How to use this calculator
- 1Enter the deck length (across the joists, the way straight boards run) and width.
- 2Choose straight or diagonal boards — the joist spacing and the extra for cuts change to suit.
- 3Enter your board width, the gap, and the length boards are sold in.
- 4Check the joist spacing against the board maker's instructions.
The formula
- 01Rows = deck width ÷ (board width + gap), rounded up
- 02Boards = rows × deck length × (1 + extra %) ÷ board length, rounded up
- 03Diagonal boards: board length = deck area ÷ (board width + gap)
- 04Joists = deck length ÷ spacing, rounded up, + 1
- 05Ledger + rim joist = 2 × deck length · Joist hangers = joists × 1 (ledger), × 2 (both ends hang) or × 0 (on beams)
- 06Screws ≈ rows × joists × 2 (diagonal: 2 at every place a board crosses a joist)
Each row of boards runs the full length of the deck, joining over joists where boards are shorter than the deck. Working in total length means offcuts from one row can start another.
The number of joist spaces is rounded up, so no gap between joists is wider than the spacing you enter — rounding down would leave the last stretch of boards unsupported. The joists are closed off at both ends — by a ledger on the house side and a rim (band) joist on the other, or by two rim joists on a freestanding deck; together that is twice the deck length. Where a joist end hangs from a ledger or a flush beam it sits in a joist hanger; where it rests on a beam it doesn't need one.
Boards laid at 45° cover the same area per metre of board, so their total length is the deck area divided by the board width plus gap. A diagonal board crosses a joist every spacing × √2 along its length. Two screws where every board crosses a joist is the usual fixing pattern; hidden-fastener systems use their own clips instead.
Worked example
A 4 m × 3 m deck with 140 mm boards and 5 mm gaps has 21 rows.
With 3.6 m boards and 10% extra, buy 26 boards. At 400 mm centres you need 11 joists, 11 joist hangers and about 462 screws.
The ledger and the rim joist at the two ends of the joists add up to 8 m; there is one hanger for each joist at the ledger.
Tips before you buy
- Stagger board joints so they don't all land on the same joist.
- Leave a gap at the house wall and follow the board maker's ventilation advice.
- Use screws rated for outdoor use and, for hardwood, pre-drill.
- Joints in diagonal boards need a joist (or a doubled joist) under both board ends — plan where they fall before you cut.
Frequently asked questions
How far apart should deck joists be?
Commonly 400 mm (16 in) centres for straight softwood boards; many composite boards need 300–400 mm, and boards laid at 45° need closer joists (Trex: 12 in / 300 mm for homes). Always follow the board maker.
How many screws per deck board?
Two at every joist the board crosses.
How many joist hangers do I need?
One for every joist end that hangs from a ledger or a flush beam. On the usual deck — joists hung from a ledger at the house and resting on a beam — that is one per joist; if both ends hang, two per joist; a freestanding deck with joists on beams needs none. Use the hanger and the nails or screws its maker lists for your joist size.
How much extra should I order for diagonal decking?
About 25 %, against 10 % for straight boards: TimberTech says a diagonal deck takes about 15 % more boards than a straight one, because boards at 45° are cut at both ends and the offcuts are often too short to reuse. Some guides allow up to 30 %.
Also searched as: deck board calculatorcomposite decking calculatordeck material calculatorjoist calculatordiagonal decking calculator
Sources
- Method: rows × deck length ÷ board length; fixing: two screws per board per joist (common practice).
- Trex Academy – How to install deck joists: 16 in on centre for residential decks, 12 in when the decking runs diagonally
- TimberTech – Diagonal decking patterns: about 15 % more deck boards than a straight installation
- American Wood Council – DCA 6-15, Prescriptive residential wood deck construction guide (ledger, rim joist, joist hangers, Figs. 1A and 1B)
Formulas last reviewed: September 30, 2026 ·How we check our formulas