# Deck calculator

> Deck boards, joists and screws or hidden clips for a rectangular deck, from its size, joist spacing, board width, gap and waste allowance.

Interactive version: https://www.calcopenly.com/construction/deck-calculator
Subject: Construction and home calculators

Boards run along the house and joists run out from it. The joist count is the deck width divided by the joist spacing, rounded up, plus one for the far end. Rows of boards are the deck depth divided by board width plus gap, rounded up. Boards to buy are rows × deck width × (1 + waste) ÷ board length, rounded up, and fasteners are counted where each row crosses each joist.

The default 16 × 12 ft deck, with joists 16 in on centre and 5½ in boards gapped ⅛ in, needs 13 joists and 26 rows: 416 linear feet, or 29 sixteen-foot boards with 10% waste, held by 676 face screws.

Rim joists, beams, posts, ledger, blocking and stair stringers are not counted. Joist size and span depend on species, grade and load; take them from the span tables in 2021 or 2024 IRC section R507 or from AWC DCA 6.

## Inputs

- **Deck width (along the house)**: Boards run this way.
- **Deck depth (out from the house)**: Joists run this way.
- **Joist spacing (on centre)**: 2021/2024 IRC Table R507.7: 5/4 wood boards 16 in (12 in diagonal), 2x boards 24 in; composites per their ASTM D7032 label.
- **Board width (actual)**: A nominal 6 in deck board is 5½ in wide.
- **Gap between boards**: ⅛ in for wood; ³⁄₁₆ in is common for composite.
- **Board length you'll buy**
- **Waste allowance**: 10% for square decks, 15% or more for diagonal boards.
- **Fastening** (options: Face screws, Hidden clips)

## Results

- Deck boards to buy (boards) — main result
- Joists (joists)
- Joist length (ft)
- Joist length (m)
- Rows of boards
- Decking before waste (ft)
- Decking before waste (m)
- Screws or clips (pcs)
- Deck area (ft²)
- Deck area (m²)

## Formula

$$
\text{joists} = \lceil W/s \rceil + 1,\quad \text{rows} = \lceil D/(b + g) \rceil,\quad \text{boards} = \lceil \text{rows} \cdot W (1 + w)/L \rceil
$$

## Worked examples

### 16 × 12 ft deck, joists 16 in on centre, 5½ in boards, ⅛ in gap

- Deck width (along the house): 16 ft
- Deck depth (out from the house): 12 ft
- Joist spacing (on centre): 16 in
- Board width (actual): 5.5 in
- Gap between boards: 0.125 in
- Board length you'll buy: 16 ft
- Waste allowance: 10%
- Fastening: Face screws
- **Joists: 13 joists**
- **Rows of boards: 26**
- **Decking before waste: 416 ft**
- **Deck boards to buy: 29 boards**
- **Screws or clips: 676 pcs**
- **Deck area: 192 ft²**
- Checked against: Python decimal: 192/16 + 1 = 13 joists; 144 ÷ 5.625 = 25.6 → 26 rows; 26 × 16 = 416 ft × 1.1 ÷ 16 = 28.6 → 29; 26 × 13 × 2 screws

### Edge: 12 × 12 ft with 6 in board modules, no waste

- Deck width (along the house): 12 ft
- Deck depth (out from the house): 12 ft
- Joist spacing (on centre): 12 in
- Board width (actual): 5.5 in
- Gap between boards: 0.5 in
- Board length you'll buy: 12 ft
- Waste allowance: 0%
- Fastening: Face screws
- **Joists: 13 joists**
- **Rows of boards: 24**
- **Deck boards to buy: 24 boards**
- **Screws or clips: 624 pcs**
- Checked against: Hand calculation: 144/12 + 1 = 13 joists; 144 ÷ 6 = 24 rows exactly; 24 rows × one 12 ft board

### Metric 4.8 × 3.6 m, 400 mm centres, hidden clips

- Deck width (along the house): 4.8 m
- Deck depth (out from the house): 3.6 m
- Joist spacing (on centre): 400 mm
- Board width (actual): 140 mm
- Gap between boards: 5 mm
- Board length you'll buy: 4.8 m
- Waste allowance: 10%
- Fastening: Hidden clips
- **Joists: 13 joists**
- **Rows of boards: 25**
- **Deck boards to buy: 28 boards**
- **Screws or clips: 325 pcs**
- **Deck area: 17.28 m²**
- Checked against: Python decimal: 4800/400 + 1 = 13; 3600 ÷ 145 = 24.8 → 25 rows; 25 × 4.8 m × 1.1 ÷ 4.8 m = 27.5 → 28; 25 × 13 clips

## Questions

### How far apart should deck joists be?

16 in on centre suits 5/4 wood boards laid square to the joists. 2021 and 2024 IRC Table R507.7 allows 5/4 boards 16 in when each board crosses three or more joists (12 in over two), and 12 in when laid diagonally (8 in over two). 2x boards may span 24 in, except 18 in for diagonal boards over two joists. Composite boards use the span on their ASTM D7032 label.

### How many deck boards do I need?

Divide the deck depth by the board width plus gap to get rows, then multiply by the deck width and add waste. For a 16 × 12 ft deck with 5½ in boards and ⅛ in gaps: 144 ÷ 5.625 = 25.6, so 26 rows; 26 × 16 ft = 416 linear feet; with 10% waste, 457.6 ft, or 29 boards 16 ft long.

### How many screws do I need for a deck?

Two face screws wherever a board crosses a joist. A 16 × 12 ft deck at 16 in centres has 13 joists and 26 rows, so 26 × 13 × 2 = 676 screws, about 3.5 per square foot. Hidden clips take one per crossing, 338 here. Closer joist spacing adds crossings: at 12 in centres the same deck needs 884 screws.

### How many joists do I need for a deck?

Divide the width along the house by the joist spacing, round up and add one. A 16 ft (192 in) deck at 16 in centres needs 192 ÷ 16 + 1 = 13 joists; at 12 in centres it needs 17, and at 24 in, 9. Rim joists, doubled joists under board splices and the beam are extra.

### How accurate is the deck calculator?

Accuracy depends on your inputs and the method's assumptions. Decimal arithmetic uses 50 significant digits, but estimates, numerical methods and source data can be less precise; the displayed rounding does not remove those limits. It is checked against 3 worked examples whose answers come from independent sources; for example, “16 × 12 ft deck, joists 16 in on centre, 5½ in boards, ⅛ in gap” is checked against Python decimal: 192/16 + 1 = 13 joists; 144 ÷ 5.625 = 25.6 → 26 rows; 26 × 16 = 416 ft × 1.1 ÷ 16 = 28.6 → 29; 26 × 13 × 2 screws.

### Where does the method come from?

American Wood Council DCA 6 — Prescriptive Residential Wood Deck Construction Guide, based on the 2015 IRC (joist spacing 12, 16, 24 in); International Code Council — 2021 and 2024 IRC section R507 (exterior decks), Table R507.7 maximum joist spacing for decking.

## Sources

- [American Wood Council DCA 6 — Prescriptive Residential Wood Deck Construction Guide, based on the 2015 IRC (joist spacing 12, 16, 24 in)](https://web-media.awc.org/wp-content/uploads/2022/02/17210514/AWC-DCA62015-DeckGuide-1804.pdf)
- International Code Council — 2021 and 2024 IRC section R507 (exterior decks), Table R507.7 maximum joist spacing for decking