Deck Calculator
Work out the deck boards, joists, beams, posts, footings, screws and railing for your deck, see a plan from above, and add up what it will cost.
Front section (sits in front of the main deck, on its left side)
Side section (against the house, to the right of the main deck)
The real width. Typical: 5½ in for a “6 in” board, or 140 mm.
Typical 3/16 in (0.1875 in) or 5 mm. Check the board maker.
Typical lengths. Best fit picks the one that wastes least.
Fixed: a ledger board on the house wall holds one end of the joists.
Beams, posts and footings
Rule of thumb about 12 ft (3.6 m) for 2×8 joists at 16 in. Check a span table.
Typical 6–8 ft (1.8–2.4 m).
For post length and the number of steps.
Below the frost line where you live.
Typical box: 350 screws. Check the pack.
Leave blank for the usual: 10% straight, 15% diagonal.
Left open in the middle of the longest side.
Typical 6 ft (1.8 m) or less.
US rule: under 4 in. UK rule: under 100 mm.
Usually 2: a top and a bottom rail.
Also picks typical stock lengths and spacings in feet or metres.
Joists, ledger, rims and beams.
Posts and rails, without the balusters.
How your deck is worked out
Length × width for the area, the width ÷ (board + gap) for the rows of boards, cut from stock lengths plus waste, then joists, beams and posts at their spacing.
- 1
Deck area
16 × 12 ft= 192 sq ft
- 2
Rows of boards
144 in ÷ 5.69 in= 25.32, round up= 26 rows
- 3
Boards + 10% waste
26 × 16 ft boards26 × 1.1 = 28.6= 29 boards
- 4
Joists and posts
192 in ÷ 16 in + 1= 13 joists3 posts, 12 bags
Read more: the formulas, a worked example, typical sizes and fixings
A deck is a floor of boards on a frame. The boards lie in rows across the joists; the joists sit on a ledger at the house and on beams; the beams sit on posts; and the posts stand on concrete footings in the ground. The calculator works out each layer in turn from the deck size and the spacings you choose.
The formula
area = length × width (add up each section)
rows of boards = width they cover ÷ (board width + gap), rounded up
boards = rows cut from stock lengths (offcuts reused), then × (1 + waste % ÷ 100), rounded up
diagonal boards = area ÷ (board width + gap) ÷ stock length, plus waste
joists = side they are spaced along ÷ joist spacing, rounded up, + 1
beam rows = joist length ÷ longest span, rounded up (+ 1 if freestanding)
posts = beam rows × (beam length ÷ post spacing, rounded up, + 1) · footings = posts
concrete per footing = π × (hole width ÷ 2)² × depth · bags = all concrete ÷ bag yield, rounded up
screws = rows × joists × 2 (+ extra at butt joints), + 5% · clips = rows × joists, + 5%
balusters per railing section = (clear gap − baluster gap) ÷ (baluster width + gap), rounded up
Worked example
A deck 16 ft along the house and 12 ft out from it, fixed to the house, with 5½ in boards and a 3/16 in gap laid along the house. The area is 16 × 12 = 192 sq ft (17.84 m²). The boards cover 12 ft = 144 in, and each board plus its gap takes 5.5 + 0.1875 = 5.6875 in, so 144 ÷ 5.6875 = 25.32, which rounds up to 26 rows. Each row is 16 ft long, so one 16 ft board does a row: 26 boards. Adding 10% waste, 26 × 1.1 = 28.6, so you buy 29 boards. With joists every 16 in along the 16 ft (192 in) side, 192 ÷ 16 = 12 spaces, so 13 joists, each 12 ft long. A 16 ft ledger holds the house end and a 16 ft rim joist runs across the far end. The joists span 12 ft, so 12 ÷ 12 = 1 beam row, 16 ft long, doubled to 32 ft of lumber. With posts every 8 ft, 16 ÷ 8 = 2, plus 1, gives 3 posts and 3 footings. Each footing hole is 12 in wide and 36 in deep: 3.1416 × 0.5² × 3 = 2.356 cu ft, so all three take 7.07 cu ft, and at 0.6 cu ft per 80 lb bag that is 11.78, or 12 bags. The boards cross the joists 26 × 13 = 338 times; at 2 screws each that is 676, and with 5% extra 709.8, so 710 screws (3 boxes of 350). The deck is 2 ft (24 in) up, so 24 ÷ 7.5 = 3.2 means about 4 steps down.
Typical sizes used
- Deck boards: “6 in” boards are really about 5½ in wide; metric boards are often 140 or 145 mm. Typical lengths: 8, 10, 12, 16 and 20 ft, or 2.4, 3.6, 4.8 and 5.4 m.
- Gap: about 3/16 in (5 mm) for dry wood and composite boards. Follow the board maker.
- Joists: 16 in (400 mm) centres for straight boards, 12 in (300 mm) for diagonal boards. Many composite boards need 16 in or less, or 12 in at 45°.
- Beams and posts: posts about 6–8 ft (1.8–2.4 m) apart, beams doubled. About 12 ft (3.6 m) is a common joist span for 2×8 joists at 16 in, but spans depend on the wood, its size and grade, and your local rules.
- Footings: holes about 12 in (300 mm) wide; depth must reach below the frost line where you live. Concrete mix: an 80 lb bag makes about 0.6 cu ft; a 25 kg bag about 0.012 m³.
- Fixings: 2 screws per board at each joist, boxes of about 350; hidden clips about 1 per board per joist, boxes of about 90 (roughly 50 sq ft at 16 in).
- Steps: risers of about 7½ in (190 mm) are comfortable. Work out the exact stairs with a stair calculator.
These are rule-of-thumb figures; products and local rules differ, so check before you buy or build. Checked 11 October 2026.
Railings and baluster gaps
In the United States, the International Residential Code asks for a guard on a deck more than 30 in above the ground, and the gaps must stop a 4 in ball passing through. In England, Approved Document K uses a 100 mm ball. Other countries have their own rules. The calculator spaces the balusters evenly so each gap is no wider than the gap you enter.
L-shapes and three sections
The main deck sits against the house. The front section sits in front of it on the left, making an L. The side section sits against the house to the right. Each section gets its own boards, joists, beams and posts; the front section is framed as freestanding, so its numbers are on the safe side.
Common questions
How many deck boards do I need?
Divide the width the boards cover by one board plus its gap to get the rows, then cut each row from stock lengths. For a 16 × 12 ft deck with 5½ in boards and a 3/16 in gap, 144 ÷ 5.6875 = 25.32, so 26 rows. Each row is one 16 ft board, so 26 boards, and with 10% waste you buy 29.
How many joists does a deck need?
Divide the side the joists are spaced along by the joist spacing and add one for the end. A 16 ft deck with joists every 16 in needs 192 ÷ 16 = 12 spaces, so 13 joists. The rim (band) joists along the ends and the ledger on the house come on top.
How far apart should deck joists be?
Joists every 16 in (400 mm) are the usual choice for boards laid straight across them. Boards laid at 45° span further between joists, so joists every 12 in (300 mm) are typical. Many composite boards need 16 in or less. Always check what the board maker and your local rules ask for.
How many screws do I need for a deck?
Use two screws in each board at every joist it crosses. A 16 × 12 ft deck has 26 rows of boards over 13 joists, which is 338 crossings, so 676 screws. With 5% extra for drops that is 710 screws, or 3 boxes of 350. Hidden clips need about one clip per crossing instead.
How much concrete do deck footings need?
Each round footing holds π × radius² × depth. A hole 12 in wide and 36 in deep holds 3.1416 × 0.5² × 3 = 2.36 cu ft. Three footings need 7.07 cu ft. An 80 lb bag of mix makes about 0.6 cu ft, so you need 12 bags. Footing size and depth depend on your soil and frost line.
What gap should I leave between deck boards?
About 3/16 in (5 mm) is a common gap for kiln-dried wood and composite boards, so water drains and the boards can move. Wet treated wood shrinks as it dries, so it is often laid tight. Composite and PVC boards have their own gap rules, which can change with the temperature on the day.
Related calculators
Made by Zulu Calculator, part of AlphaZulu LLC. Formulas are shown in full and rule figures are checked against official sources; see how we build calculators.