Deck calculator (US)
Count decking boards by row rather than by area, plan the cuts, and size the joists, bearers, posts, fixings and post-hole concrete.
Set up for United States: USD prices, Sales tax, and imperial units with local product sizes.
Works in feet and inches with US lumber sizes. Board widths default to 5.5 in, gaps to 3/16 in, and joists to 16 in on center.
US terminology throughout: lumber rather than timber, beams rather than bearers, and prices in dollars with a sales-tax field you set yourself.
Your project
Showing United States units, product sizes and Sales tax.
Quick start
Deck size
Rows build up across the width. Joists run across the length.
Boards
The default for UK and US decks. Lightest and cheapest, needs periodic treatment.
The stock length you can buy. The cut plan reuses off-cuts from this length.
3/16 in (0.1875) is typical for timber; composite gaps are set by the manufacturer.
Two screws per board at every joist.
Frame
16 in centres suit most timber boards; composite usually needs 12–16 in.
Steps and trim
Estimate updated. 22 boards of Treated softwood decking boards required.
Deck calculator
United States · Imperial units · 30 July 2026
Shopping list
7 lines- 22boardsTreated softwood decking boards5.50 in × 1.00 in × 16.00 ft lengths
- 110ftJoists — 1.50 in × 5.50 in10 joists at 16.00 in centres, each 10.00 ft (3.05 m) long
- 29ftBeams2 at 6.00 ft (1.83 m) centres, each 13.00 ft (3.96 m) long
- 6postsPosts2 beams × 3 posts at 6.00 ft (1.83 m) centres
- 462screwsDecking screws2 per board at each of 10 joists
- 4boardsFascia boardsPerimeter of 46.00 ft (14.02 m)
- 2280 lb bagsConcrete for post holes6 holes at 12 in diameter × 30 in deep
Estimate results
What you measured
- Deck area
- 130.00 ft²
- 12.08 m²
- Board rows
- 22
- Total board length
- 286.00 ft (87.17 m)
- Joists
- 10
- Posts
- 6
What to buy
10% waste included| Material | Buy | Weight |
|---|---|---|
Treated softwood decking boards 5.50 in × 1.00 in × 16.00 ft lengths Rounding: The calculation requires 17.88 boards. Adding 10% waste gives 19.66. Rounded up to 20 boards. A cutting plan that reuses off-cuts needs 22 lengths, so the order is set to that. Main material | 22boards | 209 kg |
Joists — 1.50 in × 5.50 in 10 joists at 16.00 in centres, each 10.00 ft (3.05 m) long Rounding: The calculation requires 100 ft. Adding 10% waste gives 110. Rounded up to 110 ft. Structure | 110ft | 98 kg |
Beams 2 at 6.00 ft (1.83 m) centres, each 13.00 ft (3.96 m) long Rounding: The calculation requires 26 ft. Adding 10% waste gives 28.6. Rounded up to 29 ft. Structure | 29ft | 52 kg |
Posts 2 beams × 3 posts at 6.00 ft (1.83 m) centres Rounding: The calculation requires 6 posts. No waste allowance was added. Rounded up to 6 posts. Structure | 6posts | — |
Decking screws 2 per board at each of 10 joists Rounding: The calculation requires 440 screws. Adding 5% waste gives 462. Rounded up to 462 screws. Fixings | 462screws | — |
Fascia boards(optional) Perimeter of 46.00 ft (14.02 m) Rounding: The calculation requires 2.88 boards. Adding 10% waste gives 3.16. Rounded up to 4 boards. Finishing | 4boards | 38 kg |
Concrete for post holes 6 holes at 12 in diameter × 30 in deep Rounding: The calculation requires 19.63 80 lb bags. Adding 10% waste gives 21.6. Rounded up to 22 80 lb bags. Base and membrane | 2280 lb bags | 897 kg |
Assumptions behind each line
Treated softwood decking boards
- 22 rows of boards across 10.00 ft (3.05 m), at 5.50 in wide plus a 0.188 in gap.
- 87.17 linear metres of board in total.
- Cut plan uses 22 lengths with 81.3% utilisation.
Joists — 1.50 in × 5.50 in
- Joists run perpendicular to the boards and are spaced along the 13.00 ft (3.96 m) run.
- Joist size and span must be checked against a span table for the load and the spacing used.
Beams
- Beams run parallel to the boards and carry the joists.
- Assumed to be twice the joist thickness. Size them from a span table for your post spacing.
Posts
- Post spacing drives the bearer size. Closer posts allow a smaller bearer section.
- Post height and fixing method depend on the ground and the deck height, which are not estimated here.
Decking screws
- Two screws per board at every joist crossing, which is standard practice for face fixing.
- Use stainless or coated fixings; plain steel stains treated timber.
Fascia boards
- One row of fascia around the full perimeter. Deep frames may need two rows.
Concrete for post holes
- Hole volume π × r² × depth = 55.6 litres per post.
- No deduction is made for the post itself, which gives a little spare.
- Hole depth must reach below the frost line and suit the ground conditions.
Board and joist layout
How this was calculated
Deck area
13.00 ft × 10.00 ft
= 130.00 ft² (12.08 m²)
Board rows
(10.00 ft (3.05 m) + 0.188 in gap) ÷ (5.50 in board + 0.188 in gap)
= 22 rows
- Boards are counted by row, not by area. Dividing deck area by board area ignores the gaps and under-orders.
Total board length
22 rows × 13.00 ft (3.96 m) run
= 286.00 ft (87.17 m)
Boards required
The calculation requires 17.88 boards. Adding 10% waste gives 19.66. Rounded up to 20 boards. A cutting plan that reuses off-cuts needs 22 lengths, so the order is set to that.
= 22 boards of 16.00 ft
- Cut plan: 22 lengths, 20.12 m of off-cut, 81.3% utilisation.
Joists
(13.00 ft (3.96 m) ÷ 16.00 in) + 1
= 10 joists, 100.00 ft (30.48 m) total
Beams
(10.00 ft (3.05 m) ÷ 6.00 ft (1.83 m)) + 1
= 2 × 13.00 ft (3.96 m) = 26.00 ft (7.92 m)
Posts
2 beams × ((13.00 ft (3.96 m) ÷ 6.00 ft (1.83 m)) + 1)
= 6 posts
Screws
22 rows × 10 joists × 2
= 440 screws before waste
Weight and delivery
Approximate weight
1.43 tons
Likely delivery
dump truck
- Board lengths of 16.00 ft will not fit in a car. Check the merchant's delivery vehicle can reach your access.
- The boards alone weigh about 462 lb (209 kg).
- 1.43 tons needs a dump truck. Confirm access width, overhead clearance and where the driver can tip.
- Check that the delivery route can take the weight — block paving, drainage covers and soft verges are easily damaged.
Safety and compliance
Material planning only
This estimate is for material planning only. Structural dimensions, foundations and compliance requirements should be checked by a qualified professional.
Deck framing must be designed, not guessed
Joist and bearer sizes, post fixings, footing depth and any connection to a building are structural decisions. Raised decks and decks attached to a house need particular care — a badly fixed ledger is one of the most common causes of deck collapse.
Check whether approval is required
Decks over 300 mm (12 in) above ground, decks attached to a dwelling, and decks covering more than half the garden often need approval. Confirm the requirements with your local building department before you start.
Data behind this estimate
| Assumption | Value | Source |
|---|---|---|
Treated softwood decking and framing Freshly treated timber is heavier than kiln-dried stock and can exceed the range. | 550 kg/m³range 450–650 | TRADA / BS EN 338Reviewed 2026-07-01 |
Deck joist spacing 400 mm (16 in) centres suit most 32–38 mm softwood boards laid square to the joists. | 400 mm centresrange 300–600 | Timber Decking and Cladding AssociationReviewed 2026-07-01 |
Wet concrete Normal-weight concrete. Lightweight mixes are considerably lighter. | 2,400 kg/m³range 2,300–2,500 | British Standards InstitutionReviewed 2026-07-01 |
Bagged ready-to-use concrete yield Derived from the published 80 lb (36.3 kg) ≈ 0.60 ft³ (0.0170 m³) yield. | 0 m³ per kg of dry mixrange 0–0.001 | Bagged concrete manufacturers (Quikrete, Sakrete)Reviewed 2026-07-01 |
Default waste allowance Covers cutting, breakage, setting-out error and the odd unusable unit. | 10 %range 5–15 | How Much NeededReviewed 2026-07-01 |
Cost
Enter the prices your supplier quoted. This site does not hold or estimate supplier prices, so totals appear only once you fill these in.
Sales tax varies by state and locality, so it starts at 0%. Enter your own combined rate to include it.
No prices entered yet. Everything else in this estimate — quantities, weight, delivery and the shopping list — works without them.
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How this calculator works
Every figure below is derived rather than looked up, so you can check it against your own numbers. Nothing is hidden behind a black box.
Board rows, not area
A deck is covered by whole board widths separated by gaps. The number of rows is the span across the boards, plus one gap, divided by the module of one board plus one gap, rounded up.
Dividing the deck area by the board area is the common shortcut and it always under-orders, because it treats the gaps as if they were covered by timber.
rows = ⌈(span + gap) ÷ (board width + gap)⌉
Board direction
Which way the boards run changes everything downstream: the number of rows, the length of each run, the number of joists and how much cutting there is. Both options are offered because the answer is genuinely different.
Cutting plan
The board quantity is the greater of two numbers: total linear length divided by the stock length plus waste, and the number of stock lengths a first-fit-decreasing cutting plan actually consumes when off-cuts are reused.
The second number is often higher. A 4 m run cut from 3.6 m boards leaves a 0.4 m piece from every board, and you cannot make a 4 m row out of leftovers.
Frame
Joists run perpendicular to the boards and are counted along the board run at the spacing you set, plus one for the final joist. Bearers run parallel to the boards beneath the joists, and posts are counted along each bearer.
Sizes are not designed here. Joist depth, bearer section and post fixing all depend on span and load and must come from a span table or an engineer.
joists = ⌊run length ÷ spacing⌋ + 1
Fixings
Face-fixed decking takes two screws per board at every joist crossing. Hidden fastener systems take one clip per crossing, though some need two at board ends — check the system before ordering.
screws = rows × joists × 2
Post-hole concrete
Each hole is treated as a cylinder: π × radius² × depth, using 300 mm × 600 mm in the UK and 12 in × 30 in in the US. No deduction is made for the post itself, which leaves a small margin.
Common questions
- How many decking boards do I need?
- Divide the span across the boards by the width of one board plus one gap, then round up to get the number of rows. Multiply by the length of the run for total linear metres, and divide by the board length you can buy. A 4 × 3 m deck in 144 mm boards with 5 mm gaps needs 21 rows and 84 linear metres.
- What gap should I leave between decking boards?
- 5–6 mm (about 3/16 in) for timber that has been stored dry, and less for timber that arrives wet since it will shrink. Composite gaps are set by the manufacturer and are often larger, with a separate end gap for expansion.
- How far apart should deck joists be?
- 400 mm (16 in) centres suit most 28–32 mm softwood boards laid square to the joists. Composite boards usually need 300–400 mm, and any board laid diagonally needs the spacing reduced further.
US product sizes and conventions
Decking lumber commonly stocked in the US
- 5/4 × 6 in decking (actual 1 × 5.5 in) in 12, 16 and 20 ft lengths — the standard choice.
- 2 × 6 in decking (actual 1.5 × 5.5 in) where a stiffer board is wanted.
- 2 × 4 in decking for smaller boards and tighter patterns.
- 5.5 in composite in 12, 16 and 20 ft lengths, usually needing 12–16 in joist centers.
US frame conventions
- Joists at 16 in on center for most decking laid perpendicular to the joists.
- 12 in on center is common for composite and for any board laid at 45 degrees.
- Pressure-treated Southern Yellow Pine or Douglas Fir, typically 2 × 8 in joists.
- Footings dug below the local frost line — 12 in diameter × 30 in deep is a common starting point, but the frost depth is set locally.
Buying and sales tax
- Sales tax defaults to 0% because rates vary by state, county and city. Enter your own combined rate.
- Lumber is usually priced per piece rather than per linear foot at home centers, and per board foot at lumberyards.
- Decks over 30 in above grade, and any deck attached to the house, typically require a permit and an inspection.
UK version: Decking calculator — metric units and UK product sizes →