Decking Calculator

How to Use the Decking Calculator

1. Choose your units. USA mode uses feet and inches; UK mode uses metres and millimetres.
2. Enter the deck length and width. Length is the side the boards run along. Width is the direction the joists run (usually out from the house).
3. Choose the board: timber or composite, its actual width (for example 5½ inches or 144mm), its length, and the gap between boards.
4. Set the joist spacing. 16 inches (400mm) is typical for timber; composite boards often need 12–16 inches (300–400mm), and diagonal boards need tighter spacing.
5. Choose the joist size and timber species (USA) to check whether the joists can span the deck width.
6. Choose screws or hidden fasteners, and add prices (optional) to see the cost.

How to Calculate Deck Boards

The calculator works out boards by rows rather than by area, which gives more accurate results:

1. Rows of boards = deck width ÷ (board width + gap), rounded up.
2. Linear feet (or metres) of board = rows × deck length.
3. Boards to buy = linear length × (1 + waste) ÷ board length, rounded up.

A simple area-based estimate (deck area ÷ board coverage) often comes out short because it ignores the end cuts and offcuts you can't reuse.

Waste allowance: add about 10% for straight boards and 15% for diagonal or herringbone patterns. Choosing a board length that divides neatly into your deck length (for example, 16 ft boards on a 16 ft deck) cuts waste further.

Joist Spacing for Timber and Composite Decking

Composite boards are more flexible than timber, so manufacturers set their own maximum spacing. Most major US brands, including Trex, TimberTech and Fiberon, call for 16 inches on center for straight runs and 12 inches for a 45° diagonal. Some UK composite manufacturers ask for 300mm centres. Exceeding the manufacturer's spacing usually voids the warranty and makes the deck feel bouncy.

Deck Joist Span: Will My Joists Reach?

Joists must span from the ledger (or beam) on one side to the beam on the other without sagging. In the USA, the International Residential Code (IRC) sets maximum spans for deck joists. The values below are from IRC 2021 Table R507.6, for No. 2 grade pressure-treated lumber, a 40 psf live load and a 10 psf dead load:

In our 12 × 16 ft example, the joists span 12 ft at 16 inches on center. Southern pine 2×8 reaches only 11'10", so it fails; 2×10 reaches 14'0" and passes. The calculator does this check for you and suggests the smallest joist that works.

Joists can cantilever past the beam by up to one-quarter of the adjacent span under the IRC, subject to the table limits. Your local code may differ, and many jurisdictions use the AWC's *DCA 6* deck guide, so always confirm with your building department.

UK: there's no single span table in the Building Regulations for garden decks. The Timber Decking and Cladding Association (TDCA) publishes guidance, and timber merchants provide span tables for their joists. As a conservative planning guide for C16 or C24 treated joists at 400mm centres on a domestic deck: 47×100mm up to about 1.8 m, 47×150mm up to about 2.4 m, and 47×175mm up to about 2.9 m. Add a support beam or posts in the middle for longer spans, and check with your supplier.

Screws, Hidden Fasteners and Gaps

• Face screws: 2 per board at every joist. That's roughly 320 screws per 100 sq ft of deck with 5½-inch boards at 16-inch spacing, or about 35 per m² at 400mm centres.
• Hidden clip fasteners: about 1 clip per board per joist. Composite manufacturers sell them in boxes for a set area, so check the coverage on the box.
• Side gaps between boards: about 3/16 inch (5mm) is typical for both timber and composite, but follow the board manufacturer.
• End gaps for composite: 1/16–1/8 inch or more, depending on temperature at installation, because composite expands lengthwise.
• Wet pressure-treated timber shrinks as it dries, so it's often laid with small or no gaps.

Deck Costs

USA

• Materials only: pressure-treated wood about $3–$8 per sq ft, composite $5–$20 per sq ft, PVC $5–$13 per sq ft.
• Professionally built: about $30–$60 per sq ft, with labor around $15–$35 per sq ft.
• Typical project: many decks cost $9,000–$20,000; the national average is about $14,000.

UK

• Pressure-treated softwood decking, installed: about £40–£57 per m².
• Redwood or cedar, installed: about £67–£87 per m².
• Hardwood, installed: about £87–£105 per m², rising to £125+ for teak.
• Supply-only softwood: about £10–£20 per m². A typical 30 m² garden deck costs around £1,200–£2,600 depending on materials.

Footings, stairs, railings, lighting and removing an old deck add to these costs. The Lumber Calculator helps price framing timber, and the Concrete Calculator works out post footings.

Permits and Planning Rules

USA: under the IRC, a deck is generally exempt from a permit only if it's 200 sq ft or less, no more than 30 inches above grade, not attached to the house, and doesn't serve the required exit door. Most attached decks need a permit. Decks more than 30 inches above the ground need guards (railings), normally at least 36 inches high. Local codes vary, so check before you build.

UK (England): decking counts as permitted development if it's no higher than 30cm above the ground, and together with extensions and outbuildings covers no more than 50% of the garden. Higher decks, decks at listed buildings, and decks at flats or maisonettes need planning permission. Guarding is generally needed where there's a significant drop, commonly taken as over 600mm.

Tips for a Better Deck

• Leave at least 150mm (6 inches) of clearance under ground-level decks for airflow, and lay a weed membrane.
• Use ground-contact rated timber (UC4 in the UK) for posts and any framing near the soil.
• Seal cut ends of treated timber with an end-grain preservative.
• Use joist tape on top of timber joists under composite boards to slow rot.
• Slope the deck slightly (about 1:100) away from the house so water drains.
• Pick board lengths that match the deck to avoid joints, or plan joints over a doubled joist.

Frequently Asked Questions