Decks & Fences
Decking Calculator
Enter your deck size in metres or feet — Canadian lumber is imperial but many plans are metric, and this calculator converts either way to count boards and joists.
Formula tested · Local units · No sign-up
Project inputs
Enter measurements
Use your preferred units. Results update automatically.
Show the calculation methodFormula, conversions, rounding, and assumptions+
Rows of boards = deck width ÷ (board width + gap), rounded up. Canadian deck boards are imperial stock — a nominal 6 in board is actually 5.5 in (about 140 mm) — but you can enter every dimension in metric or imperial.
Boards per row = deck length ÷ board length, rounded up; boards to buy adds your wastage allowance and rounds up.
Joists = deck length ÷ joist spacing rounded up, plus one closing joist — 16 in (about 400 mm) on-centre is the common wood-deck spacing.
Real-world example
Worked example: 4.8 m × 3.6 m deck, 140 mm boards, 3 mm gap, 4.8 m boards, 400 mm centres
- Row width: 140 + 3 = 143 mm. Rows: 3.6 ÷ 0.143 = 25.17 → round up to 26 rows.
- Boards per row: 4.8 ÷ 4.8 = 1 board.
- Boards to buy: 26 × 1 × 1.10 (10% wastage) = 28.6 → round up to 29 boards.
- Joists: 4.8 ÷ 0.4 = 12 exactly, + 1 closing joist = 13 joists.
Buy 29 boards and 13 joists. Enter your lumber-yard prices and your province's combined GST/HST or GST+PST rate for a cost estimate.
Before you start
How to measure
- Measure the deck length (board direction) and width in whichever units your plan uses — fields accept metres, feet, millimetres and inches.
- Use the actual board width: nominal 5/4 × 6 decking measures 5.5 in (about 140 mm) across.
- Enter the stock length you'll buy — 8, 12 and 16 ft boards are common at Canadian yards; a 16 ft board is about 4.88 m.
Local guidance
Notes for Canada
- Deck footings in most of Canada must bear below the local frost depth or the deck heaves seasonally — frost depth varies by region, so ask your municipal building department. Footings are not estimated by this tool.
- Most municipalities require a permit for attached or elevated decks, and many publish standard deck detail sheets — check before building.
- Pressure-treated lumber is the standard framing and budget decking choice; cedar and composite are common upgrades — the calculator handles any of them since you enter the board dimensions.
- Sales tax is 5% federal GST plus provincial tax, or a combined HST, depending on the province — enter your combined rate.
Quick reference
Common Canadian decking planning values
| Item | Common value |
|---|---|
| Board width (actual) | 5.5 in / ~140 mm |
| Drainage gap | 1/8 in / ~3 mm |
| Board stock lengths | 8, 12, 16 ft |
| Joist spacing | 16 in / ~400 mm on-centre |
Planning values only — local code, frost depth and manufacturer charts govern.
Good to know
Common mistakes to avoid
- Mixing unit systems mid-entry — a 140 mm board typed as 140 in the inch field is a 3.5 m plank.
- Skipping the wastage allowance — crowned, split or checked pressure-treated boards get culled on site.
- Building footings above the frost line — the deck racks and doors stop closing within a couple of winters.
- Skipping the permit on an attached deck — ledger failures are the classic deck collapse, and inspections exist to catch them.
Need help?
Frequently asked questions
How many boards for a 14 ft × 10 ft deck with standard 5.5 in boards?
Row width is 5.625 in, so 120 in ÷ 5.625 = 21.33 → 22 rows. With 16 ft boards that's one per row, and 22 × 1.10 = 24.2 → 25 boards with 10% wastage. Joists at 16 in: 168 ÷ 16 = 10.5 → 11, + 1 = 12 joists.
Can I plan in metric and buy imperial lumber?
Yes — enter your plan in metres and the board sizes in inches (or their metric equivalents); everything is converted internally. Just remember stock lengths are imperial: a '4.8 m' board is really 16 ft = 4.877 m.
Does this tool size my joists and footings?
No. It counts boards and joists at the spacing you enter; joist depth, beam spans and frost footings are structural decisions governed by your local building code and permit process.
Keep planning
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About this calculator
- Written by:
- BuildMeasure Editorial Team
- Technically reviewed by:
- Pending independent technical reviewer (formula unit-tested; see methodology)
- Last reviewed:
- 2026-07-16
- Formula version:
- 1.0.0
- Region reviewed for:
- Canada
- Spotted an error?
- Report a correction
Methodology
- Boards are assumed to run parallel to the deck length; joists run parallel to the width, spaced along the length. All dimensions are converted to metres internally before any arithmetic.
- Rows of boards = deck width ÷ (board width + gap), rounded UP. Boards per row = deck length ÷ board length, rounded UP — part boards count as whole boards because that's what you buy.
- Boards to buy = rows × boards per row × (1 + wastage%), rounded UP. Joists = deck length ÷ joist spacing, rounded UP, plus one closing joist. Exact multiples are not bumped up.
- The cost estimate multiplies the board and joist counts by the prices you enter, then applies the tax rate you enter. No prices are built in.
- The formula is covered by automated unit tests, including hand-calculated worked examples, and is versioned (see formula version on this page).
Sources & standards
- Board dimensions: Nominal 5/4 × 6 decking measures 5.5 in actual = 139.7 mm (1 in = 25.4 mm, exact).
- Joist spacing: 16 in on-centre is common Canadian wood-deck practice; local code and span tables govern.
This tool provides a material estimate for planning purposes only. It is not a quotation, and it does not size joists, bearers, posts, footings or ledger connections — deck structure is an engineering and permit matter. Confirm the structural design with a qualified person and quantities with your supplier before ordering.