How do I calculate square footage of a deck?

Multiply length × width in feet for a rectangle. For an L-shape: split into 2 rectangles, multiply each, then sum. For octagon: area = 2(1 + √2) × side² (where side is the length of one edge). For a circle: π × radius². DeckMath handles all 7 common shapes automatically — pick the shape, enter dimensions, and read the sqft + sqm + sqyd output instantly.

What's the average deck size in square feet?

The US average residential deck is 300-400 sqft (a 16×20 or 12×24 layout). Smaller starter decks run 100-200 sqft (a 10×12 or 8×16). Premium decks built for outdoor entertaining typically hit 500-800 sqft. Permitting threshold in most jurisdictions is 200 sqft — below that, many municipalities allow build-without-permit (verify with your local AHJ).

How many deck boards do I need per square foot?

At 16″ joist o.c. with 5/4×6 boards (5.5" face + 1/8" gap = 5.625" row pitch), you need about 0.13 boards per square foot when boards span the full length of the deck in one piece. For a 192 sqft deck: ~26 boards. For a 400 sqft deck: ~53 boards. This rule-of-thumb assumes you pick the optimal stocked length (no butt joints) and includes a 4-7% waste margin. Pattern decks (diagonal/herringbone) add 20-35% more.

How much does a deck cost per square foot?

2026 installed cost ranges: pressure-treated DIY $18-28/sqft, PT pro-installed $28-45, cedar pro-installed $38-58, mid composite (Trex Enhance) $45-68, premium composite (Trex Transcend, TimberTech AZEK) $60-95, exotic hardwood (Ipe) $85-120, frameless glass railing + premium composite $120-180. Multiply your sqft by the tier to get a budget. A 300 sqft deck in mid composite: $13,500-20,400 installed (boards + framing + footings + railing + stairs).

What's the difference between square footage and surface area?

Square footage = horizontal area of the deck (what you walk on). Surface area = total exposed area including the sides + face of fascia + risers (used for stain coverage). For a 12×16 deck: footprint = 192 sqft, but stain surface area might be 230-280 sqft once fascia and rim joists are counted. DeckMath's Stain Calculator uses the surface model; this Square Footage Calculator uses the footprint.

How do I measure an irregular deck shape?

Break it into known shapes (rectangles, triangles, semicircles) and sum the areas. For a curved deck with a kidney profile: use a tape to measure the longest dimension and the width at three points (1/4, 1/2, 3/4 along length), then average. For decks attached to round porches: use the L-shape mode plus the round mode, sum the two. Always round UP — material orders are easier to manage with 2-5% extra than to come up short.

Do I count stairs in my deck's square footage?

For most permits: yes, the tread surface counts. For typical 8-tread stair from a 36" rise: 8 treads × 36" wide × 10.5" run = 21 sqft added. For cost estimates: stairs are usually quoted separately ($1,200-2,500 for a typical 4-step deck stair) because their per-sqft labor is 2-3× higher than the deck surface. DeckMath's Deck Cost Calculator quotes stairs as a separate line item.

What's the largest deck I can build without an engineer?

Most US jurisdictions allow prescriptive (no engineer needed) deck design up to: 400 sqft, deck height ≤ 9 ft, joist span ≤ IRC R507.6 max, beam span ≤ IRC R507.5 max, footing size per IRC R507.3. Above that (or any condition that triggers a stamped drawing — coastal high-wind, snow load > 50 psf, deck > 10 ft above grade) you'll need an engineer's seal. For most homeowners, the 400 sqft × 9 ft height envelope covers 95% of projects.

Round deck — how is it calculated?

Use the round option in this calculator. Area = π × radius² (where radius = diameter / 2). For a 14-ft-diameter round deck: π × 7² = 153.9 sqft. Round decks need radial joists (joists fanned from a center beam) and curved fascia. Decking is typically installed straight across with cut waste of 20-30% at the curved edge. Total cost is 30-50% more than a square deck of the same area due to layout complexity + curve cuts.

Can I use this for patio square footage?

Yes — square footage math is identical for patios, decks, porches, and floor plans. DeckMath's cost benchmarks assume deck construction (raised, framed); for ground-level concrete patios costs are lower ($8-15/sqft installed for plain concrete, $15-30 for stamped). The shape area + perimeter calculations work for any flat surface.

area · perimeter · sqft / sqm / sqyd · 7 shapes

Square Footage Calculator

Deck area for any shape — rectangle, L-shape, U-shape, T-shape, regular octagon, regular hexagon, or round. Feet + inches inputs (1/16″ precision). Returns sqft, sqm, sqyd, perimeter LF, plus quick BoM hooks for deck boards, joists, footings, and a cost range across 5 material tiers. The universal entry point for sizing any deck project — then deep-link to the Deck Material, Cost, Board, or Joist calculators with one click.

7 shapesFeet + inches5 cost tiersDeep-link to other calcssqft + sqm + sqyd

7·Shapes supported

1/16″·Input precision

8 / 6·Sides — octagon / hexagon

5·Cost tiers

Inputs

Shape

Main rectangle

Length

Width

Length16′ 0″

Width12′ 0″

Area192 sqft

Perimeter56′

Results

Rectangle · feet + inches precision · live update

Square feet

17.84 m²

Square yards

concrete / sod

Perimeter

17.07 m

Est. cost (mid composite)

Low $8,640

Quick BoM (verify with dedicated calcs)

Deck boards

5/4×6 at 16″ o.c.

Joists

2×10 at 16″ o.c.

Footings

8′ × 8′ grid

→ Full BoM (Material Calc)→ Full cost (Cost Calc)→ Pattern + waste (Board Calc)→ Framing BoM (Joist Calc)

BoM estimates are rule-of-thumb (5/4×6 boards at 16″ joist o.c., 2×10 PT at 16″ o.c., 8×8 footing grid). For exact counts including pattern waste, hidden clips, blocking, and hangers, use the dedicated calculators above.

Cost by material tier (installed)

Pressure-treated DIY

$18-$28/sqft

$3,456 – $5,376

192 sqft × tier

Pressure-treated pro install

$28-$45/sqft

$5,376 – $8,640

192 sqft × tier

Cedar pro install

$38-$58/sqft

$7,296 – $11,136

192 sqft × tier

Composite mid (Trex Enhance)

$45-$68/sqft

$8,640 – $13,056

192 sqft × tier

Composite premium (Trex Transcend / AZEK)

$60-$95/sqft

$11,520 – $18,240

192 sqft × tier

2026 mid-market installed-cost benchmarks. DIY = materials only + your time. Pro install adds 50-90% over DIY for labor + project management. Premium tiers include hidden clips, picture-frame border, and 25+ year material warranties.

How to use

Three steps. Permit-ready output.

Pick a shape

Rectangle is the default. L-shape, U-shape, and T-shape need a primary rectangle + secondary leg. Regular octagon and hexagon take one side-length input. Round takes diameter. For irregular shapes, break into rectangles and sum the results.

Enter dimensions

Feet + inches for each. 1/16″ precision is supported (just type 192.5 for 16′ 0.5″). The calculator updates live as you type — no submit button. Bottom-bar shows feet-inches conversion for verification.

Read area + perimeter outputs

Square feet is the primary number — every contractor + supplier prices by sqft. Square meters for international + metric specs. Square yards for concrete + sod ordering. Perimeter LF for railing, fascia, ledger length.

Use the BoM hooks

Estimated board count, joist count, footing count are quick-glance numbers — verify with the dedicated calculators (Deck Material for full BoM, Deck Board for pattern + waste, Deck Joist for hangers + blocking). Cost benchmarks span 5 material tiers from PT DIY ($18-28/sqft) to composite premium ($60-95/sqft installed).

Share or save

Every input is encoded in the URL — share the link and the recipient sees your exact deck. Use the deep-link buttons to jump to other DeckMath calculators with this size pre-loaded.

How we calculate

The math, fully transparent.

The Square Footage Calculator returns deck area (square feet, square meters, square yards), perimeter (feet + meters), and quick BoM hooks for any shape — rectangle, L-shape, U-shape, T-shape, regular octagon, regular hexagon, or round. Enter dimensions in feet + inches (or pure inches) for 1/16″ precision. DeckMath uses the area to estimate deck-board count (5/4×6 at 16″ o.c.), joist count (16″ o.c.), footing count (8′ × 8′ grid), and total deck cost across 5 material tiers (PT DIY → composite premium). Use this as the entry point when starting a deck project — then jump to the Deck Material, Deck Cost, or Deck Board calculator with one click to refine the BoM.

IRC references

IRC 2021 R301.5 — Deck live load 40 psf (50 psf at stair landings) for residential
IRC 2021 R507.2 — Deck design loads — concentrated load 200 lb on any 4 sq in
IRC 2021 Table R301.2(2) — Wind exposure category — drives lateral load on guards by sqft of exposed face

Area formulas are standard geometry — rectangle L×W, L/U/T = sum of rectangles minus shared edges, regular octagon A = 2(1+√2)·s², regular hexagon A = (3√3/2)·s², circle A = π·r². Cost benchmarks are 2026-Q1 mid-market installed pricing across 5 material tiers: PT DIY $18-28/sqft, PT pro $28-45, cedar pro $38-58, mid composite (Trex Enhance) $45-68, premium composite (Transcend / AZEK) $60-95. Verified against Home Depot Pro Desk + RemodelingExpense + HomeAdvisor 2026 surveys.

Rectangle area

A = L × W

16 ft × 12 ft = 192 sqft. Most common deck shape — 60% of residential decks. Perimeter: 2(L + W) = 56 LF.

L-shape area

A = (L × W) + (l × w)

Main rectangle + leg rectangle. For a 16×12 main + 8×6 leg: 192 + 48 = 240 sqft. Perimeter accounts for shared edge: 56 + 28 - 12 = 72 LF.

Regular octagon area

A = 2(1 + √2)·s²

Where s is the side length. For 8-ft sides: 2(1 + 1.414)(64) = 309.0 sqft. Across-flats (apothem-to-apothem) = s·(1 + √2) = 19.3 ft. Perimeter: 8s = 64 LF.

Regular hexagon area

A = (3√3/2)·s²

Where s is the side length. For 8-ft sides: (3 × 1.732 / 2)(64) = 166.3 sqft. Across-corners = 2s = 16 ft, across-flats = s·√3 = 13.86 ft.

Circle area

A = π · r²

For a 14-ft-diameter round deck: π × 7² = 153.9 sqft. Perimeter = π × d = 43.98 LF. Round decks are 20-30% more expensive than square footage suggests due to curve cuts.

Square feet → square meters

sqm = sqft × 0.092903

192 sqft × 0.092903 = 17.84 m². For square yards (concrete/sod ordering): sqft ÷ 9 = sqyd. 192 ÷ 9 = 21.3 sqyd.

Save your plan

Don’t lose this estimate.

Your inputs are preserved in the URL — email it to yourself or copy the link so you can compare with contractor bids later. No account needed.

Frequently asked

Questions, answered.

Embed this calculator

One line. Any site. Free.

Drop the snippet into your contractor site, blog, or marketing page. Theme matches the parent site automatically.

<!-- Drop into any HTML, WordPress, Webflow, or contractor site -->
<script src="https://embed.deckmath.com/v1.js"
        data-deckmath-calc="square-footage-calculator"
        data-theme="auto"
        async></script>

Deck Cost Calculator

Estimate full deck project cost: materials, labor, permits, and per-square-foot pricing for any size and material.

Deck Material Calculator

Calculate exact decking boards, joists, beams, posts, footings, and fasteners for your deck — IRC 2021 compliant.

Deck Board Calculator

Pattern-aware deck board count — straight / 45° diagonal / herringbone, with optional picture-frame border. 4 profiles × 5 materials × 5 stocked lengths. Cut-waste optimizer + hidden-clip count.

Square Footage Calculator

Inputs

Quick BoM (verify with dedicated calcs)

Cost by material tier (installed)

Three steps. Permit-ready output.

Pick a shape

Enter dimensions

Read area + perimeter outputs

Use the BoM hooks

Share or save

The math, fully transparent.

IRC references

Don’t lose this estimate.

Questions, answered.

One line. Any site. Free.

More calculators you'll need.

Deck Cost Calculator

Deck Material Calculator

Deck Board Calculator