Deck Calculator
Boards, Joists, Posts & Material Estimator
Calculate deck boards, joists, posts, concrete footings, rim boards, and hardware for any wood or composite deck. Enter dimensions, board width, and joist spacing — with diagonal board options and a complete material summary with optional cost estimate.
Deck Dimensions
Decking Material
Deck Material Comparison
| Material | Cost/sq ft | Lifespan | Maintenance |
|---|---|---|---|
| Pressure-Treated Wood | $2–4 | 15–25 yrs | Annual staining/sealing |
| Cedar / Redwood | $4–7 | 15–30 yrs | Biennial sealing |
| Composite (Trex, etc.) | $6–12 | 25–30 yrs | Low — wash yearly |
| Ipe (Hardwood) | $10–20 | 40–75 yrs | Annual oiling |
Joist Spacing and Span
Joist spacing of 16 inches on-center is standard for most deck boards. Composite and PVC decking often requires 12-inch spacing due to deflection limits. For 5/4×6 pressure-treated decking, maximum unsupported span is 16 inches; for 2×6 solid lumber, up to 24 inches.
Permits and Building Codes
Most jurisdictions require a building permit for decks over 200 square feet or those attached to the house. Railing height must be at least 36 inches for decks under 30 inches above grade, and 42 inches for higher decks. Balusters must be spaced so a 4-inch sphere cannot pass through.
What Is the Deck Calculator?
This deck material calculator estimates everything the frame and surface of a deck requires: the number of deck boards for your square footage, the joists at your chosen spacing, beams and posts, and the fasteners to hold it together. It handles standard 5.5-inch-wide boards (nominal 1×6 or 5/4×6) in common lengths, 16-inch on-center joist spacing (or 12 inches for diagonal or composite installs), and the 10–15% waste factor that separates a smooth build from a mid-project lumber run.
Deck Calculator Example
A 16 × 12 ft deck (192 sq ft) with boards run across the 16-ft direction:
Each 16-ft board covers 16 ft × 0.4583 ft (5.5 in) = 7.33 sq ft. Boards needed: 192 ÷ 7.33 = 26.2 → 27, plus 10% waste → 30 boards.
Joists at 16 in OC across 12 ft: (12 × 12 ÷ 16) + 1 = 10 joists, 16 ft long, plus doubled rim joists. Fasteners at 2 screws per board per joist: 30 × 10 × 2 = 600 screws, order 700. Framing hardware: 20 joist hangers and structural screws for the ledger.
Understanding Deck
Structure from the Ground Up
Loads flow from decking to joists to beams to posts to footings. Footings must reach below local frost depth and size to soil bearing; posts (6×6 preferred) carry beams, typically doubled or tripled 2× lumber; joists span from ledger or beam at your chosen spacing; and the ledger bolts, never nails, to the house rim with flashing above it. Each connection has a code-specified fastener or hardware piece; joist hangers and structural screws are not optional accessories.
Wood or Composite Decking
Pressure-treated pine is cheapest and needs staining every 2–3 years; cedar and redwood cost more, weather to gray, and resist rot naturally; composites cost the most upfront, never need finishing, and carry 25–50-year warranties, but run hotter underfoot and demand tighter joist spacing. Lifetime cost often favors composite within 10–15 years once refinishing labor is counted; upfront budget favors wood decisively.
Build Sequence and Time
A competent DIY pair completes a simple ground-level 200 sq ft deck in 3–5 working days: footings and posts day one (plus concrete cure time), beams and joists day two, decking day three, stairs and railing after. Lay the first board perfectly straight and square, every subsequent board references it. Order all lumber at once from the calculator's list; deliveries mid-project stall builds far longer than the delivery takes.
Frequently Asked Questions
How many deck boards do I need per square foot?
A 5.5-inch board covers 0.458 sq ft per linear foot, so you need about 2.18 linear feet of board per square foot of deck. A 200 sq ft deck therefore needs roughly 437 linear feet, 28 boards at 16 ft, before waste. Gapping (1/8–1/4 inch) slightly reduces coverage and is already reflected in these practical figures.
What joist spacing does my decking require?
16 inches on-center suits most wood decking installed perpendicular. Diagonal decking and many composite brands require 12 inches on-center, composite spans less stiffly than wood, and manufacturers void warranties over wide spacing. Always check the specific product's span chart; joist spacing is a structural requirement, not a preference.
Should I use screws or hidden fasteners?
Face screws (2 per board per joist) are strongest, cheapest, and easiest to service; coated or stainless screws prevent staining. Hidden clip systems give a fastener-free surface for grooved composite boards at roughly 2–3× the cost and slower installs. Nails are no longer recommended for decking, they back out with seasonal movement.
How much does deck framing cost versus the decking surface?
As a rule of thumb the pressure-treated frame (posts, beams, joists, hardware) runs 40–50% of material cost with wood decking on top. Choosing composite decking roughly doubles the surface cost while the frame stays the same, which is why composite decks price 50–80% above wood despite identical structure.
Do I need a permit to build a deck?
Usually yes for attached decks and anything above a low height threshold (commonly 30 inches). Permits enforce genuinely structural rules: ledger attachment to the house, footing depth below frost line, post sizing, and guardrail requirements. Ledger failures are the leading cause of deck collapses, the inspection is protecting the exact connection most DIY builds get wrong.
You Might Also Like
More Engineering Calculators
Explore 450+ Free Calculators
From math and science to finance and everyday life — all free, no account needed.