Decking Calculator — Boards & Cost

Calculate decking boards, joists and screws needed. Get material cost estimates.

Last updated: April 2026 · Source: GOV.UK — Building Regulations Approved Document A

Boards

26

Joists

11

Screws

208

Area

12.0 m²

Board Cost

£208.00

Disclaimer

This calculator is provided for informational purposes only and should not be considered as financial or tax advice. All calculations are performed locally in your browser — no personal data is collected or sent to our servers. Rates and thresholds are sourced from HMRC and GOV.UK and are updated for the current tax year. Always verify results with HMRC or consult a qualified professional before making financial decisions.

How It Works

Decking board quantity is calculated by dividing the deck area by the coverage of each board. Standard softwood decking boards are 2.4 m, 3.0 m, 3.6 m or 4.8 m long and 120–145 mm wide (face width). Each board covers approximately 0.29–0.70 m² depending on size. A 5–10% waste allowance covers cuts and offcuts.

Substructure requirements include joists (typically 47 × 150 mm at 400 mm centres, supported on post-and-beam or concrete pads) and screws (approximately 20–25 stainless-steel screws per m²). The calculator also estimates joist hangers, post brackets and any concrete needed for footings.

Enter the deck dimensions. The calculator shows boards, joists, screws and an estimated cost. For treated softwood decking, expect £20–£40 per m² for materials. Composite decking is £40–£80 per m² but lasts longer and requires less maintenance. Any raised deck above 300 mm may need Building Regulations approval.

Example: Deck 4 m × 3 m, 145 mm softwood boards, 3.6 m lengths

  1. Area: 12 m²
  2. Boards (145 mm wide, 3 m deck width): 3,000 ÷ 145 ≈ 21 boards
  3. Add 10% waste: 23 boards at 4 m length
  4. Joists (400 mm centres, 4 m span): 3,000 ÷ 400 + 1 = 9 joists at 4 m
  5. Screws: 12 × 22 ≈ 264 screws (2 boxes of 200)

Source: GOV.UK — Building Regulations Approved Document A

Frequently Asked Questions

What does the Decking Calculator — Boards & Cost do?
Calculate decking boards, joists and screws needed. Get material cost estimates. All calculations are performed in your browser using official UK rates and thresholds.
Does this include waste allowance?
Yes. Most building material calculations include a standard 10% waste allowance. You can adjust this figure depending on the complexity of your project and cutting requirements.
Are prices accurate?
Material prices are indicative estimates based on average UK prices. Actual costs vary by region, supplier, brand and current market conditions. Always get quotes from local suppliers.