Deck Joist Calculator

How many joists do I need for a deck?

A deck joist calculator divides the deck length in inches by the joist spacing, rounds any fraction up, and adds one for the starting joist. Multiply the count by the joist span to get linear feet of joist lumber, then add two rim boards the length of the deck. A 16 × 12 ft deck framed at 16 inches on center takes 13 joists, 156 linear feet of joist stock, and 32 linear feet of rim.

Enter the deck length (the direction joists are spaced along), the width (the span each joist covers), and the spacing in inches.

The formula in plain words

• Joists = (deck length × 12 ÷ spacing), rounded up, + 1
• Joist linear feet = joists × deck width
• Rim linear feet = 2 × deck length

The +1 is the joist at the start. When the length divides evenly by the spacing, the last joist lands exactly on the far end. When it does not, rounding up gives the far end its own joist, and the last bay is narrower than the rest.

Worked examples

Both match the calculator.

1. Attached deck, 16 × 12 ft, joists at 16 in on center

• Length in inches: 16 × 12 = 192
• Joists: 192 ÷ 16 = 12, plus 1 = 13 joists
• Joist stock: 13 × 12 = 156 linear ft (thirteen 12 ft boards)
• Rim: 2 × 16 = 32 linear ft

2. Composite deck laid diagonally, 20 × 14 ft, joists at 12 in on center

• Length in inches: 20 × 12 = 240
• Joists: 240 ÷ 12 = 20, plus 1 = 21 joists
• Joist stock: 21 × 14 = 294 linear ft (twenty-one 14 ft boards)
• Rim: 2 × 20 = 40 linear ft

Tightening the spacing from 16 to 12 inches on the same 20 ft deck raises the count from 16 joists to 21. That is five more boards plus hangers, so diagonal decking costs more to frame.

Joist count by deck length and spacing

Deck length	12 in OC	16 in OC	24 in OC
10 ft	11	9*	6
12 ft	13	10	7
16 ft	17	13	9
20 ft	21	16	11
24 ft	25	19	13

*10 ft at 16 inches does not divide evenly (120 ÷ 16 = 7.5). The count rounds up to 8 gaps, so 9 joists, and the last bay is 8 inches wide.

What sets the spacing

Decking	Typical maximum spacing
5/4 wood decking, square to joists	16 in
2× wood decking	16–24 in
Composite or PVC, square to joists	12–16 in, per manufacturer
Any board laid at 45°	Often 12 in
Hot tub area	Engineered; usually much tighter

Composite manufacturers publish their own limits. Follow the instructions for your exact board, because spacing can affect the warranty.

How to measure

1. Length runs parallel to the ledger or beam, the line the joists are spaced along.
2. Width is the joist span from ledger to beam. Add any overhang past the beam to the board length you buy.
3. Buy in standard lengths. Joist lumber comes in 2 ft steps. A 13 ft span means 14 ft boards.
4. Add an extra joist wherever two deck boards butt end to end, under stair stringers, and around openings.

Hardware that follows the count

Each joist that hangs from a ledger needs one joist hanger, fastened with the nails or screws its maker specifies. Where a joist sits on a beam, it usually gets a tie or bracket as well. The 13 joists in example 1 need 13 hangers and about 13 ties.

Common mistakes

• Adding an extra end joist by hand when the length does not divide evenly. The count already includes it.
• Swapping length and width. Spacing along the 12 ft side instead of the 16 ft side gives a different count and the wrong board lengths.
• Ignoring blocking. Mid-span blocking on long joists stops twisting. It adds a short piece per bay.
• Using decking spacing from the wrong product. A board rated for 16 inches square-on may need 12 inches diagonally.

Related calculators

The joist spacing calculator counts joists for floors and other runs, and the decking board calculator sizes the surface. Check the frame against loads with the deck load calculator. Deck framing is structural, so confirm joist size, span, beam, and footing layout with local code and a licensed professional before you build.