How to calculate wood for a deck

Updated February 21, 2017

If you're planning to build a deck, you'll need to figure out the amount of wood required to build it before you can estimate the cost or buy the materials. Although wood decks come in a variety of styles, shapes and sizes, you still use the same basic formula to figure out the amount of wood you'll need. The same formula or process applies to composite as well as to wood decks.

Measure the length and width of the area where you want to put your deck, using a measuring tape. Write down the figures on a piece of paper.

Figure out the size of board you will need for your rim joists and ledger. Each rim joist is the length of one side of your deck. Make the width of each of the doubled-up rim joists, or outside joists, equal in inches to the length in feet the joist is crossing. For instance, if your deck is 2.4 m (8 feet), the rim joists should be constructed out of 5 cm by 20 cm (2 inch by 8 inch) timber. Double the number of sides of your deck to find out how many boards you need to create the rim joists. Add an extra board to the sum for the ledger board to attach the deck to the building, if needed.

Figure out how much timber you need for the regular joists. Subtract 10 cm (4 inches) from your overall deck width measurement. This is the length of each of your regular joist boards. Divide the length the joists are crossing by 2. Add 3 to this sum to get the board width. For instance, if the span of the deck the boards are crossing is 2.4 m (8 feet), your final board width will be 15 cm (6 inches). This means your regular joists boards are 5 cm by 15 cm (2 inches by 6 inches).

Subtract 10 cm (4 inches) from the length measurement of your deck, or the side running perpendicular to your joists. Divide the number you get by 16. This is the number of regular joist boards you'll need.

Measure or find the width of the board you're planning to use for decking. Add 3 mm to 6 mm (1/8 to 1/4 inch) to the width of each board, depending on the amount of space recommended between boards, if needed. Divide the length of the deck parallel to the regular joists by the width figure. This is the number of decking boards you'll need.

Figure one post for each corner of your deck to hold it in place. Add an additional post every 2.4 m (8 feet) along each of your rim joists, if your deck is large. For example an 2.4 m 4.8 m (8 foot by 16 foot) deck would require six posts -- one post for each corner and an additional post in the middle of each 4.8 m (16 foot) side.


Always round up when figuring out material amounts. It's better to have too much of something than not enough.

Things You'll Need

  • Measuring tape
  • Paper
  • Pencil or pen
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About the Author

Sienna Condy began writing professionally in 2001 while attending the University of Cincinnati, and she's been at it ever since. Since graduating, she's written everything from marketing materials to articles on removing stains. Today, she enjoys writing about weddings, legal issues, science, health and parenting.