A deck looks like a rectangle of boards but it’s really four layers stacked on top of each other — boards, joists, bearers, and posts on footings. Get any one layer’s spacing wrong and the math for the others falls apart. The Chippy Tools decking calculator handles the math for all four layers from a single deck length × width input — and returns the area, board count, linear metres of boards (with and without 10% waste), joist count, total joist length, bearer count, total bearer length, and even an estimate of the screws you’ll need.
What the decking calculator does
The Chippy Tools decking calculator takes six inputs:
- Deck length and deck width — the overall deck dimensions
- Board width — the width of one decking board (commonly 90mm, 140mm, or 5.5"/1×6 nominal)
- Board spacing — the gap between adjacent boards (typically 3–6mm or 1/8" to 1/4")
- Joist spacing — on-center spacing of the joists running perpendicular to the boards (typically 450mm or 16")
- Bearer spacing — on-center spacing of the bearers (beams) running under the joists (typically 1.8m or 6 feet)
And returns eight outputs:
- Deck area — total square metres or square feet
- Linear metres of boards — the total board length you need
- Linear metres with 10% waste — same figure with waste already added
- Joist count — total number of joists
- Total joist length — total linear metres of joist material
- Bearer count — total number of bearers (beams)
- Total bearer length — total linear metres of bearer material
- Screw count — estimate based on 2 screws per board per joist crossing
The app runs locally on iOS and Android with no internet required. Defaults match Australian convention (450mm joists, 1800mm bearers, 5mm board gap) in metric mode, and US convention (16" joists, 72" bearers, 1/4" gap) in imperial mode.
Deck anatomy
A residential deck is a structural sandwich. From top to bottom: deck boards form the walking surface; joists run perpendicular to the boards and carry the boards; bearers (beams) run perpendicular to the joists and carry them; posts stand vertically and carry the bearers; concrete footings spread the load from the posts into the ground.
In the diagram: (1) deck board, (2) joist, (3) bearer (beam), (4) post, (5) footing.
The Chippy Tools decking calculator handles the top three layers — boards, joists, and bearers. Posts and footings are calculated separately because their layout is structural and depends on local span tables and soil conditions; for the post-spacing math, use the Equal Spacing Calculator and confirm with your local council.
How to calculate deck boards
Board count = deck width ÷ (board width + board spacing), rounded up.
For a worked example, a 4.0m wide deck using 90mm boards with a 5mm gap divides into 4000 ÷ 95 ≈ 42.1 → 42 boards (or 43 if you allow the extra board’s gap to be absorbed into a wider end gap). Multiply by the deck length to get total linear metres: 42 × 6.0m = 252 linear metres of decking. The Chippy Tools calculator returns 252m and 277m (with 10% waste) — order the bigger figure to allow for cuts, offcuts, and rejected boards.
For an imperial example, a 20-foot wide deck using 5.5" (1×6) boards with a 1/4" gap divides into 240" ÷ 5.75 ≈ 41.7 → 42 boards. Multiply by the deck length and add waste, exactly as in the metric workflow.
How to calculate deck joists
Joist count = (deck width ÷ joist spacing) + 1, rounded up. The “+ 1” accounts for the joist at each end of the deck — N joists make N − 1 spans, so to make M spans you need M + 1 joists.
For the same 4.0m wide deck at 450mm joist spacing: (4000 ÷ 450) + 1 = 8.9 + 1 → round to 10 joists. Multiply by the deck length: 10 × 6.0m = 60 linear metres of joist material. Chippy Tools returns the joist count and total joist length directly.
Joist spacing varies by board material. Hardwood and treated pine typically use 450mm in Australia (16" in the US). Composite decking usually requires tighter 300mm (12") spacing because composite boards sag more between supports than timber. Always check the board manufacturer’s datasheet — getting joist spacing wrong is a common cause of squishy or wavy decks years after install.
How to calculate deck bearers (beams)
Bearer count = (deck length ÷ bearer spacing) + 1, rounded up. Same logic as joists, just running perpendicular and supporting from underneath.
For a 6.0m long deck at 1.8m bearer spacing: (6000 ÷ 1800) + 1 = 3.3 + 1 → round to 5 bearers. Multiply by the deck width to get total bearer length: 5 × 4.0m = 20 linear metres of bearer material.
Bearer spacing is the most code-sensitive of the three. The bearer carries all the joist load above it, transferred down to posts and footings below. Standard residential spacing is 1.8m (Australia) or up to 6 feet (US), but always confirm with span tables for your specific bearer size and timber grade — under-sizing bearers leads to deflection and the deck “bouncing” underfoot.
Calculate Decks On the Job Site
Download Chippy Tools and use the decking calculator on iOS or Android. Boards, joists, bearers, screws — all from a single deck length × width input. Offline, fast, and accurate.
Deck screws and fasteners
The Chippy Tools decking calculator estimates screw count using the standard rule: 2 screws per board per joist crossing. For a 6.0m × 4.0m deck with 90mm boards at 450mm joist spacing, the screw count works out to about 1,120 screws. Order at least 10% extra for dropped fasteners, stripped heads, and the predawn moments when you snap a screw and have to back it out.
The estimate assumes face-fixed screws. Hidden-fastener systems (composite-board clips, edge-grooved boards) typically use 1 fastener per board per joist crossing — half the face-fix figure. For boards over 140mm wide, manufacturers sometimes specify 3 screws per joist crossing instead of 2; in that case, multiply the calculator’s estimate by 1.5.
Screws should be appropriate to the board and joist material — stainless or coated decking screws for treated pine, polymer-headed screws for composite, and silicon-bronze for premium hardwoods that bleed tannins. The screw count is independent of the head type or material — just the count.
Deck board spacing and patterns
Board spacing serves two purposes: it allows the boards to expand and contract with humidity (5mm in metric, 1/4" in imperial is standard), and it lets water drain off the deck instead of pooling. Tighter gaps trap moisture and accelerate rot; wider gaps look “rustic” but become a magnet for splinters and weed growth between the boards.
For straight running boards (the most common pattern), enter the deck length as one continuous run and let the calculator return the total linear metres. For staggered (brick-pattern) layouts, the linear metres figure is the same but you’ll order longer board lengths to allow for the staggered cuts — the offcut from board A becomes the start of board B in the next row.
For diagonal or chevron patterns, the calculator’s straight-board figure is roughly correct but will under-estimate by about 5–10% depending on the angle. Treat the diagonal figure as a minimum and add a buffer if you’re using a more elaborate pattern.
Posts, footings, and balustrade — what the calculator doesn’t cover
The Chippy Tools decking calculator handles the deck surface and the framing immediately below it (boards, joists, bearers). Three structural elements that come before and after — posts, footings, and balustrade — sit in adjacent calculators:
- Post layout — equal spacing along the bearer line. Use the Equal Spacing Calculator — set the bearer length as the run and the target post spacing (typically 2.4m to 3.0m for domestic decks) as the spacing.
- Footings — the concrete pads under each post. Use the Concrete Calculator for the volume of concrete per footing once you’ve picked the footing dimensions from your local span tables.
- Balustrade and stair handrails — safety-critical baluster spacing for any deck above ~600mm. Use the Baluster Spacing Calculator for the 100mm/125mm-sphere code requirement.
The four calculators pair in the same Chippy Tools app — calculate deck boards, posts, footing concrete, and balustrade in one workflow without re-entering measurements.
One App, Whole Deck
Decking, posts, footings, balustrade, and stair stringers — Chippy Tools handles the whole deck workflow on iOS and Android. Download once, calculate anywhere — no internet required.
Decking materials
The calculator is material-agnostic — the same length × width × board width × gap math applies regardless of what you’re laying. Common options:
- Treated pine — H3 or H4 LOSP-treated softwood; 90mm or 140mm boards at 450mm joist spacing. Cheapest option; needs annual oil/stain.
- Hardwood — spotted gum, blackbutt, ironbark, jarrah, merbau; 90mm or 140mm boards at 450mm joist spacing. Mid-cost, premium look.
- Composite — Trex, ModWood, NewTechWood and similar; 90mm to 145mm boards at 300mm joist spacing. Higher upfront cost, near-zero maintenance.
- Aluminium / steel — proprietary systems with their own joist spacing specs.
For each material, plug the actual board width, gap, and recommended joist spacing into the calculator. The results — board count, linear metres, joist count, bearer count, screw count — are all material-independent calculations driven by the geometric inputs.
Imperial and metric units
Chippy Tools accepts millimetres, centimetres, metres, feet, inches, or feet and inches in any combination. The decking calculator defaults to 450mm joist spacing and 1.8m bearer spacing in metric mode, and 16" joist spacing with 72" (6 feet) bearer spacing in imperial mode. Override either default to match your local convention or the board manufacturer’s spec.
Decking is a particularly mixed-unit field — Australian builders often work in mm but buy boards by the linear metre, while North American builders work in feet and inches but buy by the board-foot. The calculator handles unit conversion internally so you can mix metric inputs with imperial outputs without losing precision.
Why use a decking calculator on your phone
The Chippy Tools app is built for tradespeople who need calculations on-site without internet. The decking calculator pairs in the same app with the Equal Spacing Calculator, Baluster Spacing Calculator, Stair Calculator, and Concrete Calculator — calculate the deck surface, the post layout, the balustrade, the stair stringers, and the footing concrete in one workflow without re-entering measurements.
Web decking calculators break in remote backyards with patchy 4G. The Chippy Tools calculator runs locally — the answer is on screen in under a second. Switch between metric and imperial without restarting and save common board/joist/bearer presets for the materials you use most often.
Related calculators
- Equal Spacing Calculator — for deck post spacing along the bearer line
- Baluster Spacing Calculator — for the deck balustrade and stair-handrail spindle spacing
- Stair Calculator — for deck stairs from the deck down to ground level
- Concrete Calculator — for the volume of concrete per footing pad