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Deck · US units

Deck Calculator — Boards and Joists

Nominal 5/4×6 is not 5.25 × 6. Home Depot pressure-treated decking and Trex both use a 1.0 × 5.5 in actual face. With no gap that is 12 ÷ 5.5 = 2.182 linear feet of board per square foot.

Lumber prices null · 2026-08-27

Rectangular deck

ft

Board count comes from this direction.

ft
ft
in

Fixture uses 0 gap → 2.182 LF/sq ft. Add 1/8 in if your install requires it.

Actual 5/4×6 = 1.0 × 5.5 in (Home Depot PT + Trex). Joists: 16 in OC along the length, spanning the width, plus two rim boards.

in OC
Waste factor

Trex’s published rule of thumb is 5%. Some retailer guides use 10% for straight layouts and 15% for diagonal — not a lumber TDS.

$

PT 5/4×6 national HD price was not verified. Field stays empty on purpose.

$

PT 2×8 national HD price was not verified. 4×4×8 posts are $10.98 in the index; joists are not.

Boards to buy

29 boards

27 before waste · 2.182 LF/sq ft · 192 sq ft

Cost: enter a local price to see a dollar total. We will not invent one.

Decking linear ft
464
Joists
13 × 12 ft
Rim boards
2 × 16 ft
Framing linear ft
188

Board and joist dollars stay blank until you type a local price. We will not invent a zip-localized lumber ticket.

What calculator.net does not show

Trex publishes the 5% extra rule and the 144 ÷ 5.5 = 27 board example for a 12 ft width. Some retailer measuring guides instead use 10% for a straight layout and 15% for diagonal. Those are not interchangeable with a PT mill TDS, so they are toggles. Pressure-treated 5/4×6 boards and 2×8 joists did not have a verifiable national Home Depot price on 2026-08-27 — zip-localized. The fields start empty. The 4×4×8 PT post in the index is $10.98; that is not a joist.

Worked US example: 12×16 deck, 16 ft boards, no gap

Area           = 12 × 16 = 192 sq ft
Rows           = 12 ft × 12 in ÷ 5.5 in = 26.182 → 27 boards
Before waste   = 27 boards (16 ft each)
Trex 5%        = 27 × 1.05 = 28.35 → buy 29 boards
10% straight   = 27 × 1.10 = 29.7 → 30 boards
15% diagonal   = 27 × 1.15 = 31.05 → 32 boards
Joists 16 in OC along 16 ft, spanning 12 ft
               = floor(192 / 16) + 1 = 13 joists × 12 ft = 156 lf
Rims           = 2 × 16 ft = 32 lf
Framing total  = 188 lf
Board $ and joist $ = user local price only

This is a rectangular takeoff, not a structural design. Confirm joist size, span, and footing schedule against IRC tables and your permit office. Composite gap and end-gap rules differ from PT; set the gap field if your manufacturer requires it.

Citations

  1. Trex. How Many Deck Boards Do I Need?. Accessed 2026-08-27. https://www.trex.com/deck-ideas/how-many-deck-boards-do-i-need-/ — Actual 5.5 in face width for typical 5/4×6 / 1×6 boards. General rule: 5% extra for waste. Trex’s own 12 ft width example: 144 / 5.5 = 26.18 → 27 boards; 5% extra rounds to 29.
  2. The Home Depot. WeatherShield 5/4 in × 6 in pressure-treated decking — actual size. Accessed 2026-08-27. https://www.homedepot.com/p/WeatherShield-5-4-in-x-6-in-x-16-ft-Standard-Ground-Contact-Pressure-Treated-Southern-Yellow-Pine-Decking-Board-253919/206968440 — Actual product thickness 1 in; actual product width 5.5 in. Matches Trex 5.5 in face width. No national online price (zip-localized).
  3. Composite Deck Direct / retailer how-tos. Composite decking measuring guides (10% straight / 15% diagonal). Accessed 2026-08-27. https://compositedeckdirect.com/blogs/composite/composite-decking-measuring-guide — Some retailer guides use 10% waste for straight layouts and 15% for diagonal. Distinct from Trex’s 5% rule of thumb. Offered as optional toggles, not as a manufacturer TDS.

These calculators estimate material takeoff for planning. They are not engineering, structural, or code advice. Confirm spans, mixes, fastener schedules, and permits with a licensed professional and the manufacturer’s current data sheet. Local shelf prices and tax are not this index.