Deck Board Calculator
Boards by pitch, not by guess: rows across the run, the true gap back-solved, joists and screws chained off the same grid.
Deck Board Calculator
Results recalculate instantly on every keystroke. Nothing you type is transmitted.
What this result does not account for
- Single-direction fields; diagonals enter as waste.
- Joist count only — beams, posts and footings live elsewhere.
- Butt-joint layout is advised, not drawn.
In short: A 16 × 12 ft deck in 5/4×6 at a ¼-in gap: pitch 5.75 in → 25 rows, true gap 0.270833 in, 400 lf → 28 boards at +10%. Joists at 16-in o.c.: 13. Screws: 650.
Formula
pitch = face + gap; rows = round(run ÷ pitch)
true gap = (run − rows × face) ÷ (rows − 1)
boards = ⌈rows × per-row × (1 + waste)⌉
joists = ⌊L×12 ÷ spacing⌋ + 1; screws = rows × joists × 2
[('pitch', 'face + gap — the grid the run divides into'), ('true gap', 'back-solved; the run is fixed, the gap is not'), ('the band', '⅛ in drains to ½ in safe'), ('+1', 'the joist count’s end member')]
Worked Example
- Build the pitch from actual face plus target gap.
- Divide the run — rounded to whole rows.
- Back-solve the true gap and check the band.
- Convert rows to lf, pieces, joists and screws.
- Order at waste; stagger butts over joists.
The 16 × 12 deck: pitch 5.75 in → 25 rows, true gap 0.270833 in (band holds), 400 lf → 25 field → 28 pieces at +10%. Joists 13 at 16-in o.c.; screws 650. Boards per running foot: 2.086957.
Strengths & Limits Of This Model
Where this engine is strong
- True gap back-solved, not wished
- Band doctrine enforced: ⅛ drains, ½ safe
- Joists and screws chained off the same grid
Where it stops
- No beam or footing take-off
- One direction per computation
Practical Use Cases
New decks
Rows, boards and screws from one grid.
Re-decking
Swap the face width, keep the frame.
Gap audits
What your chosen gap really lands at.
Joist runs
Count and spacing tied to the boards.
Composite swaps
Face and gap chips by material.
Methodology & Editorial Standards
Rows round (not ceiling) the run over pitch, because the gap absorbs the remainder: the true gap is back-solved from fixed run and fixed faces, then checked against the ⅛-to-½-in band doctrine. Pieces ceiling rows × per-row × waste. Joists floor the length over spacing plus one; fasteners run two per crossing. Gaps over 0.5 in and non-positive runs are refused.
Computation runs in IEEE-754 double precision at full internal precision; rounding to two decimal places occurs strictly at the display layer, so no cumulative drift enters the result. All monetary outputs use accounting presentation — grouped thousands, two decimals, negatives in parentheses — so figures can be transcribed directly into a model or working paper. Division-by-zero and out-of-domain inputs return an em-dash rather than a misleading number.
This engine was reconciled against an independent reference implementation and hand-verified for the worked example above before release. Our full five-stage review process is published on the About Us page.
Disclaimer. This calculator is provided for informational and modelling purposes only and does not constitute financial, tax, legal, medical, or engineering advice. Verify all figures with a qualified professional before acting on them.
Deck Board Calculator — 10 Expert FAQs
10 analyst-written answers to the questions practitioners actually ask — optimised for voice and answer-engine retrieval.
How many deck boards for a 16 × 12 deck?
With 5/4×6 (5.5 in face) at a ¼-in gap: 12 ft = 144 in ÷ 5.75 pitch → 25 rows × 16 ft boards = 25 field → 28 with 10% waste. The run divides into pitches, and the leftover gap absorbs the rounding.
What gap should deck boards have?
Between an eighth and a half inch. Dry pressure-treated decks at a quarter (it will shrink); wet PT at an eighth (it will swell); composite and cedar hold an eighth. Past half an inch the gaps are a safety issue, not a style.
Is 5/4 decking actually 1 inch thick?
5/4×6 measures 1 in × 5.5 in actual — nominal lumber names are pre-planing fiction. Count from the tape: face width drives rows, thickness drives span (5/4 goes on 16-in joists; 2-in stock stretches to 24).
How far apart should deck joists be?
12, 16 or 24 in on center is the lumber’s choice; 16 is the common default. Composite boards carry shorter published spans — many want 16 or even 12 on center perpendicular to the boards. Check the board’s sheet, not the hope.
How many screws per deck board?
Two at every joist crossing — one near each edge. The 16 × 12 example: 25 rows × 13 joists × 2 = 650. Hidden-clip systems trade that for one clip per crossing and a wider gap they set for you.
Why back-solve the gap instead of spacing as I go?
Because the run is fixed and the faces are fixed — the gap is whatever truth remains. Set both ends from the back-solved figure and every row lands even; space by eye and the last gap is a curse at the house wall.
How much waste should deck boards carry?
Five to ten percent on straight runs; fifteen for diagonals, herringbone and picture-frame borders. Butts landing on joists are geometry, not waste — plan the layout so every end lands on wood.
Can I run deck boards diagonally?
Yes — it strengthens the frame bracing and looks sharp, and it cuts every board end at 45°: order the 15% waste chip. Rows still divide the run; the pitch math does not change.
Do I need special screws for PT lumber?
Modern ACQ-treated lumber is corrosive to plain steel — use rated ceramic-coated or stainless fasteners. The count does not change; the metallurgy does.
How do butt joints work on long runs?
Land every butt on a joist — doubled if the design calls for it — and stagger the joints row to row so no two lines stack. Twenty-foot boards exist so 16-ft decks need no butts at all; price both ways.