Deck boards run across the joists, with a small gap between them for drainage and seasonal movement – about ⅛–¼ in for wood and whatever the manufacturer specifies for composite. Buying the right board length reduces both waste and the number of butt joints, which should always land on a joist. This calculator assumes boards run along the length of the deck and joists span across its width.
How to use the Decking Calculator
- Deck length (along boards)
- Deck width
- Board width (actual)
- Gap between boards
- Board length
- Joist spacing
- Waste
- Price per deck board
- Price per joist
- Price per box of 350 screws
Results update as you type, and every field accepts the units shown in its menu.
How it works
Enter the deck length (the direction the boards run) and width, the board width and gap, the length of boards you will buy, the joist spacing and waste. The calculator counts rows of boards, the boards per row, joists and face screws (two per board at every joist), and prices each item.
Tips for an accurate estimate
- Sketch the job and split irregular areas into rectangles, circles and triangles before you measure.
- Add a waste allowance – typically 5–10% for simple layouts and 15–20% for diagonal, curved or heavily cut work.
- Check the coverage or yield printed on the product, because sizes and densities vary between manufacturers.
- Buy all units of a finished material (tile, brick, siding) from the same batch so colours match, and keep a few spares for repairs.
- Confirm local prices – quarry, lumber and steel prices vary widely by region and season.
Common mistakes to avoid
- Ordering exactly the calculated amount with no allowance for breakage, offcuts and settlement.
- Forgetting to subtract doors, windows and other openings – or subtracting small ones that still need cutting around.
- Confusing US short tons (2,000 lb) with metric tonnes (1,000 kg) when comparing quotes.
Inputs explained
| Field | What to enter |
|---|---|
| Deck length (along boards) | Units: ft, m, in, yd, cm |
| Deck width | Units: ft, m, in, yd, cm |
| Board width (actual) | Choose one: 5½ in (5/4×6 or 2×6), 3½ in (2×4), 5⅜ in composite, 7¼ in (2×8) … |
| Gap between boards | Units: in, cm, mm, ft, m, mm |
| Board length | Choose one: 8 ft, 10 ft, 12 ft, 16 ft … |
| Joist spacing | Choose one: 12 in o.c., 16 in o.c., 24 in o.c., 400 mm … |
| Waste | In % |
| Price per deck board | Units: /piece |
| Price per joist | Units: /piece |
| Price per box of 350 screws | Units: /piece |
What each result means
- Deck area = [Deck length (along boards)]×[Deck width]
- Rows of boards = ceil([Deck width]÷([Board width (actual)] + [Gap between boards]))
- Deck boards = ceil([Rows of boards]×ceil([Deck length (along boards)]÷[Board length] – 0.000001)×(1 + [Waste]÷100))
- Linear length of decking = [Rows of boards]×[Deck length (along boards)]
- Joists = ceil([Deck length (along boards)]÷[Joist spacing] – 0.000001) + 1
- Face screws = [Rows of boards]×[Joists]×2
- Boxes of screws = ceil([Face screws]÷350)
- Material cost = [Deck boards]×[Price per deck board] + [Joists]×[Price per joist] + [Boxes of screws]×[Price per box of 350 screws]
Example values
How the main result changes when Deck length changes and everything else stays at the example values above:
| Deck length (along boards) (ft) | Deck boards |
|---|---|
| 8 | 29 |
| 12 | 29 |
| 16 | 29 |
| 20 | 58 |
| 24 | 58 |
| 32 | 58 |
Formula
Rows of boards = ceil(deck width ÷ (board width + gap))Boards = rows × ceil(deck length ÷ board length) × (1 + waste)Joists = ceil(length ÷ spacing) + 1; screws = rows × joists × 2Worked example
Inputs — Deck length (along boards): 16 ft; Deck width: 12 ft; Board width (actual): 5½ in (5/4×6 or 2×6); Gap between boards: 0.1875 in; Board length: 16 ft; Joist spacing: 16 in o.c.; Waste: 10 %; Price per deck board: $28 /piece; Price per joist: $22 /piece; Price per box of 350 screws: $45 /piece.
Results — Deck area = 192 ft²; Rows of boards = 26; Deck boards = 29; Linear length of decking = 416 ft; Joists = 13; Face screws = 676; Boxes of screws = 2; Material cost = $1,188.00.
Frequently asked questions
How many deck boards do I need for a 12 × 16 deck?
With 5½ in boards and ⅛–3/16 in gaps, about 25 rows of 16 ft boards, plus 10% waste – roughly 28 boards.
How far apart should deck joists be?
16 in on centre for most wood and composite decking; 12 in for diagonal boards or some composites.
How many screws per deck board?
Two at every joist; for 5½ in boards at 16 in spacing that is about 350 screws per 100 ft².
What gap should I leave between deck boards?
About ⅛ in for kiln-dried wood, close-butted for wet pressure-treated lumber (it shrinks), and the manufacturer's spacing for composite.
What formula does the Decking Calculator use?
Rows of boards = ceil(deck width ÷ (board width + gap)).