Decking Calculator
Plan your deck build — total area, boards, joists, screws, and cost — in one go. Supports multiple deck sections, board gap, and joist spacing.
Deck sections
Deck plan preview (to scale, with boards)
1 unit = 0.1 ftBoard
Common: 5.5" wide × 12 ft, 1/8" gap (imperial) · 140 mm × 3.6 m, 4 mm gap (metric).
Framing & waste
16" / 400 mm is the most common residential spacing.
Typical: 5–10% straight, 10–15% with picture-frame border or diagonal pattern.
Your estimate
About Decking Calculator
The Decking Calculator helps you plan a deck build from a rough sketch to a clean materials list. Enter the length and width of every deck section, your board size and gap, and joist spacing — and the calculator returns total area, deck boards needed, joist count and total linear length, and an estimated screw count. Add a board price to get an instant cost estimate.
It works for any common decking material — pressure-treated pine, cedar, ipé, composite, or PVC — and supports both imperial (feet, inches) and metric (meters, millimeters) units. The to-scale preview shows your sections side-by-side with simulated board stripes so you can sanity-check the layout before buying.
How to Use Decking Calculator
- 1 Pick your unit system. Toggle Feet / Inches or Meters / mm at the top. The calculator switches all defaults and inputs to match.
- 2 Enter every deck section. For a simple rectangular deck, one section is enough. For an L-shaped or multi-level deck, click "Add another section" and add each rectangle separately — the calculator sums them.
- 3 Set your board specs. Enter the actual board length you'll buy (commonly 8, 12, or 16 ft / 2.4, 3.6, or 4.8 m), the face width (5.5" / 140 mm is standard), and the gap you want between boards (1/8" / 4 mm is typical for drainage).
- 4 Choose joist spacing. Tap one of the chips — 16" / 400 mm is the residential standard. Use 12" / 300 mm for heavy-duty or diagonal boards, 24" / 600 mm only for very light loads.
- 5 Adjust waste and (optionally) price. 10% waste is a safe default. Tick "Include" under Cost estimate and type the per-board price for an instant board-cost total.
- 6 Read the green estimate card. It always shows total area, deck boards to buy, joist count and total joist length, and approximate screws. Bring this list to the lumber yard.
Frequently Asked Questions
How does the calculator decide how many deck boards I need? ▾
For each section it works out how many rows of boards fit across the width (deck width ÷ (board width + gap), rounded up) and how many board lengths it takes to cover the deck length (deck length ÷ board length, rounded up). Multiplying gives the raw board count; the waste percentage is then applied and the result rounded up.
What waste percentage should I use? ▾
5–10% is enough for a simple rectangular deck with straight boards. Add 10–15% if your deck has a picture-frame border, mitred corners, diagonal boards, or lots of cut-outs (stair landings, post wraps, hot-tub openings). Always round up — leftover boards are useful for repairs.
Why is the board width in inches but the section length in feet? ▾
Decking is sold using two scales: boards are measured in inches (face width like 5.5") and millimetres (like 140 mm), while deck dimensions are usually in feet or meters. The calculator converts automatically — just use the unit shown beside each input.
How is the joist count calculated? ▾
Joists run perpendicular to the deck boards. The calculator divides the deck length by the joist spacing, rounds up, and adds one for the end joist (joists at both edges). Each joist's length equals the deck width, so total linear length is joist count × deck width. For multi-section decks the result is the sum across sections.
Does the calculator handle L-shaped or multi-level decks? ▾
Yes — split the deck into rectangles and add each one as a separate section. An L- shape is two rectangles, a T-shape is three. Stair landings or a hot-tub platform can be their own sections too. The totals (area, boards, joists, screws) combine automatically.
Does it include posts, beams, or railings? ▾
No — this calculator focuses on the deck surface (boards), framing (joists), and fasteners (screws). Posts, beams, ledger boards, footings, railings, balusters, and stair stringers are sized from your deck's height, span, and local code. Use the results here as your boards-and-joists list and add structural items separately.
Is my data saved or sent anywhere? ▾
No. Everything runs in your browser. Your section dimensions, board specs, and prices never leave your device and nothing is stored on a server.