Deck Calculator — m, boards, joists, cost
How to use this calculator
Enter deck dimensions in metres, centimetres or millimetres using the unit dropdown. Choose rectangle or L-shape, select the decking profile and run direction, joist size and on-centre spacing, beam build-up and post layout, then set a waste allowance for off-cuts and the picture-frame border. The plan and side section redraw on every change so you can see joist runs and post bays before ordering.
Cost — enter price per board, per joist, per beam and per post, and pick your currency (EUR, GBP, PLN, CZK, SEK, etc.).
Hardware — joist hangers, post bases, deck screws (kg) and coach bolts are derived from the framing layout per EN 1995-1-1.
Saved Calculations
| Time | Shape | Area m² | Boards | Joists | Beams | Posts | m³ | Total |
|---|
How to Calculate Materials for a Timber Deck
Set the deck footprint as a rectangle or L-shape, then pick a decking profile, joist section and on-centre spacing, beam build-up and post layout. Joist and beam sizing follows EN 1995-1-1 (Eurocode 5) deflection and bending limits for C16/C24 softwood; spacing of 300, 400 or 600 mm matches standard joist-hanger and decking-fixing centres. Board counts are rounded up per row, and the waste slider covers off-cuts, twisted boards and the picture-frame border. For decking laid at 45°, set waste to 15–20% because every board is double-cut.
Estimate the cubic-metre lumber bill with the lumber volume calculator, verify the joist section against the floor joist span calculator, and size a guard or privacy screen with the deck railing calculator. A skirt or step framed like a stud bay can be checked with the stud wall calculator, and a ground-level boundary with the fence calculator.
Formulas
Rows = deck span perpendicular to boards ÷ (board face width + gap), rounded up. Boards per row = run ÷ board length, rounded up, so a 6 m run on 3.6 m stock needs 2 boards per row.
Joists = floor(span ÷ spacing) + 1, plus 2 rim joists for a rectangle (4 for an L). Timber volume = width × depth × length for every piece, summed in m³ for decking, joists, beams and posts.
Screws ≈ 22 per m² (about 200 stainless 5 × 60 mm screws per kg). Joist hangers = 1 per field joist. Coach bolts = 2 per beam ply at every post.
On install day
Three things deck builders forget at the merchant. Use class UC4 per EN 335 — joists, beams and posts in ground or splash contact must be pressure-impregnated to UC4 (look for the EN 351 tag) or a naturally durable species (oak, larch, thermowood); ordinary garden-grade UC3 is a callback within two seasons. Picture-frame border — most decks finish with a perimeter board on top of the joists, which adds ~10% to the board count and needs noggins under every mitre. Hidden hardware — joist hangers and post bases run €3–15 each and add up fast; budget €8–14/m² for hardware alone. Typical 2025 EU installed cost is €180–320/m² for treated softwood, €280–480/m² for composite. 45 × 190 mm joists span up to ~3.6 m and 45 × 240 mm up to ~4.8 m at 400 mm centres per EN 1995-1-1 deflection limits.
FAQ
What joist spacing for composite decking? Most composite brands require 400 mm o.c. for perpendicular installation and 300 mm o.c. for 45° diagonal layouts.
How much waste for a simple rectangle? 10% covers off-cuts and the picture-frame border; use 15–20% for diagonal or herringbone patterns.
Should I use a doubled rim/end joist? Double the rim where the deck is wider than the joist span allows and where stair stringers or a hot tub frame into the rim.
