Deck Board Calculator
Calculate how many deck boards, joists and fasteners a deck needs from its dimensions, including gap spacing, waste allowance, board length optimisation and total material cost.
Last reviewed by the Radiatus Cloud team
Need this done properly for your business?
Radiatus delivers secure cloud, DevOps & compliance engineering.
Boards, gaps and the arithmetic in between
Counting deck boards is not the deck width divided by the board width. Every board needs a drainage gap beside it, typically three to six millimetres for a composite board and a little more for timber that will shrink as it dries. Across a four metre deck those gaps consume most of a board's worth of width, so ignoring them leaves you short by one board and a trip back to the merchant.
Length matters more than area
Boards come in fixed lengths, and a deck slightly longer than a standard board means every run needs a joint, doubling the number of boards and requiring a joist under every joint. A deck a few centimetres shorter than the board length wastes almost nothing. That is why deck dimensions are worth adjusting to the material rather than the other way round, and why this calculator reports the offcut per board so an unfortunate dimension is obvious before anything is ordered.
Joists, fasteners and the rest of the list
Joist spacing follows the board: four hundred millimetre centres for most timber decking laid perpendicular, reduced to three hundred for boards laid diagonally or for composites that sag between wider spans. Fastener count follows from two screws per board per joist crossing, which adds up faster than people expect. All of it is included here alongside a configurable waste allowance, because the standard ten percent is too little on a deck with many cuts and too much on a simple rectangle.
Related tools
- User Agent Parser — Parse a User-Agent string into browser, engine, operating system and device. Explains why UA strings are unreliable and what to use instead.
- QR Code Generator — Generate QR codes for URLs, text, Wi-Fi and contact details. Adjustable error correction and size, produced entirely in your browser.
- Credit Card Validator — Validate a card number with the Luhn algorithm and identify the issuing network from its prefix. Runs locally, nothing is transmitted.
- Text Case Converter — Convert text between camelCase, PascalCase, snake_case, kebab-case, CONSTANT_CASE, Title Case and sentence case. Runs entirely in your browser.
Frequently Asked Questions
What gap should I leave between deck boards?
Timber laid green or damp shrinks, so a 3 to 5 millimetre gap is common. Kiln dried timber and composite boards do not shrink and need 5 to 6 millimetres for drainage and thermal movement. Check the manufacturer instruction for composites, since it affects the warranty.
How far apart should the joists be?
400 millimetre centres is standard for timber boards laid perpendicular to the joists. Reduce to 300 for a diagonal layout, since the effective span increases, and follow the manufacturer figure for composite boards, which is often 300 or less.
How much waste should I allow?
Ten percent for a simple rectangle, fifteen for a deck with cuts around posts or an angled edge, and twenty for a diagonal layout where every board ends in an angled cut. The calculator lets you set the figure rather than assuming.
How many screws will I need?
Two per board at every joist crossing is the standard for face fixed boards. Hidden fastener clips are one per crossing instead. Either way the quantity is larger than most people estimate, which is why it is worth counting.
Does the calculator handle a diagonal layout?
Yes. Selecting it increases the board length required by about 1.41 times, tightens the joist spacing recommendation and raises the default waste allowance, all of which a diagonal deck genuinely needs.
Privacy & Security
Everything runs in your browser; nothing is uploaded.
How to Use
Enter the deck dimensions and board size to get a full material list with waste allowance.
Disclaimer: This tool is provided "as is" without warranty of any kind. Results are for educational and utility purposes.
Related Tools
User Agent Parser
UtilityParse a User-Agent string into browser, engine, operating system and device. Explains why UA strings are unreliable and what to use instead.
QR Code Generator
UtilityGenerate QR codes for URLs, text, Wi-Fi and contact details. Adjustable error correction and size, produced entirely in your browser.
Credit Card Validator
UtilityValidate a card number with the Luhn algorithm and identify the issuing network from its prefix. Runs locally, nothing is transmitted.