Deck Board Calculator
Count the boards, the joints on the joists, and the screws or clips to buy.
Enter the deck length along the boards and the width across them.
- Length along boards
- —
- Width across boards
- —
- Board
- 5/4x6 wood
- Board width
- 5 1/2"
- Gap
- 1/8"
- Lengths
- 8, 10, 12, 14, 16, 18, 20
- Joist spacing
- 16 in
- End overhang
- 0
- Extra
- 10%
- Fastener
- Face screws
- Stagger joints
- On
- Price
- —
2 to 100 feet. This is the direction the boards run.
2 to 100 feet, measured across the rows.
Board
2 to 12 inches. Editing this switches the preset to Custom.
0 to 1/2 inch. Wood is often 1/8 inch. Trex is often 3/16 inch.
Joist spacing
0 to 2 inches past each end joist. Composite boards warn above 3/4 inch.
0 to 30 percent spare for defects and mis-cuts. Straight runs often use 10.
Fastener
Optional. Leave blank to skip the cost.
Price unit
Enter the deck length along the boards and the width across them.
Code reference
-
Piece span
At least two joist spaces
AWC DCA 6
Each board piece has to bear on three joists.
-
Last row
Under 2 inches warns
Rule of thumb
A rip this narrow is hard to fasten. A rule of thumb, not a code limit.
-
Gap
Over 1/2 inch warns
Trex
Trex says no deck-board gap should exceed 1/2 inch.
-
Composite overhang
Over 3/4 inch warns
Trex
Trex limits the end overhang of a composite board to 3/4 inch.
Code checks reference the 2021 International Residential Code (section numbers for the 2024 edition shown in parentheses). States and cities adopt different editions and often amend them, so limits in your area may differ.
Based on the IRC model code. Your local jurisdiction may adopt a different edition or amendments — confirm with your building department.
How to measure
Length along the boards is the direction the deck boards run, from the outside of one end joist to the outside of the other. Width across the boards is the other dimension, from the outer edge of the first row to the outer edge of the last row. Measure both on the finished frame. Record feet and inches. A board that hangs past an end joist is the end overhang, measured once for each end, and it is not part of the deck length. The gap is the open space between two board edges. A 5/4x6 or 2x6 wood board is 5 1/2 inches wide, and a very wet pressure-treated board can start with the gap closed. Trex 1x6 is also 5 1/2 inches wide, and the usual gap is 3/16 inch. A grooved TimberTech board is 5.36 inches wide, with a gap from 1/8 inch to 3/16 inch. Pick every stock length the yard will sell, from 8 feet through 20 feet in steps of 2 feet. Joist spacing is the on-center distance: 12, 16, or 24 inches. This page covers one rectangle. A deck made of two rectangles needs one calculation for each rectangle.
Formula
The row count is the deck width plus the gap, divided by the board width plus the gap, rounded up. The last row is the width that remains. A last row under 2 inches is a warning. Each row is the deck length plus the overhang at both ends. When that length fits on one stock board, the page buys the shortest board that covers it. When it does not, every joint lands on a joist centerline, and each piece must bear on three joists. The layout buys the least footage, then the fewest pieces. With stagger on, neighboring rows use two different layouts of that same cost when both exist. Screws are two at every board and joist crossing, plus two for each board end at a joint. Clips are one at each crossing, one starter on each joist, and two at each joint. The area figure is a check only.
A 16 by 12 foot deck with 5.5 inch boards and a 3/16 inch gap needs 26 rows, a last row of 1.8125 inches, 416 linear feet, 29 boards after 10 percent extra, 676 screws, and about 405.10 linear feet by area.
Common mistakes
- Measuring across the joists and calling that the board length.
- Selecting only a 16 foot board for a row that is longer than 16 feet.
- Opening a gap past 1/2 inch on a composite board.
- Turning stagger off and then lining every joint up on one joist.
- Buying the installed count and leaving out the extra percent.
FAQ
How do I measure the deck for this page?
Measure length along the boards, from the outside of one end joist to the outside of the other. Measure width across the boards, from the outer edge of the first row to the outer edge of the last. Record feet and inches. A board that hangs past an end joist is the end overhang, and it is a separate number. This page covers one rectangle. A deck made of two rectangles needs one run for each rectangle.
Why do the joints have to land on a joist?
A deck board end that misses a joist has nothing to fasten to. The page places every joint on a joist centerline. Each piece must also reach across two joist spaces, so it bears on three joists. That is the DCA 6 decking rule this page checks. If the lengths you selected cannot do that, the result fails the span check or says no layout is possible. Add a longer board, or a shorter one that still reaches three joists.
What does stagger joints change?
Stagger is on by default. The page first finds the layouts that buy the least footage and then the fewest pieces. When two of those layouts put the joint in different places, odd rows use one and even rows use the other. The 24 foot example uses 12 plus 12 on one row and 16 plus 8 on the next. If every cheapest layout shares the same joint, the page says the rows cannot stagger and uses that one layout.
Why does the last row show a warning?
The row count rounds up, so the last row is whatever width is left after the full boards and the gaps. A last row under 2 inches is hard to fasten and easy to split. The warning is a rule of thumb, not a code limit. Rip the first row narrower, or change the gap, until the last row is a piece you can screw down.
How are screws and hidden clips counted?
Face screws are two at every place a board crosses a joist, plus two more for each board end at a butt joint. Hidden clips are one at each crossing, plus one starter clip on each joist for the first row, plus two at each butt joint. The shopping list uses the fastener you select. A brand may sell a special clip for the butt joint. Check that installation guide before you order, especially for Trex or TimberTech grooved boards.
Does a passing check mean the deck meets my local code?
A passing row means each board piece spans at least two joist spaces, which is the model detail printed beside it. The last-row warning, the gap warning, and the composite overhang warning are separate from that pass. Joist span, beam size, and footing size are not checked here. Your city may adopt another code edition or change a number. A pass is not a permit. Confirm the adopted code and the board installation guide before you buy.
Related tools
- Size the outdoor stair off this deck Count stringers and tread boards for an outdoor stair.
- Space balusters on the open edge Space balusters on an open stair or deck edge.
- Price the framing lumber in board feet Price sawn boards, with waste and lumberyard tally.
- Add the boards to a deck shopping list Walk a deck from size to a combined shopping list.
By Lumber Calc Editorial Team · Verified against 2021/2024 IRC & AWC DCA 6 · Last updated Oct 24, 2026