How it works
The calculator counts how many rows of boards fit across the deck, then how many boards each row needs, and adds your waste allowance.
Rows = ⌈ (Span + Gap) ÷ (Board width + Gap) ⌉
Rows per board = ⌊ Board length ÷ Run ⌋
Boards = ⌈ Rows ÷ Rows per board ⌉ if a board covers at least one row,
otherwise Rows × ⌊ Run ÷ Board length ⌋ + ⌈ Rows ÷ Pieces per board ⌉,
where Pieces per board = ⌊ Board length ÷ Leftover piece ⌋
Boards with waste = ⌈ Boards × (1 + Waste ÷ 100) ⌉
Linear length = Boards with waste × Board length
The run is the side the boards run along; the span is the other side. ⌈ ⌉ rounds up and ⌊ ⌋ rounds down. One gap fewer than rows fits between the boards, which is why the gap is added to the span before dividing. When a row needs joints, each row gets whole boards plus one shorter piece, and those pieces are cut from extra boards only as many times as they fully fit – an offcut that is too short is not counted as usable.
Worked example
Inputs
- Deck length
- 16 ft
- Deck width
- 12 ft
- Board direction
- Boards run along the deck length
- Board width
- 5 1/2 in
- Gap between boards
- 3/16 in
- Board length
- 16 ft
- Waste allowance
- 5 %
Result
- Deck area
- 192 sq ft
- Rows of boards
- 26
- Boards
- 28
- Total board length
- 448 ft
Common mistakes
- Using the nominal board width. A board sold under one width often measures less; use the actual face width from the product data.
- Leaving out the gap. Over many rows the gaps add up to a noticeable share of the deck width, and the manufacturer’s gap also matters for drainage and expansion.
- Buying boards that are slightly too short. A board exactly as long as the deck leaves nothing for trimming the ends; check whether you need the next length up.
- Forgetting picture-frame borders, stair treads and fascia boards. Count those separately.
Assumptions and limits
- The deck is a rectangle and the boards run straight, parallel to one side. Diagonal and herringbone layouts are not handled.
- If one board covers one or more full rows, each board is cut into as many full rows as fit and the rest is offcut. If boards are shorter than a row, offcuts are reused in other rows.
- Joist positions are not known, so the joint layout is not planned. Real joints must fall on framing, which can use more material.
- The waste allowance is applied to the board count and rounded up. Framing, fasteners and trim are not included.
Frequently asked questions
Which way should the boards run?
Choose the direction your framing is designed for – deck boards run across the joists. Pick “along the deck length” if the boards run parallel to the long side. The calculator then counts rows across the width.
What gap should I use?
Use the gap in your decking manufacturer’s installation guide. Trex’s guide specifies a 3/16 in (5 mm) side-to-side gap for its decking; other products, especially wood that may shrink, can differ.
Why is the linear footage more than the deck area suggests?
Linear footage is the number of boards × the board length you buy, including offcuts and the waste allowance. Dividing deck area by board width alone ignores the gaps and the unused ends of boards.
Can I mix board lengths?
The calculator uses one board length. To mix lengths, run it for the main length and adjust by hand, making sure every joint lands on a joist.
Sources
- 2026 Trex Decking Installation Guide (p. 24); Trex Enhance product page — Trex Company. Minimum width-to-width gap of 3/16 in (5 mm); Trex Enhance boards are 5.5 in wide.
Method reviewed November 2, 2026