Surface measure
Surface measure (SM) is the surface area of a board in square feet. To determine surface measure, multiply the width of the board in Inches by the length of the board in feet and divide the sum by 12 rounding up or down to the nearest whole number. The percentage of clear wood required for each grade is based on the surface measure, not the board feet, and because of this all boards, No matter what the thickness, are graded in the same way.
Some examples for surface measure calculations are as follows:
6 1/2" x 8' ÷ 12 = 4 1/3" = 4' SM
8" x 12' ÷ 12 = 8' SM
10" x 13' ÷ 12 = 10 10/12 = 11' SM
Example of SM and BF:
Surface MeasureThe board above is a 2" thick, 6 1/4" wide, and 8' long. 6 1/4" x 8' ÷ 12 = 4 1/4 , thus the SM is 4'. Multiply the SM by the thickness 2" and the BF is 8'.
When preparing a bundle tally for export, the boards are recorded by their width and length. Random widths above or below the half inch are rounded to the nearest whole inch. Board widths falling exactly on the half inch are alternatively rounded up or down. Lengths that fall between whole foot increments are always rounded down to the nearest whole foot. For example a board 5 1/4" width and 8 1/2' long is tallied 5" and 8'.
板面量度(SM)是指板材的表面積，以平方英尺為單位。它是通過將板材寬(英寸)和板材長(英尺)的乘積除以 12，四捨五入後取整數得出的。各個等級中規定的淨劃面的百分比就是根據表面量度而非板尺數來決定的，並且正因如此，不同厚度的板材的分等方式是相同的。
以下是幾個計算板面量度的例子：
61/2" × 8' ÷ 12 = 41/3 = 4' SM
8" × 12' ÷ 12 = 8' SM
10" × 13' ÷ 12 = 1010/12 = 11' SM
板面量度板面量度和板尺的例子：
上圖所示的板材厚 2英寸，寬 6英寸，長 8英尺。
61/4" × 8' ÷ 12 = 41/4因此板面量度是 4'，將其乘以厚度 2英寸得 8個板尺。
一捆出口板材的材積是按照板材的長度和寬度來計算的。寬度不足或超過0.5英寸時都四捨五入取整。當板材寬度正好等於0.5英寸時進位取整或退位取整均可，而當長度不是整數英尺時，一般退位取整。例如：一塊寬5英寸、長8英尺的板材分別計作 5英寸和 8英尺。
