How to use the deck calculator
A deck is a small structure with a long materials list, and most of it follows from a few layout decisions: how big the deck is, which way the boards run, how far apart the joists are and how many posts hold up the beams. This calculator assumes the common layout where the boards run along the house, the joists run out from the house at right angles to the boards, and one or two beams on posts carry the joists.
- Choose units and the deck type. An attached deck hangs its joists from a ledger on the house and has one beam at the outer edge. A freestanding deck has a beam at each side.
- Enter the length along the house and the depth the deck projects from it.
- Enter the board face width, gap and stock length. A 5/4 × 6 deck board is about 5.5 in wide; many metric boards are 140 or 145 mm.
- Pick the joist spacing from the decking maker's instructions, and the maximum post spacing your beam can span.
- Enter the post height and footing size to get post lumber and concrete, then read the full list.
Deck framing formulas
Board rows = ⌈depth ÷ (board width + gap)⌉
Boards = ⌈rows × ⌈length ÷ board length⌉ × (1 + waste)⌉
Joists = ⌈length ÷ spacing⌉ + 1, each as long as the depth
Posts per beam = ⌈length ÷ max post spacing⌉ + 1
Screws = rows × joists × 2 × 1.05
Footing concrete = posts × π × (diameter ÷ 2)² × depth
Boards per row are counted without reusing offcuts, so a 20 ft deck with 16 ft boards needs two boards per row with a 12 ft offcut each. That is deliberate: offcuts are often too short to reach the next joist, and butt joints need to land on a joist, ideally a doubled one. Choosing a stock length that divides the deck length neatly is the easiest way to cut waste.
| Member | Typical US size (estimate) | Typical metric size (estimate) |
|---|---|---|
| Decking | 5/4 × 6 or 2 × 6 | 28 × 140 mm or 32 × 145 mm |
| Joists | 2 × 8 to 2 × 10 at 16 in | 47 × 150 to 47 × 200 mm at 400 mm |
| Beam | Doubled 2 × 10 or 2 × 12 | Two 47 × 200 or 47 × 225 mm |
| Posts | 6 × 6 (4 × 4 on low decks) | 100 × 100 or 150 × 150 mm |
These are common starting points, not a structural design. Sizes depend on span, species, grade, loads and local rules.
Worked examples
A 16 × 12 ft attached deck
With 5.5 in boards and 3/16 in gaps, each row takes 5.6875 in, so 144 ÷ 5.6875 = 25.3 means 26 rows of 16 ft boards, or 29 boards with 10% waste (416 linear ft). At 16 in centres the ledger carries 13 joists, each 12 ft long. One doubled beam needs 32 ft of lumber and three posts at 8 ft spacing. Three 12 × 36 in footings hold about 7.1 ft³ of concrete, or 12 bags of 80 lb, and the boards need about 710 screws.
A 4.8 × 3.6 m freestanding deck
With 140 mm boards and 5 mm gaps, 3,600 ÷ 145 = 24.8, so 25 rows of 4.8 m boards and 28 boards with waste. Joists at 400 mm centres come to 13, each 3.6 m long. Two doubled beams use 19.2 m of timber and six posts at 2.4 m spacing. Six 300 mm footings 600 mm deep take about 0.25 m³ of concrete, or around 20 bags of 25 kg, and the boards need about 683 screws.
Picking the board length for a 20 ft deck
A 20 × 10 ft deck with joists at 12 in centres has 22 rows and 21 joists. With 16 ft boards each row needs two boards, so 44 boards (49 with 10% waste) and 704 ft of decking. With 20 ft boards it is 22 boards (25 with waste) and 440 ft, which saves roughly a third of the decking cost even if 20 ft boards cost more per foot. Screws come to about 971 either way.
Who this calculator helps
- Architects and landscape designers drawing up deck options and early cost estimates.
- Deck builders and carpenters producing a quick takeoff for a quote or lumber order.
- Builders' merchants and lumber yards turning a customer's sketch into a materials list.
- Homeowners and DIY builders pricing timber against composite before committing.
- Students learning how decking, joists, beams and posts fit together.
Practical tips
- Check the joist span for the depth you've entered with the joist and rafter span calculator; a deep deck may need larger joists or a mid-span beam.
- Get more detail on footing concrete, including waste and other bag sizes, from the concrete calculator, and convert timber quantities to board feet with the board foot calculator.
- Add stairs with the stair calculator; most codes require guards once a deck is more than about 30 in (760 mm) above grade, but check your local rules.
- Flash the ledger and bolt it to the house rim with through-bolts or structural screws; never nail a ledger.
- Order joist hangers, hurricane ties and post bases in the same corrosion-resistant finish as your fasteners.
Want to show the client the deck before it's built? Add it to a photo of the garden with Landscape.
