This page explains the exact equation behind board foot estimates, plus the shortcuts most woodworkers and lumber buyers use when thickness and width stay in inches and length stays in feet.
Main Equation
If thickness, width, and length are all measured in inches, use the full equation below:
Board Feet = (Thickness x Width x Length x Pieces) / 144
The number 144 comes from the fact that 1 board foot equals 144 cubic inches.
Shortcut
Many lumber buyers leave thickness and width in inches but measure length in feet. In that case, use the shorter version:
Board Feet = (Thickness x Width x Length in feet x Pieces) / 12
This shortcut works because converting feet to inches already accounts for one factor of 12.
Examples
Multiply the single-board result by the number of pieces to get your order total.
Conversion
If you already know the coverage area and the board thickness, use this relationship:
Board Feet = Square Feet x Thickness in inches
This only works when thickness is known. If thickness changes, square footage alone is not enough for pricing.
Cost
Once you have total board feet, estimating cost is simple:
Total Cost = Total Board Feet x Price Per Board Foot
That is why the main calculator includes both a board footage result and a price result.