Deck Calculator — ft, boards, joists, cost

How to Calculate Materials for a Wood Deck

Set the deck footprint as a rectangle or L-shape, then pick a decking profile, joist size and on-center spacing, beam build-up and post layout. Joist spacing, allowable span, beam lines, posts and connections must be taken from the exact adopted table, engineered design or approved framing plan; this page performs a quantity takeoff from those inputs. Board counts are rounded up per row, and the waste slider covers off-cuts, defective boards and the picture-frame border most builders add. For decking that runs at 45°, bump waste to 15–20% because every board is cut twice.

Estimate the lumber bill before pricing it with the board feet lumber calculator, verify your joist size against the floor joist span calculator, and if the deck sits low and gets a privacy screen or guard, size the rail with the deck railing calculator. A skirt or step framed like a stud bay can be checked with the stud wall calculator, and ground-level fascia or perimeter fencing 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 20 ft run on 16 ft stock needs 2 boards per row.
Field joists = ceil(layout width ÷ spacing) + 1 for each rectangular framing zone. Field joist LF is count × run length; rim/boundary LF is separate. Board feet = (nominal T × nominal W × length ft) ÷ 12, summed for decking, joists, beams and posts.
Hangers, fasteners and bolts are entered from the selected connection details; the calculator does not infer connector schedules from deck area.

On install day

Three things deck builders forget at the lumberyard. Ground-contact rating — joists, beams and posts in soil contact must be UC4A pressure-treated (look for the AWPA tag); standard above-ground PT is a callback in 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 blocking under every miter. Hidden hardware — joist hangers (Simpson LUS or equivalent) and post bases run $2.50–14 each and add up fast; budget $0.80–1.20/ft² for hardware alone. Typical 2025 USA installed cost is $30–55/ft² for PT lumber, $45–80/ft² for composite. Per IRC R507.5 and R507.8, beams must bear directly on the post (not be side-bolted) and footings must extend below the local frost line.

FAQ

What joist spacing for composite decking? Most composite brands require 16″ o.c. for perpendicular installation and 12″ 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.
Do I need double rim/end joists? The IRC requires a doubled rim where the deck is wider than the joist span allows and where stair stringers or a hot tub frame into the rim.