How this deck calculator works
Most deck calculators divide square feet by board square feet and call it a day. Boards don’t install by the square foot — they install in rows, and a deck that needs 25.04 rows needs 26, because that last sliver still takes a board you have to buy. This deck calculator counts the way the deck actually goes down:
joists = length × 12 ÷ spacing + 1 · screws = rows × joists × 2
Deck width ÷ board coverage (face plus gap) sets the rows; deck length ÷ stock length sets boards per row; waste rounds the pile up to whole boards. Under the surface, joists space out along the length with one extra to close the run, two rim boards band the frame, and every board-joist crossing takes two screws. At the defaults — a 16 × 12 ft deck in 5/4×6 boards — that’s 29 boards, 13 joists, 676 screws and about $1,142 in materials; the line-by-line math is in the worked example below.
What are the parts of a deck?
A deck is five layers of structure: concrete footings and posts in the ground, a beam across the posts, joists across the beam and ledger, deck boards across the joists, and the fasteners and railings that finish the job. This calculator counts the top layers — decking, joists, rim and screws — and the glossary below tells you what the rest adds.
Footings and posts
The free edges of the deck bear on posts — typically 6×6s — standing on concrete poured below frost depth. Pier-and-tube footings are the usual residential answer: size the concrete with the sonotube calculator, or the footing calculator if your jurisdiction wants spread pads.
Beam (girder)
The beam collects the joists and carries them to the posts — usually two or three 2×10s or 2×12s fastened together. It’s priced by lumber volume like the rest of the frame; the board foot calculator turns beam stock into board feet and dollars.
Ledger board
On an attached deck, the ledger bolts the frame to the house and carries roughly half the load. It gets lagged or through-bolted — never just nailed — and flashed so water can’t sit behind it. Ledger failures are the classic deck collapse, which is why inspectors read this connection first.
Joists and rim boards
Joists — 2×8s at 16 in centers in the default — span from ledger to beam and give the deck boards something to land on. The spacing you frame is the span your decking has to bridge, which is why the surface material sets it: 16 in for wood, tighter for some composites. Rim (band) boards close the open sides of the frame; the calculator carries 2 automatically.
Deck boards (decking)
The walking surface. The standard board is a 5/4×6 — about 1 in thick with a 5.5 in face — laid perpendicular to the joists with a ¼ in gap so rain drains and wood can move. Rows stack across the deck’s width, and every fraction of a row rounds up to a bought board.
Deck screws and hidden fasteners
Two coated deck screws at every board-joist crossing is the standard schedule, and it’s exactly what this calculator counts. Grooved composite swaps face screws for hidden clips between boards — figure roughly half as many clips as screws, per the brand’s chart — with screws still needed on the first and last boards.
Railings and stairs
Any deck surface more than 30 in above grade needs a guard — 36 in minimum height in most of the country under the IRC. Stairs bring their own rise-and-run math and their own inspection line items; the stair calculator lays out a code-checked stringer for the walk down to the yard.
Wood vs composite decking: which boards should you buy?
Pressure-treated pine is the budget pick, cedar the classic, composite the low-maintenance pick, PVC the premium version of that, and tropical hardwood the lifetime surface — the board price roughly doubles at each step. Whatever goes on top, the frame underneath stays pressure-treated lumber.
Pressure-treated wood decking
Treated southern yellow pine is the cheapest board on the rack and takes fasteners well, but it moves: expect some cupping and checking, plan on re-staining every couple of years, and cull the twisted boards at the store. Honest maintenance buys a 15-year surface.
Cedar and redwood
Naturally rot-resistant, straight, light, and the best-looking board per dollar the day it goes down. They’re soft — chair legs and dog claws leave marks — and they silver fast without UV sealer. Availability is best out west; east of the Rockies cedar often costs composite money.
Composite decking
Wood flour and plastic capped in a polymer shell: no rot, no splinters, no staining, and 25-year fade-and-stain warranties. The trade-offs are heat underfoot in full sun and less stiffness between joists — check the brand’s span chart before you frame, and drop to 12 in centers when it says to. Grooved-edge boards take clips instead of face screws.
PVC decking
All plastic, no wood flour: the lightest boards, the best stain and moisture resistance, and the right answer for a wet, shaded site. Lighter colors stay walkable in summer sun. It flexes more between joists than composite and costs the most of the mainstream options.
Hardwood decking (ipe, cumaru, garapa)
Dense tropical hardwoods outlast everything — 40-year surfaces are routine — and look like furniture. The price is paid in labor: every hole pre-drilled, every cut end sealed, stainless fasteners mandatory. Price the install time along with the lumber.
| Material | Typical range |
|---|---|
| Pressure-treated pine | $25–40 |
| Cedar | $45–70 |
| Composite (capped) | $55–90 |
| PVC | $75–120 |
| Ipe / tropical hardwood | $90–140 |
| PT 2×8 joist stock (frame) | $18–28 |
How many deck boards do I need by deck size?
Read your deck off Table 2 for boards and Table 3 for the frame and screws — every row generated by this calculator’s engine at the default 5.5 in board, ¼ in gap, 16 in joist centers and 10% waste.
| Deck | Rows | Boards |
|---|---|---|
| 12 × 10 ft | 21 | 24 |
| 12 × 12 ft | 26 | 29 |
| 16 × 12 ft | 26 | 29 |
| 20 × 14 ft | 30 | 33 |
| 16 × 16 ft | 34 | 38 |
| Deck | Joists | Screws |
|---|---|---|
| 12 × 10 ft | 10 | 420 |
| 12 × 12 ft | 10 | 520 |
| 16 × 12 ft | 13 | 676 |
| 20 × 14 ft | 16 | 960 |
| 16 × 16 ft | 13 | 884 |
Spacing moves the screw pile more than the joist pile: reframe the same 16 × 12 deck at 12 in centers for composite and the count goes to 17 joists and 884 screws — nearly a third more fasteners from one field change.
How to measure a deck for decking
Two tape pulls and four decisions — the calculator does the rest. Dimension fields take feet-inches (16, 16 ft 3 in, 16′3″) or metric (5 m) and convert on the fly.
- Measure the length — the direction the boards will run, usually parallel to the house wall. Boards must land perpendicular to the joists, so decide the look before you frame.
- Measure the width — the direction the rows stack across, ledger out to the rim. This is the number that sets the row count.
- Pick the board and gap. 5.5 in face and ¼ in gap for standard 5/4×6 wood decking; grooved composite takes the clip gap from the brand chart — ¼ in or a touch under.
- Set the stock length. Match it to the deck length for clean single-board rows; shorter stock adds butt joints, which must break on a joist.
- Set the joist spacing — 16 in on center for wood decking, the brand chart’s number for composite (often 12 in).
- Leave waste at 10% for straight decking; go 15% for diagonals or a picture-frame border, which throw longer offcuts.
- Enter your prices. The defaults are placeholders — put in the board and joist quotes from your yard and the cost lines reprice instantly.
Worked example: materials for a 16 × 12 ft deck
The defaults, worked line by line — 16 ft of board run, 12 ft of width, 5/4×6 boards with a ¼ in gap on 16 ft stock, joists at 16 in centers, 10% waste, $28 boards over $22 joists:
coverage = 5.5 + 0.25 = 5.75 in
rows = (12 × 12) ÷ 5.75 = 25.04 → 26 rows
boards = 26 × 1 × 1.10 = 28.6 → 29 boards
joists = (16 × 12) ÷ 16 = 12, + 1 = 13 + 2 rim boards
screws = 26 × 13 × 2 = 676
cost = 29 × $28 + 15 × $22 = $812 + $330 = $1,142
Notice what the row method caught. A pure area method covers 192 ft² at 7.67 ft² per board — 25.0 boards, × 1.10 = 27.5, rounded to 28 — one board short, because it lets the waste factor absorb the fractional row instead of buying it first. Rows round up before waste is applied; that 29th board is the one you’d otherwise drive back for.
Per square foot, the surface-and-frame package lands at $1,142 ÷ 192 ft² ≈ $5.95/ft² in materials — before the posts, footings, rails and hardware listed below.
What this deck estimate doesn’t include
The calculator prices the deck surface and the joist layer it fastens to — everything that holds those up, and everything bolted on after, is a separate list. Budget these before calling the total a budget:
- Footings and posts. Concrete below frost depth plus 6×6 posts and post bases — the sonotube calculator linked above prices the pour per pier.
- Beam, ledger and hardware. The girder, ledger bolts, flashing, joist hangers and hurricane ties. Hangers run a few dollars each, and every joist end at the ledger wants one.
- Railings. Guards commonly run $40–80 per linear foot installed in wood, more in composite or metal — often the second-biggest number on the job.
- Stairs. Stringers, treads and a landing pad with their own footing and their own inspection — the stair calculator handles the rise-and-run.
- Permit and inspections. Most jurisdictions permit any attached or elevated deck — commonly $100–500 — with footing, framing and final visits.
- Labor. Hiring the build out adds roughly $15–35 per ft² over materials in 2026; all-in installed decks commonly land at $30–60 per ft².
- Tear-off and delivery. Demo and dump fees for an old deck, plus lumberyard delivery for anything that won’t roof-rack.
Deck FAQ
How many deck boards do I need?
Deck width ÷ board coverage gives the rows: a 16 × 12 ft deck in 5.5 in boards with a ¼ in gap needs 26 rows — 26 sixteen-foot boards, ordered as 29 with 10% waste. Length and stock length set how many boards make each row.
How many deck boards do I need for a 12x12 deck?
A 12 × 12 ft deck takes 26 rows of 5.5 in boards with a ¼ in gap — 29 twelve-foot boards after 10% waste. That is the same board count as a 16 × 12, because rows come from width alone; the boards are just 4 ft shorter. Frame it on 10 joists at 16 in centers plus 2 rim boards, fastened with 520 screws.
How many joists do I need for a deck?
One every 16 inches along the length plus one to close: 13 field joists for the default 16 ft run, plus 2 rim boards. Composite decking often wants 12 in centers — the spacing field recounts instantly.
How many screws per deck board?
Two at every joist crossing: 26 rows × 13 joists × 2 = 676 screws for the default deck — right on the classic 350-per-100 ft² rule. Buy coated deck screws by the box; grooved composite swaps screws for hidden clips.
What joist spacing for composite decking?
12 inches on center for many composite brands laid straight (16 for some, 8–12 on diagonals) against the standard 16 for wood decking. The brand’s span chart is the law here — check it before framing.
How much does it cost to build a deck?
The default 192 ft² surface-and-frame package is about $1,142 in materials — decking $812, joists and rim $330 — before posts, footings, hardware and rails. Built out, decks commonly run $30–60 per square foot installed in 2026.
What size are deck boards?
The standard 5/4×6 board has a 5.5 in face, sold in 8–20 ft lengths. Gap wood boards about ¼ in for drainage; grooved composite spaces itself with clips — enter the brand’s clip gap, usually ¼ in or a touch under.
Which direction should deck boards run?
Deck boards must run perpendicular to the joists — the usual look is boards parallel to the house wall, with joists running out from the ledger. Set “length” in this calculator to the direction the boards run, and bump waste to 15% if you lay them diagonally.
Sources and standards
The framing rules this deck calculator and page lean on come from the standards inspectors actually check against:
- 2021 IRC, Section R507 — Exterior Decks: joist spans and spacing, beam sizing, ledger attachment and post requirements.
- IRC Section R312 — Guards: required where the surface sits more than 30 in above grade; 36 in minimum height.
- AWC DCA 6, Prescriptive Residential Wood Deck Construction Guide: the span tables and connection details most building departments hand out at the permit counter.
- PS 20, American Softwood Lumber Standard: why a nominal 5/4×6 board actually measures about 1 × 5.5 in.
- Composite manufacturers’ installation charts: span, gap and fastener schedules that override the wood defaults for their boards.
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