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Deck board calculator

Boards across the span, joists along the length

A deck board calculator separates two directions. Boards run one way and are laid side by side the other way. Joists run under the boards, perpendicular to them, with a joist at each end. Enter the length of each board, the distance the boards have to cover when laid side by side, the actual board width, the gap, and the joist spacing. The count rounds up. Measure a real board; a nominal 6 inch deck board is often 5.5 inches wide.

Inputs

Result

Enter the deck size and the actual board width.

Disclaimer: This is an estimate for planning. Measure the real space, read the product label, and confirm anything structural, legal, or financial with a qualified person. Nothing you type is sent to a server. Joist size, span rating, footings, and ledgers are structural questions this count does not answer.

What the decking costs per square foot of deck, after waste, is on the SqftKit cost calculator. Concrete for footings stays on the concrete calculator.

How to use

  1. Decide which way the deck boards will run. Enter that length as the length of each board.
  2. Enter the other dimension as the distance those boards must cover when placed side by side.
  3. Measure one board’s actual width and decide the gap. An eighth of an inch is 0.125. Sixteen inch joist spacing is only the starting value.
  4. Press Calculate. Board count is how many pieces. Linear feet includes waste. Joists include both ends, and each joist is as long as the side-by-side dimension.

How it's calculated

Boards = ceiling( span in inches ÷ (board width + gap) )
Linear feet = boards × board length × (1 + waste ÷ 100)
Joists = ceiling(board length in inches ÷ spacing) + 1

The joist count puts a joist at the start and at the far end. If the board length is an exact multiple of the spacing, the spaces are equal. If it is not, the last space is shorter and you still have the end joist. That is the same idea as the fence posts.

Footings under those joists may be concrete. Volume for a round hole is on the concrete calculator. Pricing the decking per square foot, once you know the area, can be checked on the SqftKit cost calculator. This page does not look up a span table for how far a joist can run.

Worked example

Boards 16 feet long, covering a 12 foot distance side by side, 5.5 inches wide with a 0.125 inch gap. Each board claims 5.625 inches. The 12 foot span is 144 inches, so the board count is ceiling(144 ÷ 5.625) = 26. With 10 percent waste the linear feet are 26 × 16 × 1.10 = 457.6 feet. Joists at 16 inches on center along the 16 foot board length: 192 ÷ 16 = 12 spaces, so 13 joists, each 12 feet long.

Assumptions and sources

Board count is the side-by-side span divided by width plus gap, rounded up. Joist count is the board-length direction divided by the on-center spacing, with an extra joist for the far end. The 5.5 inch starting width is the actual width often sold for a nominal 6 inch board; measure yours. The 16 inch spacing is a common layout, not a span rating. No ledger, beam, or footing size is calculated. Waste applies to linear feet of boarding, not to the joist count.

FAQ

Why measure the board instead of using the store name?

Nominal sizes are larger than the finished board. A board called a 6 is often 5.5 inches. Using the name overstates coverage and you come up short.

Does 16 inch spacing mean the joists are strong enough?

No. Spacing is how far apart you asked to place them. Whether that joist can span the distance is a span-table and load question this page does not answer.

What is the joist length?

Each joist runs the side-by-side direction, so its length is the dimension you entered as the distance the boards cover.

Where do footings go?

This count does not place footings. If you will fill holes with concrete, take the hole size to the concrete calculator.

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