Boards Perpendicular to the Joists

• 1). Measure the width of the deck across the joists in feet, using a tape measure, or note the length from a plan, if you haven't started building yet. Add 1 inch to 2 inches extra for overhang. Round up the result to the closest length of boards available, to calculate what length of boards to purchase.
  
  
• 2). Measure the length of the deck along the joists in feet. Convert the result to a decimal by dividing the excess number of inches by 12 and adding the result to the feet. For example, a deck that's 12 feet, 6 inches would be 12.5 feet.
  
  
• 3). Divide the result from Step 2 by .47 if you're using boards labeled 6 inches wide. Divide the result by .3 if you're using boards labeled 4 inches wide. The result will be the number of boards you need of the length you determined in Step 1. Add 10 to 15 percent extra to allow for waste.
  
  

Boards at a 45-Degree Angle to the Joists

• 1). Measure the width of the deck in feet. Convert the result to a decimal by dividing the excess number of inches by 12 and adding the result to the feet.
  
  
• 2). Measure the length of the deck in feet and convert the result to a decimal the same way.
  
  
• 3). Multiply the length by the width. Divide the result by .47 if you're using boards labeled 6 inches wide. Divide the result by .3 if you're using boards labeled 4 inches wide. The result is the number of linear feet of boards you'll need.
  
  
• 4). Draw out a plan for the deck on graph paper, or measure diagonally across the deck and choose the most economical length of boards to purchase in feet. For example, if distances across joists usually measure 6 feet to 8 feet, 8-foot boards would be the most economical.
  
  
• 5). Divide the result from Step 3 by the length of the boards you want to purchase to calculate how many boards you'll need. Purchase at least 10 to 15 percent extra to allow for waste.