Decking calculator: boards, joists, bearers and posts

How it works

Enter the deck dimensions and the board size printed on the product you plan to buy. Add the centre-to-centre spacing specified for your joists, bearers and posts. The calculator uses the joist lines to estimate board fixings, the bearer lines to estimate posts, and the post count to estimate footing concrete.

Board rows cover the deck width using board width plus the gap. Each row uses enough boards to cover the deck length. Stagger joined boards between neighbouring rows where joints are needed, this estimate does not optimise cut-offs or create a cutting plan, so check the layout before ordering.

Composite boards need the board maker’s own installation spacing. The maximum guidance cited here is a check for the referenced product guide, not a universal rule for every composite board. Timber joist, bearer and post spacings are inputs from your supplier, builder or engineer.

Formula

• Count board rows and boards

  Rows = ceil((deck width + board gap) ÷ (board width + board gap)) Boards = ceil(rows × ceil(deck length ÷ board length) × (1 + waste %))

  Each result is rounded up to whole boards. The estimate does not optimise offcuts.

• Count the frame supports

  Joists = ceil(deck width ÷ joist spacing) + 1 Bearers = ceil(deck length ÷ bearer spacing) + 1 Posts = bearers × (ceil(deck width ÷ post spacing) + 1)

  Enter the spacings from your product guidance or project design.

• Estimate footing concrete

  Footing volume = posts × footing width² × footing depth Bags = ceil(footing volume ÷ 0.01 m³ per 20 kg bag)

  Screw, clip and joist-tape quantities use the selected fixing method and the cited product guidance.

Worked example

1. 1

   Calculate the board layout

   ceil((3,000 mm + 5 mm) ÷ (140 mm + 5 mm)) = 21 rows 21 × ceil(4 m ÷ 3.6 m) = 42 boards

   This sample deck is 4 m × 3 m and uses 3.6 m × 140 mm boards with a 5 mm gap.

2. 2

   Count joists, bearers and posts

   Joists: ceil(3 m ÷ 0.4 m) + 1 = 9 Bearers: ceil(4 m ÷ 1.2 m) + 1 = 5 Posts: 5 × (ceil(3 m ÷ 1.2 m) + 1) = 20

   The spacings here are example inputs. Use the values specified for your board and deck design.

3. 3

   Estimate the footing bags

   Posts × 0.3 m × 0.3 m × 0.45 m = 0.81 m³ ceil(0.81 ÷ 0.01) = 81 bags

   Each example post has a 300 mm square footing, 450 mm deep. Bag yield is taken from the country-specific product sheet.

Assumptions and safety

The deck height is collected for project context, this estimator does not design the frame or check loads, foundations, connections, guards or local building rules. It contains no joist, bearer or post span tables. Spacing values are not span approvals. Have the design and spans checked by a qualified builder or engineer before you build.

The concrete estimate uses a square footing at each calculated post and the approximate yield printed in the linked product sheet. Actual excavation dimensions, soil, waste, mixing and product yield can change the amount. Buy the matching product only after checking its current bag and instructions.

Board quantities are a basic rectangular take-off. They do not include picture-frame borders, stairs, fascia, breaker boards, diagonal layouts, double joists at board joins, unusual cut-outs, or a cut-optimised staggered layout. Clip and screw requirements vary by product and fixing system, confirm the exact count and compatible fixings with the product instructions before ordering.

Frequently asked questions