What do I need to calculate to build a deck, from frame to balusters?
Published · By Sai Nath
Building a deck takes six sums, in order: the frame's diagonals, footing concrete, joists, boards with their gap, stair risers and baluster spacing. For a 4800 × 3600 mm (15′ 9″ × 11′ 9 3/4″) deck: 6000 mm diagonals, 0.66 m³ for eight footings, 12 joists at 450 mm and 39 rows of 90 mm boards.
What does a deck building calculator need to work out?
The whole deck, in build order. One deck runs through every step: 4800 mm along the house by 3600 mm out, 1100 mm (3′ 7 5/16″) off the ground, boards running the long way, every number from the app's own formulas. Post spacing, footing and joist sizes are example inputs; yours come from the plans and span tables.
| Step | The sum | In MeisterCalc | Free on this site |
|---|---|---|---|
| 1. Square the frame | diagonal = √(length² + width²) | Check Square (free) | Square-up calculator |
| 2. Posts and footings | equal bays, then π × radius² × depth × holes + 10% | Equal Spacings, Concrete ▸ Holes (free) | Concrete calculator |
| 3. Joists | ⌈length ÷ spacing⌉ + 1 | Stud Counter, Running Measurements | — |
| 4. Boards | ⌈width ÷ (board + gap)⌉ rows × length | Decking | — |
| 5. Stairs | risers = rise ÷ ((600 − going) ÷ 2), rounded | Stairs (free) | Stair calculator |
| 6. Balusters | every gap under the limit | Baluster Spacing | — |
How do I set out and square the deck frame?
Measure both diagonals: on a square 4800 × 3600 mm frame each is √(4800² + 3600²) = 6000 mm (19′ 8 1/4″). A 16 × 12 ft frame reads 20 ft on both.
Square the side lines off the house first with a 3-4-5 triangle as big as the lines allow (the 3-4-5 rule), then check the bearers and rim before any joist goes in. Say the diagonals read 6012 and 5988 mm, a 24 mm gap. Push the corners at the ends of the long diagonal together until both read the same; the long one comes in by half the gap, 12 mm, to 6000 mm (how to square a deck frame).
How many footings, and how much concrete do they take?
Split each bearer line into equal bays no longer than the plans allow, add one post, and work each hole as a cylinder plus 10%. With an example maximum of 1800 mm, 4800 ÷ 1800 = 2.67, so 3 bays of 1600 mm (5′ 3″) and 4 posts. Equal Spacings (Overall Length 4800 mm, Target Spacing 1800 mm) shows Actual Spacing 1600.0 mm and Pieces Needed 4. It rounds to the nearest number of spaces, so check the Actual Spacing stays under your maximum (how to space deck posts evenly).
Two bearer lines of four make eight footings. At 400 mm (15 3/4″) across and 600 mm (23 5/8″) deep, one hole is π × 0.2² × 0.6 = 0.0754 m³, eight are 0.6032 m³, and with 10% you order 0.6635 m³ (0.87 yd³). From the makers' sheets that is 72 bags of Boral Concrete Mix 20 kg (108 per m³) or 40 bags of 80 lb QUIKRETE Concrete Mix (0.60 ft³ each). Footing size and depth depend on the soil, the load and local rules: take them from the plans.
How many joists do I need, and how far apart?
Divide the length by the spacing, round up, and add one for the closing joist. At 450 mm centres: 4800 ÷ 450 = 10.67, so 11 + 1 = 12 joists of 3600 mm, 43.2 m. At 16 in (406.4 mm): 4800 ÷ 406.4 = 11.81, so 13 joists, 46.8 m. In Stud Counter, read studs as joists: Wall Length 4800 mm, On-Centre Spacing 450 mm and Stud Height 3600 mm show Number of Studs 12, Total Linear 43200.0 mm. Rim joists, doubled joists and blocking are extra (how many joists do I need).
The board sets the widest spacing: WoodSolutions' Technical Design Guide 21, Table 2, gives 500 mm for 19 mm hardwood decking, and composite boards follow their maker. The joist size for the span comes from span tables: AS 1684, NZS 3604, or the IRC's deck section (R507) as your state or city adopts it. See joist spacing in Australia and in New Zealand.
How many decking boards, and why is the last board a sliver?
Rows = width ÷ (board + gap), rounded up; lineal metres = rows × length. With 90 mm boards and 4 mm gaps across 3600 mm: 3600 ÷ 94 = 38.3, so 39 rows, 39 × 4.8 = 187.2 m, and 205.9 m to order with 10%. Decking (Deck Length 4800 mm, Deck Width 3600 mm, Board Width 90 mm, Board Spacing 4 mm) shows Boards Across 39, Lineal Metres 187.2 m and Order (+10%) 205.9 m. The 4 mm is inside the 3 to 4 mm that the same guide's Table 4 recommends for seasoned hardwood 86 to 90 mm wide (decking board gap).
Boards Across counts the last part row as a whole board. Here 38 full boards and 38 gaps take 3572 mm, so the 39th row is a 28 mm (1 1/8″) sliver. Fix it before the first board goes down (last deck board too narrow):
- If the plans don't fix the width, size the frame to whole boards: 38 boards and 37 gaps is 3568 mm (11′ 8 1/2″); 39 and 38 is 3662 mm.
- Or rip the first and last boards equally: 37 full boards and 38 gaps leave 3600 − 3330 − 152 = 118 mm, two edge boards of 59 mm.
- Opening the gaps to fit 38 boards needs 4.86 mm: outside that 3 to 4 mm range, so not for this board.
How many steps down from the deck?
Divide the rise by a target riser of (600 − going) ÷ 2 and round. With a 280 mm (11″) going the target is 160 mm, and 1100 ÷ 160 = 6.88, so 7 risers of 1100 ÷ 7 = 157.1 mm (6 3/16″) and 6 treads. The treads cover 6 × 280 = 1680 mm (5′ 6 1/8″) of ground from the bottom riser to the deck edge, and 2R + G = 314.3 + 280 = 594.3 mm. In Stairs, Rise Only, the Total Run reads 1960 mm: it counts one going per riser, one more than the treads cover.
That stair passes all four countries' riser and going figures in the table below; the 2R + G of 594.3 mm is inside the 550 to 700 mm the NCC and Approved Document K use.
How far apart do balusters go?
Close enough that the test sphere can't pass: 125 mm in Australia, 100 mm in New Zealand, 4 in in the US. At 1100 mm off the ground this deck needs a barrier under both the NCC and the IRC (see the table below).
Between balustrade posts 1600 mm apart on centres, 90 mm posts leave a clear run of 1510 mm (4′ 11 7/16″). With 42 mm (1 5/8″) balusters, Baluster Spacing gives:
- Max Spacing 125 mm: 12 balusters, 77.4 mm (3 1/16″) between, 119.4 mm on centres.
- Max Spacing 4″ (101.6 mm): 14 balusters, 61.5 mm (2 7/16″) between, 103.5 mm on centres.
The app's count keeps every gap well under the limit; it isn't the fewest that would pass. Worked by hand, 9 balusters leave (1510 − 9 × 42) ÷ 10 = 113.2 mm gaps in the 125 mm case. More in balustrade gaps in Australia and in New Zealand.
Which mistakes cost deck builders the most?
- Leaving the gap out of the board count. 3600 ÷ 90 = 40 boards, but 38 gaps of 4 mm take 152 mm, nearly two boards: only 38 full boards fit, plus the 28 mm sliver.
- A sliver last board. A 28 mm strip splits and looks like a mistake. Choose two 59 mm edge boards, or a frame sized to whole boards, before the first board goes down.
- A frame not square to the house. The 24 mm between those diagonals is about a quarter of a degree: 3600 × (6012² − 5988²) ÷ (4 × 4800 × 3600) = 15 mm of lean across the 3600 mm side, visible along every board end.
Which rules apply to a deck in Australia, New Zealand, England and the US?
Each country sets its own, and states and councils amend them. These are the figures as each document states them, checked on 1 November 2026.
| Where | Barrier | Opening | Stair riser and going | Joists |
|---|---|---|---|---|
| Australia | Where a fall of 1 m or more is possible; at least 1 m high on a deck (NCC 11.3.3, 11.3.4) | No 125 mm sphere | 115–190 mm, 240–355 mm (NCC Table 11.2.2a) | AS 1684 span tables; board maxima in TDG 21 |
| New Zealand | Where people could fall 1 m or more (F4.3.1) | No 100 mm sphere in housing (F4/AS1 1.2.1) | Main private stairway: riser 190 mm or less, going 280 mm or more (D1/AS1) | NZS 3604 |
| England | External balconies: 1100 mm high (Approved Document K, Diagram 3.1) | No 100 mm sphere where children under five may be present; ask building control about decks | Private stair: 150–220 mm, 220–300 mm | TDCA/TRADA span tables (TDCA) |
| United States | More than 30 in above floor or grade: guard at least 36 in (2018 IRC R312.1, quoted in the Knox County guide) | No 4 in sphere; 4 3/8 in at the sides of stair treads | Riser 7 3/4 in or less, tread 10 in or more (IRC R311.7.5) | IRC R507 as your state or city adopts it |
Read each with its notes and exceptions. The approved plans and your council, certifier, building control or building department decide what applies; MeisterCalc does the arithmetic and checks none of it.
Which MeisterCalc calculators does a deck builder use?
Check Square, Equal Spacings, Concrete, Stud Counter, Running Measurements (to mark joists out), Decking, Stairs and Baluster Spacing. MeisterCalc has 16 calculators. Concrete, Stairs and Check Square are free; the other 13 unlock with a weekly subscription (3-day free trial) or a one-time lifetime unlock. For feet and inches, choose Settings ▸ Units ▸ Default units ▸ Imperial (ft/in). It runs on Android 7.0 or later, offline.