Deck Calculator
Count the deck boards, joists and screws your deck needs.
What you'll need to order
- deck boards
- pressure-treated joists
- joist hangers
- deck screws
- joist tape
Prices vary by region and haul distance. Get quotes from at least two local suppliers.
How this calculator works
board rows = deck width × 12 ÷ (board width + 0.125)
The 1/8 inch gap lets water drain and allows for movement. Pressure-treated boards installed wet will shrink, so butt them tight; kiln-dried and composite boards need the gap from day one.
Worked example
With deck length 16 ft, deck width 12 ft, deck board width 5.5 in, joist spacing 16 in, this calculator returns 28.16 .
Waste allowance
The headline figure includes a 10% allowance for waste and over-ordering. The exact requirement is shown alongside it.
Deck boards by board width
Figures below assume deck length 16 ft × deck width 12 ft × joist spacing 16 in. Change any measurement in the calculator to get your own numbers.
| Deck board width | Deck boards needed | Joists | Deck screws |
|---|---|---|---|
| 3.5 in | 43.70 | 13.00 | 1,033 |
| 5.5 in | 28.16 | 13.00 | 665.6 |
| 7.25 in | 21.48 | 13.00 | 507.7 |
A 16 by 12 ft deck. Narrower boards mean more pieces, more fasteners and more labour, but a more traditional look.
Materials for common deck sizes
| Job | Size | Deck boards needed |
|---|---|---|
| Small balcony, 8x8 | 8 ft × 8 ft × 5.5 in × 16 in | 18.77 |
| Standard deck, 12x16 | 16 ft × 12 ft × 5.5 in × 16 in | 28.16 |
| Large deck, 16x20 | 20 ft × 16 ft × 5.5 in × 16 in | 37.55 |
| Composite at 12 in centres | 16 ft × 12 ft × 5.5 in × 12 in | 28.16 |
| Wraparound, 24x10 | 24 ft × 10 ft × 5.5 in × 16 in | 23.47 |
Most composite decking requires 12 inch joist spacing when laid diagonally, and some brands require it everywhere. Check before framing.
What does a deck cost?
Material typically runs $25 to $70 per square foot installed. Installed, including framing, decking and railings. Pressure-treated pine sits at the bottom, capped composite and hardwood at the top.
What moves the price:
- Decking material, which spans a three-fold range
- Height above grade — anything needing stairs and guards costs considerably more
- Footings, which depend on frost depth and soil
- Railings, often 15 to 25% of the total on their own
- Permit and inspection, required for most attached decks
Prices are US averages and move with fuel, season and haul distance. Always get two local quotes before budgeting.
Decking materials compared
| Type | Typical life (years) | Notes |
|---|---|---|
| Pressure-treated pine | 15.00 | Cheapest; needs sealing every 2 to 3 years |
| Cedar | 20.00 | Naturally rot-resistant, weathers grey |
| Uncapped composite | 25.00 | Low maintenance, can stain |
| Capped composite | 30.00 | Stain and fade resistant; the mainstream premium |
| PVC | 35.00 | Lightest, no wood content at all |
| Ipe hardwood | 50.00 | Extremely durable; needs pre-drilling throughout |
Life expectancy depends far more on the framing beneath than the boards on top. Most decks are condemned for rotted joists and failed ledger flashing, not worn decking.
Frequently asked questions
How many deck boards for a 12x16 deck?
About 42 boards of 5.5 inch width running the 16 ft direction, allowing for a 1/4 inch gap. Add 10% if you plan to lay them diagonally.
How much does a 12x16 deck cost?
$4,800 to $13,400 installed, depending on material and height. Pressure-treated at ground level is the cheap end; capped composite with railings and stairs is the other.
How many joists do I need?
Deck width in inches divided by the spacing, plus one. A 12 ft deck at 16 inch centres needs 10 joists, or 13 at 12 inch centres for diagonal decking.
How many deck boards do I need?
Divide the deck width in inches by the board width plus a 1/8 inch gap, then add 10% waste. A 16 by 12 deck with 5/4x6 boards needs about 29 rows.
What gap should I leave between deck boards?
An eighth of an inch for kiln-dried and composite boards. Wet pressure-treated lumber should be butted tight — it will shrink to roughly that gap as it dries.
What joist spacing does decking need?
Sixteen inches on centre for most boards laid perpendicular. Drop to 12 inches for a diagonal layout or for composite decking, which sags more than wood between supports.
How many screws per deck board?
Two at every joist crossing. A 16 ft board over joists at 16 inch centres takes 26 screws. Hidden fastener systems use clips instead and change the count entirely.