Deck Calculator
- Materials
- Labor
- Profit
Rough count for decking and joists only. Beams, posts, rim joists, stairs and railings are extra; size spans to your local building code.
Turn this into a quote
A one-page PDF for your customer, filled in from the numbers above. Your markup isn't shown.
How the deck calculation works
Find the deck area
Length × width. A 16 × 12 ft deck is 192 sq ft, with the boards running the 16 ft length.
Convert area to linear feet of decking
Divide the area in square inches by the board width plus gap. 192 sq ft × 12 ÷ (5.5 + 0.125 in) = 409.6 linear feet.
Add waste and divide by the board length
409.6 ft plus 10% is 450.6 ft, which is 28.2 boards of 16 ft, so you buy 29.
Count joists from the spacing
Joists run across the boards. 16 ft at 16 in on center is 12 spaces, so 13 joists including both ends.
Add labor and markup for the quote
Enter crew hours and rate, or a price per sq ft, plus your markup for overhead and profit.
Worked example. A 16 × 12 ft deck with 5.5 in boards and a 1/8 in gap needs 409.6 linear feet of decking, or 29 boards of 16 ft with 10% waste ($638 at $22). At 16 in spacing it needs 13 joists ($325), plus $250 of hardware, for $1,213 in materials. With 32 hours at $55 ($1,760) and a 20% markup ($594.60), the price to quote is $3,567.60.
Deck FAQ
How many deck boards do I need?
Divide the deck area in square inches by the board width plus the gap to get linear feet, add 10% waste, and divide by the board length. A 16 × 12 ft deck needs 29 boards of 16 ft with 5.5 in boards.
How many joists do I need for a deck?
Divide the deck length in inches by the joist spacing, round up and add one. A 16 ft deck at 16 in on center needs 13 joists.
What joist spacing should I use?
16 in on center is common. For composite boards, follow the maker's span chart: Trex lists 16 in on center for its 1 × 6 decking, 2 in less at a 60° angle and 4 in less at 45°, so a diagonal deck needs joists at 12 in. Trex also says each board must span at least three joists. Your local building code sets the limits for wood decking and the framing.
How many joists do I need at 12 in spacing?
A 16 ft deck at 12 in on center has 16 spaces, so it needs 17 joists, four more than at 16 in. At $25 a joist that adds $100 to the material cost.
How much should I charge to build a deck?
Add decking, framing, hardware and footings, then your labor and a markup for overhead and profit. Stairs, railings and permits are usually priced as separate line items.
How big a gap should I leave between deck boards?
Follow the board maker's guide. Trex requires at least 3/16 in between boards side to side (3/8 in under trees, never more than 1/2 in) and 1/8 in end to end above 40°F. Wet pressure-treated lumber shrinks as it dries, so ask your supplier how tight to lay it. Enter your gap in the calculator; a wider gap means slightly fewer boards.
How much does it cost to build a deck?
Add decking, joists, hardware and footings, then labor and markup. With the default prices, a 16 × 12 ft deck comes to $1,213 in materials and $3,567.60 to quote with 32 hours of labor at $55 and a 20% markup, about $18.58 per sq ft. Lumber prices and labor rates vary a lot by area, so enter your own.
Sources: Trex Decking Installation Guide: joist spanning (p. 22) and gapping (p. 24)