Deck & Fence Calculator
Get deck boards and joists from area, and fence posts, rails, and pickets from length — with waste allowance.
Materials needed
- Total board length
- 154.8 m
- Joists
- 14
Effective board width = width + gap. Includes 10% waste. Joists calculated when side length is entered.
What this tool does
The deck and fence calculator computes deck board count and joists from area, and fence posts, rails, and pickets from length. It reflects construction variables like board gap and joist spacing and includes a waste allowance, ready for woodworking and landscaping DIY orders.
Who uses this
- Estimate deck boards for a build
- Calculate joist count
- Garden fence posts and pickets
- Fence material quote
- Woodworking DIY ordering
How to use
- 1Choose the calculation type (deck/fence).
- 2For deck: area, board width/length, gap. For fence: length, post spacing, picket spacing.
- 3See the boards/posts needed and joists/rails/pickets. Includes 10% waste.
Material quantity formula
[Deck] Effective board width = width + gap Boards = area / (effective width × length) × (1 + waste) Joists = deck side length / joist spacing + 1 [Fence] Posts = ceil(length / post spacing) + 1 Rails = sections × 2 (top/bottom) Pickets = length / picket spacing
Real examples
Example 1: 20m² deck, 140mm board 3.6m
Effective width 145mm, 0.522m² per board. 20/0.522 = 38.3 × 1.1 = 43 boards. Joists: 5m side at 400mm = 14.
Example 2: 20m fence, 2m post spacing
10 sections + 1 = 11 posts. 20 rails (2 per section). At 100mm picket spacing, 200 pickets.
Example 3: Narrow boards (90mm)
Narrower effective width needs more boards for the same area — denser look and higher cost.
Frequently asked questions
Why a board gap?+
Wood expands/contracts with moisture and needs drainage/ventilation, so leave a 3-6mm gap between boards. The gap adds to effective width, affecting board count.
What joist spacing is right?+
Standard deck boards use 400mm (16in) joist spacing; heavy loads or thin boards use 300mm. Tighter spacing is stronger but needs more joists.
Is 10% waste enough?+
Enough for simple rectangular decks; diagonals, stairs, and complex shapes warrant 15-20%. It covers cutting loss and defective boards.
Why posts = sections + 1?+
Each gap between posts is a section, and you need posts at both ends, so one more than the section count. A 10-section fence needs 11 posts. Corners and gates may need extra posts.
Pressure-treated vs composite (WPC)?+
Pressure-treated is cheaper but needs periodic oiling; composite (WPC) is pricier with less maintenance. The count is the same but board dimensions may differ.
Are deck footings calculated?+
This tool computes boards, joists, and fence materials only. Deck footings (concrete piers, posts) require separate design based on area and load.
Cautions
- •10% waste default — complex shapes 15-20%.
- •Standard joist spacing 400mm; thin boards 300mm.
- •Board gap affects the count.
- •Deck footings (piers, posts) require separate design.
- •Check pressure-treated vs composite board dimensions.
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Last reviewed: 2026-05-30