Decking Calculator - How Many Boards for Your Deck

Decking Calculator - before you order

A 20 x 12 ft deck with 5.5 in composite boards at 3/16 in gaps needs roughly 58 boards and 696 linear feet of material. Enter your own dimensions and the calculator handles row count, end joints, waste percentage and cost - so you order the right quantity on the first trip.

How to use this calculator - step by step

Deck length - measure the dimension in the direction boards will run. For a 20 x 12 ft deck with boards running lengthwise, enter 20.
Deck width - the perpendicular dimension. The calculator divides this by the board width + gap to count rows.
Board length - the individual board length you plan to buy. Common stock sizes: 8 ft, 12 ft, 16 ft, 20 ft. Choose a length that spans the deck in one piece when possible - fewer end joints mean a cleaner look.
Board width - actual face width in inches. Standard composite boards (Trex, TimberTech, Fiberon) are 5.5 in. Pressure-treated lumber: 5.5 in (5/4 x 6) or 3.5 in (2 x 4).
Board gap - the spacing between boards for drainage and expansion. Wood: 1/8 in. Composite: 3/16 in. Hidden clip systems set this automatically.
Price per linear foot - optional. Enter the per-foot price from your supplier to get a material cost estimate.
Waste allowance - extra boards to cover cuts and mistakes. 10% is standard for rectangular decks.
Read the results: total board count, linear footage, row layout and estimated cost.

Decking material comparison

Material	Cost ($/lf)	Lifespan (years)	Maintenance	Notes
Pressure-treated pine	$1-3	10-15	Stain every 1-2 years	Cheapest option, requires sealing
Cedar / Redwood	$3-6	15-25	Oil every 1-3 years	Natural rot resistance, weathers to gray
Composite (Trex, TimberTech)	$3-8	25-50	Wash annually	No staining or sealing, 25-year warranty typical
PVC (Azek, Wolf)	$6-12	30-50+	Virtually none	100% synthetic, highest durability
Ipe / Tropical hardwood	$5-10	40-75	Oil every 1-2 years	Extremely hard, requires pre-drilling

Practical examples

Example 1: Small patio 10 x 8 ft - 12 ft composite boards (5.5 in wide, 3/16 in gap), 10% waste
Rows: 17, boards per row: 1, net: 17, with waste: 19 boards, 228 lf

Example 2: Standard deck 16 x 12 ft - 16 ft pressure-treated (5.5 in, 1/8 in gap), 10% waste
Rows: 26, boards per row: 1, net: 26, with waste: 29 boards, 464 lf

Example 3: Large deck 24 x 14 ft - 12 ft composite (5.5 in, 3/16 in gap), 10% waste
Rows: 30, boards per row: 2, net: 60, with waste: 66 boards, 792 lf

Example 4: Narrow walkway 20 x 4 ft - 8 ft cedar (3.5 in, 1/8 in gap), 5% waste
Rows: 14, boards per row: 3, net: 42, with waste: 45 boards, 360 lf

Example 5: Pool deck 30 x 16 ft - 20 ft composite (5.5 in, 1/4 in gap), 15% waste
Rows: 34, boards per row: 2, net: 68, with waste: 79 boards, 1,580 lf

Board gap reference

Material	Recommended gap	Why
Pressure-treated lumber	1/8 in (3 mm)	Installed wet - shrinks as it dries
Kiln-dried wood (cedar, redwood)	3/16 in (5 mm)	Starts dry - expands in humidity
Composite (capped)	3/16 - 1/4 in (5-6 mm)	Thermal expansion - follow manufacturer specs
PVC	1/4 in (6 mm)	Highest thermal expansion coefficient
Ipe / hardwood	1/8 in (3 mm)	Dense wood - minimal movement

FAQ - Frequently asked questions

How many deck boards per square foot?

With standard 5.5 in boards and a 3/16 in gap, one board covers about 0.47 sq ft per linear foot. That works out to roughly 2.1 linear feet of board per square foot of deck. For a 200 sq ft deck, expect approximately 420 linear feet before waste.

What is the best board length to minimize waste?

Choose a board length that is equal to or slightly longer than your deck dimension in the run direction. A 12 ft board on a 12 ft deck means zero end joints and zero trimming waste. If the deck is 14 ft, use 16 ft boards - you lose 2 ft per board but avoid seams.

How much waste should I add for a diagonal layout?

Diagonal (45-degree) patterns create angled cuts at every edge, generating 15-20% more waste than straight layouts. Herringbone or chevron patterns can push waste to 20-25%. For picture-frame borders, add an extra 5% on top of the pattern waste.

Does the calculator account for joist spacing?

This calculator focuses on the deck surface (boards). Joist spacing (16 in on center is standard, 12 in for diagonal boards) determines substructure, not board count. A separate framing calculator handles joists, beams and posts.

Is composite decking cheaper than wood over 10 years?

Usually yes. A 200 sq ft pressure-treated deck costs roughly $600-1,200 in boards plus $200-400 in stain every 2 years. Over 10 years that is $1,600-3,200. Composite costs $1,200-3,200 upfront with near-zero maintenance. Break-even is typically 5-8 years.

Can I use this calculator for elevated decks?

Yes. The board quantity depends only on the deck surface area, board dimensions and gap. Whether the deck is at ground level or 10 ft up does not change the number of boards. The substructure (posts, beams, joists, stairs) is a separate calculation.

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