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What is a board foot?         What is S4S?

A typical lumber yard or home center sells softwood and hardwood lumbers that are termed S4S (Surfaced Four Sides).  What is sold as a 1 x 6 is actually 3/4" x 5 1/2" in true measure.  These boards are sold by the lineal (running) foot. Included in this price is all of material that was milled away and the labor involved to mill the boards all to a specific size.  

At a lumber dealer who sells lumber by the board foot, on the other hand, the lumber is not all milled the same so you actually get more of the material that you are paying for.  A board foot is a unit of measure that includes the thickness, width, and length.  Below is a series of examples on how you can easily compute the board footage of any size board using a pocket calculator. 

When buying lumber by the board foot, the boards can be in any number of  milled stages.  Some may be completely rough - as it was when it came out of the sawmill.  Others have 1 straight edge milled on them and possibly have been surfaced on 2 sides(S2S) to a thickness generally above the typical finished thickness.  See the chart below for typical finished thickness.  Regardless of how much milling has been done, lumber sold by the board foot is more economical than S4S lumber.

How to determine board feet

You can calculate board feet either in inches or feet.  Use one of these two simple formulas to calculate the board feet in your boards.

T" x W" x L" / 144 = board feet    1" x 6" x 96" / 144 = 4 board feet

T" x W" x L' / 12 = board feet    1" x 6" x 8' / 12 = 4 board feet

When calculating board feet the board footage is always rounded up to the nearest quarter of a foot (i.e. 3.8 bf is 4 board feet), except exotic lumber which is measured by the inch and calculated using 2 decimal places.

What does 4/4, 5/4, 6/4, etc mean?

The thickness of hardwood lumber is measured in quarters of an inch, such as 4/4 (four-quarters or 1").  This is the thickness before surfacing and is what you would use to calculate the board feet.  This is also the thickness that you would pay for, even though the planed thickness may be only 3/4" of an inch thick. The chart below shows the typical thickness before and after planning for hardwood lumber.

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