Deck Calculator
Estimate the square footage, decking boards, and joists you need to build a deck based on its length, width, and board width.
How this is calculated
Deck area is length × width. Decking linear footage is the area divided by the board's actual face width, which approximates board count while ignoring the small gaps typically left between boards for drainage. We add your selected waste percentage for angled cuts, staggered seams, and defects, then convert to a board count assuming standard 12 ft boards. Joist count assumes standard 16 in on-center spacing across the deck's width, with joists running the full length — actual framing should be confirmed against local building code and span tables for your joist size and species.
The material weights, waste allowances, and rounding rules behind every calculator are documented on our methodology page.
Example calculation
Decking needed for common deck sizes
Most backyard decks fall into a handful of standard footprints. The table below uses the same assumptions as the calculator: 5.5 in boards, 12 ft lengths, 10% waste, and joists every 16 in across the deck's width (the shorter side, listed second). Use it for a quick sanity check before you enter your own dimensions.
| Deck size | Surface area | Decking (with waste) | 12 ft boards | Joists |
|---|---|---|---|---|
| 10 × 10 ft | 100 ft² | 240 ft | 21 | 9 |
| 12 × 12 ft | 144 ft² | 346 ft | 29 | 10 |
| 16 × 12 ft | 192 ft² | 461 ft | 39 | 10 |
| 20 × 14 ft | 280 ft² | 672 ft | 56 | 12 |
| 20 × 16 ft | 320 ft² | 768 ft | 65 | 13 |
| 24 × 20 ft | 480 ft² | 1152 ft | 96 | 16 |
Board counts are for decking only. Framing lumber, fasteners, railing, and stairs are separate line items.
How board width changes the count
Decking is sold by nominal size, but the face you walk on is smaller. A 5/4×6 or 2×6 board is about 5.5 in wide, a 2×4 is only 3.5 in, and a 2×8 is 7.25 in. Narrower boards mean more pieces, more gaps, and more fasteners for the same square footage, which is why a 2×4 deck costs more labor even when the lumber is cheaper per foot.
The table shows the same 16 × 12 ft deck (192 ft²) with 10% waste at three common widths. Switching from 5.5 in to 3.5 in boards adds roughly 60% more linear footage.
| Board | Decking (with waste) | 12 ft boards |
|---|---|---|
| 2×4 (3.5 in actual) | 724 ft | 61 |
| 5/4×6 or 2×6 (5.5 in actual) | 461 ft | 39 |
| 2×8 (7.25 in actual) | 350 ft | 30 |
Choosing a decking material
Material affects price, maintenance, and how long the deck lasts far more than it affects the quantity you order. The quantities from this calculator apply to any of them, but you may need different joist spacing depending on the product.
| Material | Typical lifespan | Upkeep | Relative cost |
|---|---|---|---|
| Pressure-treated pine | 10–20 years | Seal or stain every 1–3 years | $ |
| Cedar or redwood | 15–25 years | Seal or stain every 1–2 years | $$ |
| Composite | 25–30 years | Occasional cleaning | $$$ |
| PVC (cellular) | 25–30+ years | Occasional cleaning | $$$$ |
| Tropical hardwood (ipe) | 25–40 years | Oil yearly to keep the color | $$$$ |
Lifespans are typical ranges and depend on climate, drainage, and sun exposure. Warranty terms vary by manufacturer.
Pressure-treated pine is the usual budget choice and works well when it is sealed on schedule. Composite and PVC cost more up front but remove most of the recurring staining work, so many owners compare total cost over ten or fifteen years rather than the purchase price alone. Hardwoods like ipe are extremely dense, so pre-drilling and carbide blades are essential.
Layout choices that change your material order
- Diagonal installs run boards at 45°, which creates many angled cuts. Plan on 15% waste or more, and check whether the manufacturer requires 12 in joist spacing.
- Picture-frame borders add a perimeter ring of boards and extra miter cuts. Count the border boards separately and subtract that area from the field.
- Board length matters. If your deck is 16 ft long, buying 16 ft boards removes the butt joints entirely, while 12 ft boards force a seam on every row and add waste from offcuts.
- Stagger seams so adjacent rows never end on the same joist, and put every seam over the center of a joist so both board ends have solid support.
- Leave a 1/8 to 1/4 in gap between boards for drainage, or follow the manufacturer's gap for composite, which changes with temperature at installation.
What this estimate doesn't include
The calculator covers surface boards and a joist count. A complete material list for a ground-level or raised deck also needs the items below. Ask your supplier or the building department for span tables before you finalize any of them.
- Footings, posts, and beams sized for your deck height, span, and soil.
- A ledger board with flashing if the deck attaches to the house, plus approved ledger fasteners.
- Joist hangers, blocking or bridging, and rim (band) boards around the perimeter.
- Deck screws: roughly 330–350 per 100 ft² for 5.5 in boards on 16 in centers (two screws at every joist crossing), or hidden fasteners sold by the box.
- Railing, balusters, and post caps where required, plus stairs and stringers.
- Concrete for footings. Use the concrete calculator to size each footing hole.
Permits, railings, and code basics
Requirements vary by city and county, so treat the points below as common patterns rather than rules for your property. Many jurisdictions require a permit for attached decks, for decks more than about 30 in above grade, or for decks above a certain area such as 200 ft². Inspections are usually required at the footing and framing stages.
In the United States, the International Residential Code requires guardrails on walking surfaces more than 30 in above the ground, with a minimum height of 36 in for residential guards. Stair rails, baluster spacing, and landing sizes have their own rules. A short call to your local building department before you buy lumber can save a failed inspection later.
Related calculators
Related guides
Frequently asked questions
10% is typical for a simple rectangular deck. Diagonal patterns, picture-frame borders, or decks with many cutouts often need 15% or more.