Lumber & Decking Calculator,
down to the last board.

Field guide

How to calculate decking boards and why the gap matters more than you think.

Step 1: Measure your deck area accurately

For a rectangular deck, the area is simply length × width. Measure to the nearest inch or centimetre, and always measure the finished surface — not the subframe. If your deck has cutouts around posts or built-in planters, you can subtract their footprint from the total area.

For L-shaped or irregular decks, break the area into rectangles, calculate each section separately, and add them together. In general, it is better to measure slightly generously: running short of decking mid-project forces a second order that may not match the same lot or moisture content.

Step 2: Know your board's actual dimensions

Lumber is sold by nominal dimensions, not actual dimensions. A 2×6 does not measure 2 inches by 6 inches — it measures approximately 1.5 in × 5.5 in after milling. A 5/4×6 (the most popular decking profile) is typically 1 in × 5.25 in actual. Using the wrong width will throw off your board count by up to 10%.

Common decking boards and their actual widths:

• 2×4: actual 1.5 in × 3.5 in
• 2×6: actual 1.5 in × 5.5 in (most common)
• 5/4×6: actual 1 in × 5.25 in (lightweight, popular for residential decks)
• 2×8: actual 1.5 in × 7.25 in
• Composite boards: actual dimensions vary by brand — check the manufacturer spec sheet

Step 3: Choose the right gap

A spacing gap between deck boards is not optional — it is a structural requirement. Gaps allow water to drain off the surface instead of pooling, provide room for wood to swell with moisture changes, and prevent debris accumulation that accelerates rot.

Standard gap recommendations:

• Pressure-treated or green lumber: install boards touching or with a 1–2 mm gap. They will shrink as they dry, opening to a 4–6 mm gap naturally.
• Kiln-dried lumber: install with a 4–6 mm (3/16–1/4 in) gap. The wood is already at its minimum moisture content and will not shrink further.
• Composite decking: follow the manufacturer's specification exactly — it varies by product. Most specify 3–6 mm, and the gap also affects the warranty.

Even a 1 mm change in gap spacing across a 12-foot-wide deck adds or removes one full board every 3–4 feet of width. This is why gap selection meaningfully affects your material order.

Step 4: Always add a waste allowance

Board ends must be squared off, knots and defects must be cut out, and boards meeting the house ledger or stairs must be trimmed to fit. The standard recommendation is 10% extra for a straight-run layout, 15% for a diagonal or picture-frame border, and 5% only when an experienced carpenter is doing precision work with consistent stock.

Order extra of the same lot number (also called a batch or run number). Wood stain and grain variation differ between manufacturing batches; boards cut weeks or months later will rarely be an exact colour match, especially for pressure- treated wood that weathers inconsistently.

Structural framing is separate

This calculator covers surface decking only. A complete deck project also requires:

• Ledger board: a pressure-treated board bolted to the house rim joist to anchor the deck. Usually the same width as the joists.
• Beam: one or more horizontal beams spanning between posts, supporting the joists. Typically 2× or 4× lumber doubled or tripled.
• Joists: horizontal boards running from ledger to beam, spaced 12–16 inches on centre. Typical size is 2×8 or 2×10.
• Posts: vertical supports that carry the beams down to footings. Size and count depend on beam span and local code.
• Footings: concrete piers below frost depth. Calculate these separately with a concrete calculator.
• Hardware: joist hangers, post bases, structural screws, lag bolts, and flashing — often an underestimated cost.

Decking material comparison

The right decking material depends on budget, maintenance tolerance, and climate:

• Pressure-treated pine: lowest upfront cost, requires annual sealing to look good, lifespan of 15–25 years with maintenance.
• Cedar and redwood: naturally rot-resistant, lighter and more stable than pine, premium appearance, 20–30 year lifespan.
• Hardwoods (ipe, cumaru, tigerwood): extremely dense and durable, 25–40 year lifespan, but require pre-drilling and specialised fasteners. Higher cost.
• Composite decking: low maintenance, consistent colour, resistant to splinters and staining. Higher upfront cost but minimal annual upkeep. Most products carry 25-year warranties.
• PVC decking: fully synthetic, highly resistant to moisture and insects, but can feel hollow underfoot and is among the most expensive options.

Fastener options

Fastener choice affects both appearance and longevity. Surface screws (stainless or coated) are the easiest to install and easiest to replace. Hidden fasteners (clip systems installed between boards) give a cleaner look with no visible screw heads, but cost more and take longer to install. For composite decking, most manufacturers require their own proprietary hidden fastener system to maintain the warranty.

Allow approximately 2–3 screws per board end for surface-fastened decking. For a full deck project, fastener quantity can be calculated based on linear feet of decking and joist spacing.

Disclaimer

Results are estimates based on the dimensions you enter. Actual board count may vary due to board defects, site conditions, and cutting losses not covered by the waste percentage. Always verify quantities with your lumber supplier before ordering.