Decking Calculator
Enter your deck dimensions, board size, joist spacing, and laying pattern to get an instant count of boards, screws, and hidden fasteners - plus an estimated material cost. Switch between imperial and metric units, choose from six common decking materials, and see a full cost breakdown that updates as you type.
How many deck boards do you need?
The number of boards depends on four factors: your deck area, the face width of each board, the gap you leave between boards, and how much extra you add for waste. The board pitch (face width plus gap) is divided into the deck width to find how many runs of board you need. Each run is then divided by the board length to find how many boards go end to end. Multiply runs by boards per run to get a net count, then apply a waste factor: 10% for a straight perpendicular layout, 15% for diagonal, and 20% for herringbone. Always round up to whole boards. Most residential decks use 5/4 x 6 boards (5.5-inch actual face width) on 16-inch joist centres, which is a good starting point for any estimate.
Choosing the right board size, length, and gap
Board size is listed nominally (2x6, 5/4x6), but the actual face is narrower after milling - a 6-inch nominal board measures about 5.5 inches. Wider boards mean fewer runs and fewer joints to seal, but they are heavier and can cup more if they dry out. Board length affects the number of end joints: 16-ft boards give a cleaner look than two 8-ft boards on a 14-ft deck, but only if your supplier stocks them and you can transport them. Gap size balances drainage and aesthetics: 1/4 inch (0.25 in) is standard for pressure-treated boards because they shrink as they dry; composite and pre-dried boards often use 1/8 inch. A small gap has a big impact on board count because every fraction of an inch changes how many runs fit across the deck.
Screws and hidden fasteners
Deck screws are installed from the top at an angle into the joist. As a rough rule, plan for about 220 screws per 100 square feet of deck at standard 16-inch joist spacing, or 275 at 12-inch. Two screws per board per joist is common for 5/4 material; use three for 2-inch stock. Hidden fasteners clip onto the side of each board and slide under the next, leaving a screw-free surface. They cost more per clip but eliminate surface holes that can collect water. Many builders use hidden fasteners in the main field and screws at the perimeter where clips would be visible. This calculator estimates both counts so you can mix and match.
Estimating total project cost
The calculator shows board-only material cost because that is the number most shoppers compare between suppliers. Add roughly 30-50% for the framing package (joists, beams, posts, ledger board, concrete), 10-15% for fasteners and hardware, and 100-150% of material cost for labor if hiring a contractor. A fully installed pressure-treated deck typically runs $15-25 per square foot; composite is $25-40; exotic hardwoods can exceed $60. Getting quotes from two or three local contractors and asking for itemised breakdowns will give you a much tighter number than any online estimate.
Common decking material comparison
| Material | Cost ($/lf) | Lifespan | Maintenance | Sustainability |
|---|---|---|---|---|
| Pressure-treated pine | $0.50-0.90 | 15-25 yrs | Annual seal/stain | Moderate |
| Western red cedar | $1.20-1.80 | 20-30 yrs | Every 2-3 years | Good |
| Redwood | $1.80-2.50 | 25-35 yrs | Every 3-5 years | Good |
| Composite | $2.20-3.20 | 25-30 yrs | Annual cleaning | Excellent |
| PVC decking | $2.50-3.20 | 30+ yrs | Annual cleaning | Moderate |
| Ipe hardwood | $3.50-6.00 | 40-75 yrs | Annual oiling | Varies |
2025 approximate retail pricing; installed costs are typically 2-3x the material price.
Frequently asked questions
How many deck boards do I need for a 12x16 deck?
A 12 x 16 ft deck has 192 sq ft. Using 5/4 x 6 boards (5.5-inch face) with a 1/4-inch gap gives a pitch of 5.75 inches. Across 12 ft you need about 25 runs; along 16 ft you need one 16-ft board per run. That is 25 boards net, plus 10% waste = about 28 boards for a straight layout. Diagonal adds 15%, giving about 29 boards. These counts assume you can get 16-ft boards so every run is one continuous piece.
What is the best gap between deck boards?
For pressure-treated lumber, 1/4 inch (about 6 mm) is standard. Green or wet lumber will shrink as it dries, so you may gap it at 3/8 to 1/2 inch if installing wet. Pre-dried composite boards use 1/8 inch to look tight while still draining. A nail or a 5/16-inch bolt shank makes a quick spacer. A larger gap drains better but can snag chair legs and drop small objects, while no gap at all traps moisture and accelerates rot.
Does joist spacing affect how many boards I need?
Joist spacing does not change the board count - it changes the number of fasteners and the minimum board thickness you need. At 16-inch joist centres, 5/4 decking is strong enough for most foot traffic. At 24-inch centres you should use 2-inch stock or composite rated for that span. At 12-inch centres you can use thinner boards or carry heavier loads like a hot tub. The screw count does change with joist spacing, since each board must be fastened at every joist.
How much extra should I buy for waste?
Add 10% for a straight perpendicular pattern, 15% for a diagonal pattern, and 20% for herringbone or complex patterns. If your board length does not divide evenly into the deck length - for example, 16-ft boards on a 14-ft deck leave 2 ft of offcut each run - add another 5-10% to account for those specific offcuts. Also consider buying a few extra boards to keep as spares for future repairs.
What is the difference between nominal and actual board size?
Lumber is sold by nominal size (what the mill starts with) but arrives already planed to a smaller actual dimension. A nominal 2x6 measures 1.5 inches thick by 5.5 inches wide; a 5/4 x 6 (popular for decking) measures 1 inch thick by 5.5 inches wide. This matters for your gap calculation: face width is the actual 5.5 inches, not the nominal 6 inches. Using nominal dimensions in your calculation will make each board appear wider than it is, causing you to underestimate your board count.
How much does it cost to build a deck?
For materials alone (boards, framing, fasteners, and concrete), a pressure-treated deck runs about $8-15 per square foot; cedar and redwood are $15-25; composite is $20-38; exotic hardwoods start around $30. Fully installed by a contractor, add another $15-30 per square foot for labor, depending on complexity and local rates. A simple 200 sq ft pressure-treated deck might cost $3,000-6,000 in materials and $6,000-12,000 fully installed. Permits add $500-1,500 in most jurisdictions.