How it works
The calculator counts the places where a deck board crosses a joist and multiplies by the fasteners per crossing.
Rows = ⌈ (Deck side across the boards + Gap) ÷ (Board width + Gap) ⌉
Joists = ⌈ Board run ÷ Joist spacing ⌉ + 1
Crossings = Rows × Joists
In boards mode: Crossings = Boards × (⌈ Board length ÷ Joist spacing ⌉ + 1)
Fasteners = ⌈ Crossings × Per crossing × (1 + Extra ÷ 100) ⌉
Boxes = ⌈ Fasteners ÷ Count per box ⌉
⌈ ⌉ means rounding up. Per crossing is 2 for face screws and 1 for hidden clips.
Where the numbers come from
Trex’s 2026 decking installation guide says to “always use two screws per joist or stair stringer” when face fastening, and to install hidden fasteners “on every joist”. One hidden clip sits in the gap between two boards at each joist, so the count of crossings is also the count of clips. Trex’s 90-clip box of Hideaway universal fasteners is listed as covering 50 sq ft with joists 16 in on center; the calculator’s count for that layout comes out a little below that, so the box figure includes some spare.
Worked example in words
A 16 × 12 ft deck with boards along the 16 ft side: the 12 ft (144 in) side holds (144 + 3/16) ÷ (5.5 + 3/16) = 25.35, so 26 rows. Along 16 ft (192 in) with joists 16 in on center there are 12 spaces and 13 joists. 26 × 13 = 338 crossings, and with two screws at each, 676 screws.
What is not included
Start clips and the face screws at the first and last board, picture-frame and border boards, fascia, stairs and butt joints all add fasteners. The joist spacing and the framing itself come from your plans – this calculator only counts fasteners.
Worked example
Inputs
- Count by
- Deck size
- Deck length
- 16 ft
- Deck width
- 12 ft
- Boards run along
- The deck length
- Board width
- 5 1/2 in
- Gap between boards
- 3/16 in
- Joist spacing (on center)
- 16 in
- Fastening
- Face screws (2 per joist)
- Extra fasteners
- 0 %
Result
- Deck area
- 192 sq ft
- Rows of boards
- 26
- Board–joist crossings
- 338
- Screws or clips
- 676
Common mistakes
- Counting one screw per joist. Face-fastened composite decking takes two screws at every joist; Trex’s guide says so in so many words.
- Forgetting the joists at both ends. A 16 ft board on joists 16 in on center crosses 13 joists, not 12.
- Buying hidden clips by area without checking the joist spacing. Trex’s box coverage of 50 sq ft assumes joists 16 in on center; closer joists need more clips.
- Leaving out stairs, picture-frame borders and fascia. Each has its own fastening – count them separately.
- Using screws that are not meant for the decking. Composite and treated wood need screws approved by the board maker and corrosion-resistant for treated lumber.
Assumptions and limits
- Boards run in straight rows at right angles to the joists, with one fastener position wherever a board crosses a joist.
- In deck mode, the number of rows is the deck side across the boards divided by board width plus gap, rounded up; the number of joists is the board run divided by the spacing, rounded up, plus one.
- In boards mode, each board crosses its length divided by the joist spacing, rounded up, plus one joist.
- Face fastening uses two screws per crossing, hidden fastening one clip per crossing. Extra joists at butt joints, stair stringers and borders are not counted.
- Boxes are rounded up to whole boxes after any extra.
Frequently asked questions
How many screws do I need for a 16 × 12 deck?
With 5.5 in boards, a 3/16 in gap, boards along the 16 ft side and joists 16 in on center, there are 26 rows of boards crossing 13 joists – 338 crossings. At two screws per joist that is 676 screws.
How many deck screws per joist?
Trex’s 2026 installation guide says to always use two screws per joist or stair stringer when face fastening. Wood decking and other brands have their own guides – check the one for your boards.
How many hidden clips do I need per square foot?
Trex says a box of 90 Hideaway universal clips covers 50 sq ft when the joists are 16 in on center – about 1.8 clips per sq ft. The calculator counts the clips from the actual rows and joists instead, which also works for other spacings.
How many screws come in a box?
It depends on the screw length and the package, so the calculator does not assume a number. Enter the count printed on the box to get the number of boxes.
Sources
- 2026 Trex Decking Installation Guide (pp. 27, 34) — Trex Company. Face screws: "Always use two screws per joist or stair stringer" (p. 34); hidden fasteners: "Continue to install fasteners on every joist" (p. 27).
- Trex Hideaway Universal Hidden Fasteners (90 Pack) — Trex Company. "Box contains 90 universal fasteners and 1 star bit"; "Clips cover 50 square feet of decking when deck boards are installed 16-inches on-center".
- 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.
- NIST Handbook 44, Appendix C – General Tables of Units of Measurement — National Institute of Standards and Technology. Exact unit definitions: 1 in = 2.54 cm, 1 yd³ = 27 ft³, 1 US gal = 231 in³, 1 lb = 0.45359237 kg.
Method reviewed November 5, 2026