Deck Calculator — ft, boards, joists, cost
How to use this calculator
Enter deck dimensions in feet, inches, and eighth-inch fractions. Choose rectangle or L-shape, select decking board size and run direction, joist size and on-center 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 for a material subtotal.
Hardware — joist hangers, post bases, deck screws (lbs) and carriage bolts are derived from the framing layout.
Saved Calculations
| Time | Shape | Area ft² | Boards | Joists | Beams | Posts | BF | Total $ |
|---|
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 follows IRC R507.6 span tables (12″, 16″ or 24″ o.c.); beam and post sizing follows the IRC R507.5/R507.8 beam-span and footing provisions. 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.
Joists = floor(span ÷ spacing) + 1, plus 2 rim joists for a rectangle (4 for an L). Board feet = (nominal T × nominal W × length ft) ÷ 12, summed for decking, joists, beams and posts.
Screws ≈ 3.5 per ft² (about 75 #8 × 2½″ screws per lb). Joist hangers = 1 per field joist. Carriage bolts = 2 per beam ply at every post.
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.
