Square Footage Calculator

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).

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.

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).

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.

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.

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.

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.

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.

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.