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Deck Joist Calculator

Enter deck size and spacing to estimate joists and beams.

How it is calculated

Joists run across the width (the span) and are spaced along the length. The tool converts the on-center spacing from inches to feet, then counts: joists = floor(length ÷ (spacing ÷ 12)) + 1. The extra one is the joist at the starting end. Total joist lumber is joists × width, and the side beam estimate is 2 × length. Length and width are in feet, spacing is in inches. Results are shown as whole numbers, rounded to the nearest foot. The count only covers the joists themselves: the last joist may sit short of the far end, and ledger boards, end rim joists, blocking, hangers and waste are not included.

Worked example

Take a deck 16 ft long and 12 ft wide with joists 16 in on center. Spacing in feet: 16 ÷ 12 = 1.33 ft. Spaces that fit along the length: 16 ÷ 1.33 = 12. Add the starting joist: 12 + 1 = 13 joists. Each joist is 12 ft long, so 13 × 12 = 156 ft of joist lumber. The two side beams add 2 × 16 = 32 ft. Switching the spacing to 24 in drops the count to 9 joists and 108 ft.

Try this example →

Common mistakes

  • Entering the spacing in feet or centimetres. The field expects inches, so 16 means 16 in, not 1.3 ft.
  • Swapping length and width. Joists are counted along the length and each one is as long as the width, so reversing them changes both the joist count and the lumber total.
  • Treating the lumber total as a purchase list. It is a plain sum of joist length, so stock lengths, cut waste, doubled joists and the ledger board still have to be added.
  • Measuring spacing edge to edge instead of center to center. On-center means from the middle of one joist to the middle of the next.
  • Assuming one spacing suits every decking. The right spacing depends on the decking material and the manufacturer's span rating, which the tool does not check.

About this calculator

This calculator estimates how many joists a deck frame needs by dividing the deck's length by the chosen on-center spacing (commonly 12in, 16in or 24in) and adding one for the starting joist, then figures the required lumber length for each joist plus the side beams (rim joists) that run along the deck's perimeter to tie the frame together.

DIY deck builders and contractors use this before ordering lumber, since spacing directly affects how much the decking material can span without sagging underfoot — closer 12in spacing is typical under composite decking, while solid wood decking can often use wider 16in or 24in spacing, and getting the joist count and beam lengths right up front avoids under- or over-ordering material.

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Comments (1)

  • Editorial team

    The count here does not depend on lumber size, so it will not tell you whether a 2x8 or a 2x10 can carry your width at the spacing you chose. That comes from a span table or your local code, and the answer changes if a beam supports the joists partway across. Settle the lumber size first, then use the count to price the order. Which joist size and spacing are you planning?

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