Hong Life CalculatorsUnderstanding the deck boards and screws calculator — concept and principle
This calculator estimates the number of deck boards, joists and screws for a rectangular deck. Enter the deck length and width, the board width, the gap between boards, the stock board length and the joist spacing. It gives a material list for budgeting and ordering timber or composite decking.
How it is calculated
Boards are assumed to run along the deck length, with joists running across the width beneath them.
- Rows of boards = ⌈width (mm) ÷ (board width + gap)⌉.
- Boards per row = ⌈deck length ÷ board length⌉.
- Boards = ⌈rows × boards per row × 1.05⌉ (5% waste).
- Joists = ⌈length (mm) ÷ joist spacing⌉ + 1.
- Screws = rows × joists × 2.
The principle
Each board plus its gap takes up a fixed slice of the deck width, so dividing the width by that slice gives the number of rows. If the deck is longer than one board, each row needs a second board, and the joint between them should land on a joist. Joists are spaced like fence posts: n gaps need n + 1 supports. Every board is screwed to every joist it crosses, normally with two screws, one near each edge, which is why screws scale with rows × joists.
Worked example
Default inputs: deck 4 m × 3 m, boards 140 mm wide, 5 mm gap, 3.6 m long, joists at 400 mm.
- Rows = 3,000 ÷ 145 = 20.69, rounded up to 21.
- Boards per row = 4 ÷ 3.6 = 1.11, rounded up to 2.
- Boards = 21 × 2 × 1.05 = 44.1, rounded up to 45 boards.
- Joists = 4,000 ÷ 400 + 1 = 11 joists.
- Screws = 21 × 11 × 2 = 462 screws.
| Board width (mm) | Boards |
|---|---|
| 35 | 158 |
| 70 | 84 |
| 105 | 59 |
| 140 | 45 |
| 175 | 36 |
| 210 | 30 |
| 280 | 24 |
How to read the result
The board count is conservative when the deck is only slightly longer than a board: here each row needs 4 m, so every second board leaves a 3.2 m offcut. Choosing longer boards (for example 4.2 m) or staggering joints and reusing offcuts can cut the count substantially. Joist spacing of about 400 mm is common for timber decking; composite boards and diagonal layouts often require closer spacing, so check the product instructions. Add extra joists where board ends butt together, plus beams, posts, rim joists and hidden fixings if used.
Common mistakes
- Forgetting the gap, which changes the row count.
- Running boards the wrong way relative to the joists.
- Ignoring the extra screws at butt joints and on the outer frame.
- Using spacing from timber decking for composite boards.
Frequently asked questions
What gap should I leave between deck boards?
Typically 3–6 mm for timber, depending on moisture content; composite manufacturers specify their own gaps.
How many screws per square metre of decking?
With 140 mm boards on 400 mm joists, roughly 35–40 per m², about 462 for this 12 m² deck.
Can I use one screw per joist?
Two per crossing is standard for boards wider than about 100 mm, to stop cupping and twisting.
Sources: American Wood Council, DCA 6 — Prescriptive Residential Wood Deck Construction Guide · International Residential Code (IRC), Section R507 — Exterior Decks