This calculator counts the decking boards for a rectangular deck. Enter the deck width across the boards and the length along them, the board face width, the gap between boards, the board length and a waste allowance; you get the boards to buy, the rows of boards and the linear feet.
Formula
Rows = deck width (in) ÷ (board width + gap), rounded up. Boards per row = deck length ÷ board length, rounded up. Boards to buy = rows × boards per row × (1 + waste ÷ 100), rounded up. Linear feet = rows × deck length.
- w: deck width measured across the boards, in feet
- l: deck length along the boards, in feet
- bw: actual face width of a board (a nominal 6 in board is usually 5.5 in)
- gap: spacing between boards in inches
- bl: length of the boards you buy, in feet
- waste: allowance for cuts and rejected boards
Worked Example
A 12 ft × 16 ft deck, 5.5 in boards with a 0.25 in gap, 16 ft boards, 10% extra.
- Rows = 144 ÷ 5.75 = 25.04, rounded up to 26 rows
- Boards per row = 16 ÷ 16 = 1
- Boards = 26 × 1 × 1.10 = 28.6, rounded up to 29 boards
- Linear feet = 26 × 16 = 416 ft
How to use the result
Choose a board length that matches the deck length or divides it evenly to reduce joints and waste. Where boards must be joined, butt joints should land on a joist; staggering them looks better and spreads the joints.
Limits and common mistakes
- Use the actual board width, not the nominal size.
- The gap depends on the material and its moisture; follow the decking maker’s instructions.
- This counts surface boards only, not joists, beams or fasteners, and it does not size the frame; structure needs local code guidance.
Frequently Asked Questions
How many deck boards for a 12 x 12 deck?
With 5.5 in boards and a 0.25 in gap, 26 rows of 12 ft boards, plus waste.
What is the actual width of a 5/4 x 6 deck board?
Usually about 5.5 in; check the board you buy.
How much waste should I allow?
About 5-10% for a simple rectangle, more for angles and picture framing.
Does it count joists?
No, only the deck surface boards.
Formula used
in w | Deck width (across the boards) | ft | 12 | 0 | 1000 in l | Deck length (along the boards) | ft | 16 | 0 | 1000 in bw | Board face width | in | 5.5 | 0.5 | 24 in gap | Gap between boards | in | 0.25 | 0 | 2 in bl | Board length | ft | 16 | 1 | 40 in waste | Extra for cuts and defects | % | 10 | 0 | 50 out buy | Boards to buy | boards | ceil(round(ceil(round(w * 12 / (bw + gap), 6)) * ceil(round(l / bl, 6)) * (1 + waste / 100), 6)) | 0 out rows | Rows of boards | rows | ceil(round(w * 12 / (bw + gap), 6)) | 0 out lf | Linear feet of decking | ft | rows * l | 0
Each line is one input (in, sel, list) or one result (out: label, unit, formula, decimals). How the calculators work
Method: the standard formula for this calculation, rounded as each result states. See how the calculators work or report an error.