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Calculation details
Planning estimate only. Verify dimensions, waste, coverage, product specifications, and local requirements before purchasing materials.
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What does this calculator estimate?
Decking calculator: estimate deck boards and linear footage needed from deck dimensions, board width, gap spacing, and waste. Free, private, no signup.
How this calculator works
Worked example
A 20 x 12 ft deck with 5.5-inch boards and 1/8-inch gaps: 26 courses, 520 linear feet, 572 feet with 10% waste, or 48 twelve-foot boards.
Frequently asked questions
What is a decking calculator and how does it work?
A decking calculator is a free, browser-based tool that estimates the materials you need for your deck project. You input your deck dimensions and it calculates the square footage, board lengths, and fastener quantities, no signup required and all processing happens in your browser.
Is my personal data safe when using a decking calculator?
Yes. This decking calculator runs entirely in your browser with no server-side data collection. No signup is needed, nothing is stored or transmitted, and your measurements stay private on your device.
Can I use the decking calculator for any deck shape?
The standard decking calculator works best for rectangular decks. For L-shaped or irregular decks, break the area into rectangles, calculate each section separately, and add the totals together for an accurate material estimate.
Cite this tool
BoringToolsKit. “Decking Calculator.” boringtoolskit.com/decking-calculator/. Free to reference in articles, syllabi, and answer posts with a link.
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Worked example: a 10 by 12 foot deck
Take a deck 10 ft wide by 12 ft long, 5.5 inch wide boards, a 0.25 inch gap between boards, 12 ft boards, and 10% waste. The formula works in inches across the width: 10 ft x 12 = 120 inches of deck to cover. Each board plus its gap consumes 5.5 + 0.25 = 5.75 inches, so courses = ceil(120 / 5.75) = ceil(20.87) = 21. The gap matters here: without it, 120 / 5.5 = 21.82 would also round up to 22 courses, so a real gap saved one row.
Linear feet is then courses x deck length: 21 x 12 = 252 linear feet. Adding 10% waste gives 252 x 1.10 = 277.2 linear feet. Finally, boards required = ceil(277.2 / 12) = ceil(23.1) = 24 boards at 12 ft each. Notice the waste add did not change the board count in this case (23 boards without waste also rounds to 24), but on a 16 ft board run it often does: 277.2 / 16 = 17.33 rounds to 18 boards either way, while a case landing at exactly a whole number, like 288 linear feet on 12 ft boards, buys 24 boards with no waste and 27 with 10% waste.
One subtlety: the calculator counts courses across the deck width and runs boards along the length. If your boards run across the deck instead, swap the two dimensions before entering them, otherwise the course count and linear feet will be wrong.
Boundary cases and what the formula does not cover
At 0% waste the result is a theoretical minimum with zero allowance for cut-offs, defects, or the board you split driving a screw. Composite decking vendors commonly suggest 5%, pressure-treated pine 10%, and diagonal or herringbone layouts 15% or more. Entering a negative waste percentage is accepted by the formula and silently shrinks the order: 252 linear feet at -10% becomes 226.8 ft, an order that cannot even cover the deck, so never enter a negative value.
The other fragile input is board length. The formula divides by max(0.01, boardLength), so entering 0 produces about 27,720 boards instead of an error; entering 1 ft is technically valid but yields 278 boards, which no supplier stocks. Match boardLength to what you can actually buy: 12, 16, and 20 ft are the common lengths, and buying a mix (for example 12s for fill-in pieces and 16s for long runs) is often cheaper but is not modeled, since the calculator assumes one uniform length.
The calculation also ignores framing lumber, joist count, fasteners, stair treads, and railing, and it assumes every course is a single full-length board. Decks wider than the longest board you can buy need butt joints or staggered seams, which add real waste the flat percentage does not capture. Treat the output as a board-count floor, then add your own judgment for layout complexity.