Decking calculator

DIY complexity

Advanced

Building the structural frame (joists, beams, posts, footings) requires accurate levels and secure fixings, and incorrect spacing or footing depth can compromise safety over time.

Introduction

This calculator estimates the boards, joists, beams, posts, footings and fixings needed to build a timber deck, from a small ground-level platform to a large raised family deck. It works from the deck's length and width, the board width and gap you select, and the length of stock boards your supplier sells, because deck boards almost always need to be cut and joined and rarely divide evenly into a plan. Board direction matters: decking boards normally run across (perpendicular to) the joists, so the board-direction span and the joist-direction span are handled separately, not as one flat area. Below the boards, the tool sizes the substructure independently — joist spacing, beam spacing and post spacing follow structural rules, not the board layout — along with footing diameter and depth, since frost depth and soil bearing vary by region and drive how deep a footing needs to be. Guard rails are flagged once deck height crosses typical trigger heights used in many jurisdictions. Results are rounded up to whole boards, joists and bags of concrete after your chosen waste allowance. It cannot verify structural loading, local frost depth, or replace a site-specific engineering check.

Quick answer

For a 5 × 4 m deck with 140 mm boards, a 6 mm gap and 8% waste, running boards across the 4 m width needs about 43 boards of 3.6 m stock length, plus the independently sized joists, beams, posts and footings.

Calculation methodology

01Enter the deck's overall length and width, and decide which direction the boards will run (usually across the shorter span, perpendicular to the joists).
02Calculate the number of board rows needed to cover the width, using board width plus gap as the row pitch.
03Calculate the total linear board length required per row from the board-direction span, then divide by the stock board length to get pieces per row, rounding up for cut losses at joints.
04Apply the waste percentage to absorb offcuts, damaged boards and staggered end joints, then round the total up to whole boards.
05Size the substructure independently: joist count from joist spacing across the width, beam count from beam spacing, post count from post spacing along each beam, and footing volume from footing diameter and depth.
06Add fixings (screws or clips per board), fascia boards and, where deck height exceeds the relevant trigger height, a guard rail allowance.

Formulas

boards = ceil(rows × board_direction_length × (1 + waste) ÷ stock_length)

The number of board rows crossing the deck's width is multiplied by how far each row must span, inflated by waste for cuts and joints, then divided by the stock board length to get whole boards to buy.

Variables

rows = number of board rows = width ÷ (board_width + gap), rounded up
board_direction_length = span each row must cover (m or ft)
waste = cutting and joint allowance (%)
stock_length = length of one board as sold (m or ft)
boards = whole boards to purchase

Rounding rule: Board count is always rounded up to a whole board per row segment — a deck cannot be finished with a fractional board, and joints must land on solid support.

Limitation: Assumes a rectangular deck with boards running in one consistent direction; picture-frame borders, diagonal patterns or curved edges need to be measured and entered as separate zones.

posts = ceil(beam_length ÷ post_spacing) + 1 per beam; footing_volume = π × (foot_dia ÷ 2)² × foot_depth

Joists, beams and posts are counted from their own maximum allowable spacing, not from the board layout above them. Each footing's concrete volume is calculated from its diameter and depth as a cylinder.

Variables

beam_length = length of the beam a row of posts supports (m or ft)
post_spacing = maximum allowable distance between posts (m or ft)
foot_dia = footing diameter (m or ft)
foot_depth = footing depth, driven by local frost depth and soil bearing (m or ft)
posts = whole posts along that beam

Rounding rule: Post and joist counts round up to whole units; footing volume is not rounded to a unit but is later converted to whole bags of pre-mixed concrete or delivery volume.

Limitation: Maximum joist, beam and post spacing, and required footing depth, depend on board thickness, species, span tables and local frost line — these are not universal constants and must come from the selected product's span table and local building code.

Inputs that matter

Board direction and widthDetermines how many rows are needed across the width and how far each row must span lengthwise; changing direction changes both numbers.
Board gapAdds to the effective row pitch for drainage and expansion; even a few millimetres per gap changes the row count on a wide deck.
Stock board lengthSuppliers sell fixed lengths (commonly around 3.6 m or 4.8 m); a mismatch between span and stock length drives extra joints and waste.
Joist, beam and post spacingStructural spacings set by span tables for the chosen timber, not by the board layout — using a generic spacing instead of the product's span table risks under-building the frame.
Footing diameter and depthDepth must reach below the local frost line and onto adequate bearing soil, both of which vary by region and are not something the calculator can determine for you.
Deck height and guard rail triggerAbove a jurisdiction-specific height, a guard rail (and often a different baluster spacing) becomes required; check the local threshold rather than assuming the calculator's flag is a legal ruling.

Worked examples

Ground-level deck, 5 × 4 m

A 5 m long, 4 m wide ground-level deck, 140 mm boards with a 6 mm gap running across the 4 m width, 3.6 m stock boards, 8% waste.

rows = ceil(4000 ÷ (140 + 6)) = ceil(27.4) = 28 rows. Each row spans the 5 m length: pieces per row = ceil(5 ÷ 3.6) = 2, so 28 × 2 = 56 raw pieces before waste share adjustment; applying waste to the linear total (28 × 5 = 140 m × 1.08 = 151.2 m) ÷ 3.6 m = ceil(42) = 43 boards.

56 pieces is the theoretical count from simple per-row division; 43 boards is the optimised purchase quantity once cut lengths are pooled across rows and waste is applied to the linear total rather than row by row — the actual on-site cutting plan should still be checked by whoever lays out the joints.

⚠ 43 boards assumes joints are pooled efficiently across rows on a straight rectangular deck; a real cutting plan with staggered joints on joists may need slightly more boards than this simplified count.

Raised family deck with full structure, 6 × 4 m

A 6 m × 4 m raised deck (24 m² plan area), joist spacing 0.4 m, beam spacing 2 m, post spacing 2 m, footing diameter 0.35 m and depth 0.7 m, guard rail required around the perimeter.

Joists across the 4 m width: ceil(4 ÷ 0.4) + 1 = 11 joists. Beams along the 6 m length: ceil(6 ÷ 2) + 1 = 4 beam lines. Posts per beam line: ceil(4 ÷ 2) + 1 = 3, so 4 × 3 = 12 posts. Footing volume each = π × (0.35 ÷ 2)² × 0.7 ≈ 0.067 m³; total footing concrete ≈ 12 × 0.067 = 0.81 m³.

The +1 in the joist and post formulas reflects that a run divided into n bays needs n+1 supports at the ends and between; 11 joists and 12 posts are whole-unit structural counts, separate from the 43-style board count and from the 0.81 m³ of footing concrete, which is then converted to whole bags or a delivered volume.

⚠ 0.4 m joist spacing, 2 m beam and post spacing, and 0.7 m footing depth are editable example values, not universal defaults — they must come from the chosen timber's span table and the local frost depth and soil bearing, which this calculator cannot determine.

Board and concrete purchase with local pricing

Using the 43-board simple deck from the first example, boards are sold individually at a local price, and footing concrete is bought as 25 kg bags each yielding about 0.0125 m³ of mixed concrete, for a total footing volume of 0.3 m³ across 4 footings of 0.075 m³ each.

Bags needed = ceil(0.3 ÷ 0.0125) = 24 bags. If one board costs the local currency's unit price P and one bag costs Q, total material cost = 43 × P + 24 × Q, before fixings, joists, beams, posts and fascia are added as separate line items.

24 bags for 0.3 m³ leaves a small surplus because bags round up to whole units, which is expected and not a calculation error; this is a material and concrete cost only, separate from labour, delivery and any permit fees.

⚠ Changing the selected currency only changes the displayed symbol — it does not convert a price you already entered. Enter current local supplier prices per board and per bag in the currency you have selected, and confirm sales tax separately.

Reading the results

The results separate the board count (surface finish) from the substructure counts (joists, beams, posts) and from the footing concrete volume, because each follows its own spacing rule or formula rather than one combined area calculation. A board count that looks high relative to the plan area is normal once gaps, stock lengths and waste are factored in — it is not a sign of double-counting. Footing concrete is shown as both a raw volume and a rounded bag count, so it stays clear which figure is the theoretical requirement and which is what you actually buy.

Common mistakes

Treating the board count as a simple area-over-coverage division instead of accounting for board direction, gap and stock length separately.
Copying joist, beam or post spacing from an unrelated project instead of using the span table for the actual board thickness and timber species chosen.
Setting footing depth from habit or a rule of thumb rather than the local frost line and soil bearing capacity for the specific site.
Forgetting that boards normally run across the joists, not parallel to them, which flips which dimension is the 'board-direction length' and which drives row count.
Ignoring the guard rail requirement once deck height crosses the local trigger height, or assuming a generic baluster spacing is compliant everywhere.
Not staggering board end joints over different joists, which concentrates all butt joints on one line and weakens the deck surface even though the calculator's board count is correct.

Adapting to your country

Deck board profiles, widths, stock lengths and fixing systems vary significantly by manufacturer and region — composite, hardwood and softwood boards each have different recommended joist spacing, gap and fixing requirements from their own technical data sheet. Metric markets typically size boards in millimetres and sell stock lengths such as 3.6 m, 4.2 m or 4.8 m; imperial markets commonly sell 8, 12 or 16 ft lengths. Concrete for footings is sold as bagged pre-mix with a stated yield per bag, or as ready-mix by volume, and packaging sizes differ by supplier and country. Selecting a different currency changes only the displayed symbol — enter current local supplier prices for boards, timber, fixings and concrete, and confirm applicable sales tax separately since it is not built into the calculator's cost output.

Limitations and verification

This calculator produces a planning estimate, not a structural design, engineering certification or contractor quotation. It does not verify joist, beam or post span capacity against actual loads, wind or seismic requirements, connector and fixing specifications, or compliance with local building regulations. Footing depth and diameter you enter are used to compute volume only — the calculator cannot determine the correct depth for your site's frost line and soil bearing capacity. Guard rail and stair requirements vary by jurisdiction and must be verified locally. Always confirm span tables, spacing, footing design and permit requirements with a qualified professional and your local building authority before construction.

Last reviewed: 2026-08-24

Frequently asked questions

Do deck boards run in the same direction as the joists?

No. Boards normally run across (perpendicular to) the joists, which is why the calculator treats the board-direction span and the joist spacing as two independent inputs, not one.

Why is the calculated board count higher than plan area divided by board width?

Because gaps between boards, cut losses at end joints, and the mismatch between stock board length and the actual span all add material that a pure area division ignores.

How is footing depth determined?

The calculator uses the diameter and depth you enter to compute concrete volume, but it cannot determine the correct depth itself — that must come from the local frost line and soil bearing capacity, which vary by region.

Are joist, beam and post spacing based on the board layout?

No. They are structural spacings that come from a span table for the chosen timber species, thickness and grade, entered independently of how the boards are laid out.

When is a guard rail required?

Once the deck's height above the ground exceeds a threshold that varies by jurisdiction. The calculator can flag this based on the height you enter, but the applicable threshold and rail height must be confirmed against local regulations.

Can I use this calculator for a raised or multi-level deck?

Yes, but each level with different dimensions, spacing or footing depth should be entered and calculated as a separate zone, then the results combined.

Does changing the currency convert prices I already entered?

No. Changing currency only changes the displayed symbol and formatting; enter a current local supplier price in the currency you select.

Is the result enough to submit for a building permit or order materials directly?

No. It is a planning estimate. Verify span tables, frost depth, guard rail rules and any permit requirements with local regulations and a qualified professional before ordering or building.

Decking calculator.

Project geometry

Product & installation

Purchase prices