How the deck estimate works
Measure the deck length in the direction the boards will run. Width is the dimension across those boards. Enter the actual face width of your selected product, stock length and specified gap. Switching between metric and imperial converts dimension inputs so the physical project stays the same.
Boards per row = round up (deck length ÷ stock board length)
Order boards = round up (rows × boards per row × (1 + waste ÷ 100))
A worked example
For a 4 m × 3 m deck, 140 mm boards, 5 mm gaps and 4 m stock lengths, the estimate is 21 rows and 21 boards before waste. At 10% waste, order 24 boards. This may be conservative if a cut plan lets you reuse offcuts. The final row may need ripping to fit.
What the framing numbers mean
The indicative number of joist lines is the deck length divided by your spacing, rounded up, plus one edge line. Multiply this by deck width for a linear quantity. Fixings are rows × crossings × your selected fixings per crossing, with two extra crossings per butt joint and the waste allowance, then rounded to packs. This is a screw-planning model; proprietary clip systems need their manufacturer’s fastening schedule. Joist widths and end setbacks are not modelled.
Do not use these quantities to size structural timber or decide support locations. A real frame needs a design for spans, loads, ground conditions, connections and local rules. Extra joists at board joints, blocking and picture-frame borders may materially change the quantities.
Before you order
- Confirm board orientation, actual dimensions and the cut plan.
- Check end-to-end gaps, side gaps and temperature requirements for the chosen product.
- Price posts, beams, footings, rails, stairs, fascia and delivery separately.
- Confirm the right fastener type and corrosion rating for your materials.
Product guidance: Trex board-quantity planning and Trex board-spacing guidance. These references are manufacturer-specific; use the instructions supplied with your own product.
Common questions
Does this include the entire deck structure?
No. The board estimate covers straight rectangular decking. Framing and screw counts are indicative. Posts, beams, foundations, stairs and railings need separate planning.
Can I use this for a diagonal deck?
The calculation assumes straight boards. A diagonal pattern, borders or irregular shapes need a specific layout and cutting plan; a higher waste percentage alone does not resolve every cut.
Why did changing the board length increase the order?
Each row is rounded up to full stock boards. The estimate does not share offcuts across rows, so short stock boards or awkward lengths can increase the conservative order.