soil inside the bed and read it at the top edge of the boards. Take off whatever gap you plan to leave below the rim.

4. 4

   Multiply and divide

   Inches long x inches wide x inches deep ÷ 1,728 gives cubic feet. 93 x 45 x 11 ÷ 1,728 = 26.6.

If you want 4 by 8 ft of soil, the frame has to grow by 3 inches each way to 51 by 99 inches. The long sides are then 99 inches, longer than an 8 ft board, so you'd buy 10 ft boards and cut them. If 8 ft boards suit you better, plan the bed by its outside and accept 93 by 45 inches inside.

Source: NIST, PS 20-20 American Softwood Lumber Standard, checked October 10, 2026.

UGA Extension says lumber comes in 8, 10 and 12 ft lengths and suggests keeping beds to multiples of 2 or 3 feet to cut waste. An outside size does that best, because the long sides stay whole.

Source: UGA Extension C 1027-4, raised garden bed dimensions, checked October 10, 2026.