Deck Calculator

A deck calculator turns length and width into a material list. A 16 × 12 ft deck is 192 square feet, which needs about 461 linear feet of 5.5-inch decking after a 10% waste factor, roughly 29 boards at 16 feet, plus 13 joists at 16 inches on center and 3 footings if the deck is attached to the house. Enter your dimensions to size boards, joists, footings and screws.

How the deck calculation works

Everything starts with area, then converts to the way materials are actually sold. The core formulas are:

Board width is entered in inches and divided by 12 to get its coverage in feet. The waste factor (about 10%) covers cuts and defects. Joist spacing is the on-center distance in inches; dividing by 12 gives the spacing in feet, and the "+1" accounts for the joist at the starting edge. The board coverage ignores the roughly 1/8-inch gap DCA 6 calls for between boards, which makes the estimate about 2% generous for 5.5-inch boards.

A worked example with real numbers

Take a 16 ft × 12 ft deck with 5.5-inch boards that come in 16-foot lengths, framed at 16 inches on center:

• Area: 16 × 12 = 192 sq ft.
• Board coverage: 5.5 ÷ 12 = 0.458 ft per board.
• Linear feet: 192 ÷ 0.458 = 419 ft base, × 1.10 waste = ~461 ft.
• Boards: 461 ÷ 16 = 28.8, rounded up to 29 boards.
• Joists: 16 ÷ 1.33 = 12, + 1 = 13 joists, each spanning 12 ft; DCA 6 Table 2 allows a Southern Pine 2x10 to span 14 ft 0 in at 16 inches on center (a 2x8 only 11 ft 10 in).
• Footings: attached deck, one beam line, posts at most 8 ft apart: 16 ÷ 8 = 2, + 1 = 3 footings (6 if freestanding).
• Screws: 144 ÷ 5.5 = 26.2, so 27 board rows × 13 joists = 351 crossings × 2 = 702 screws.

Order whole boards, so you would buy 29 sixteen-foot boards. If you instead chose 12-foot boards, you would need about 39 of them but with more end seams to plan around.

How many joists do I need by deck length and spacing?

Joists run perpendicular to the decking. Count them along the length the boards run: divide that length by the on-center spacing, then add one for the starting joist. The table below is computed as ceil(length ÷ spacing) + 1 for the three standard spacings. IRC Table R507.7 allows 16 inches under 5/4 wood decking and 24 inches under 2-inch decking when boards run perpendicular to the joists (12 and 16 inches for diagonal boards); composite decking follows its label.

Counts include the joist at both edges. They size the number of joists, not their span; confirm the joist size and maximum span against a code span table or the American Wood Council DCA 6 deck guide (the calculator shows the DCA 6 Table 2 size for your width).

How to measure your deck

Accurate inputs make the estimate useful. Work through these steps:

1. Measure length and width of the finished deck surface in feet. In the calculator, length is the side the boards run along (along the house on an attached deck) and width is the joist span (from the house out to the beam).
2. Pick your board width. Wood decking in a nominal 6-inch width is dressed to 5-1/2 inches under the American Softwood Lumber Standard; composite widths vary by brand. Use the actual coverage, not the nominal size.
3. Choose a board length that minimizes seams, ideally one that spans the deck in a single piece. Common lengths are 8, 10, 12, 16 and 20 feet.
4. Set joist spacing. 16 inches on center is the IRC limit under 5/4 wood decking laid square to the joists; use 12 inches for diagonal 5/4 boards, and follow the label for composites.
5. Confirm the waste factor. Keep 10% for a rectangle; raise it to 15 to 20% for diagonal or picture-frame designs.

Who this calculator is for

It is built for anyone turning a deck idea into a shopping list:

• DIY builders pricing a weekend project before heading to the store.
• Homeowners sanity-checking a contractor's material estimate.
• Remodelers swapping old wood decking for composite and re-figuring board counts.
• Budgeters who want a quick cost baseline by multiplying boards and joists by unit price.

Key terms explained

• Square footage: the deck surface area, length × width, that drives every other number.
• Linear feet: the total running length of decking, regardless of width: the unit decking is sold and ordered by.
• Board feet: a volume measure from the nominal size (thickness × width × length) used for lumber pricing, not for ordering decking; convert framing lumber with the Board Feet Calculator.
• On center (OC): the distance from the center of one joist to the center of the next; 16" OC is the IRC limit under 5/4 wood decking laid square to the joists.
• Waste factor: the percentage of extra material added for cuts, trimming and defects.
• Footing: the concrete base under each post that transfers the deck's load to the ground.
• Cubic yard: the unit you'll buy ready-mix concrete in for footings (27 cubic feet), handy when you price the concrete portion with a Concrete or Cubic Yard Calculator.

How many linear feet of decking per 100 sq ft by board width?

Board width is the biggest lever on how much decking you buy. This quick reference shows the linear feet needed to cover 100 square feet at each common face width, computed as 100 ÷ (width ÷ 12), plus a 10% waste allowance. Multiply the "+10% waste" column by your deck's square footage ÷ 100 to get your order.

A wider face means fewer boards and fewer fasteners for the same area, but each board costs more, so the total linear feet, not the board count, is what drives material cost.

Deck board and joist counts by deck size

This matrix is computed with the tool's own formula: 5.5-inch decking, a 10% waste factor, and 16-inch on-center joists. The board count assumes a board length that spans the deck in one piece (shown next to each count). Read it as a starting order for common rectangular footprints.

The 20-foot rows assume 20-foot boards are available; if you can only get 16-foot lengths you would buy more boards and plan for one row of end seams. Bump the linear-feet column by another 5 to 10% for diagonal patterns.

Three common scenarios

Different layouts change the count even for the same footprint:

• Small 10 × 10 platform (100 sq ft): about 240 linear feet with waste, roughly 24 ten-foot boards, and 9 joists at 16" OC. A clean single-row layout with almost no seams.
• Standard 16 × 12 deck (192 sq ft): about 461 linear feet, 29 sixteen-foot boards, 13 joists and 3 footings if attached: the example above.
• Large 20 × 16 diagonal deck (320 sq ft): bump the waste to 18% for the angled cuts, giving roughly 825 linear feet, and tighten joists to 12" OC (21 joists), because IRC Table R507.7 limits 5/4 wood decking laid diagonally to 12-inch joist spacing.

Factors that change the result

• Board width: a wider board covers more area, so you need fewer boards and fewer fasteners.
• Board layout: diagonal and herringbone patterns add 15 to 20% waste and require tighter joist spacing.
• Joist spacing: moving from 16" to 12" OC adds joists and screws but stiffens the deck.
• Attached or freestanding: a ledger on the house replaces one beam line, so a freestanding deck needs twice as many footings.
• Deck shape: L-shapes, curves and cut-outs raise waste; break complex decks into rectangles and add them up.
• Board length vs. deck span: choosing a length that spans the deck in one piece cuts seams and offcuts.

Tips for ordering

• Always order about 10% extra (15 to 20% for diagonal or framed patterns) and keep a few spares for future repairs.
• Buy boards long enough to span the deck in one piece where possible; fewer seams look better and waste less.
• Match dye lots on composite boards by ordering all at once, since color can vary between production runs.
• Add fasteners to the order: two screws at every joist crossing (about 700 for a 16 × 12 ft deck), or about one hidden clip per crossing for grooved boards.
• Don't forget framing and trim: beams, ledger, fascia, joist hangers and structural screws are separate from the deck boards.

Limitations and assumptions

This is a planning estimate, not an engineered design:

• It assumes a rectangular deck; split irregular shapes into rectangles and total them.
• The footing count is a planning estimate (posts at most 8 ft apart along each beam); the actual post layout depends on beam size, span tables, soil and code.
• It sizes the deck surface and joists and checks joist size against DCA 6 Table 2 for Southern Pine; it does not size beams, railings, stairs, ledger or fascia.
• Composite and PVC products follow the manufacturer's span and fastening tables, which override generic spacing.
• Local building codes and permits govern structural decks, so always verify before you build.

Related materials and calculators

These tools each size a different building surface: use this deck calculator for boards, joists and footings; the Concrete Calculator for the footing volume under the posts; the Paint Calculator for stain or sealer on the finished deck; and if the deck is covered, the Roofing Calculator with the Roof Pitch Calculator to size the shingles and slope of the roof above it.

Sizing the deck surface is one piece of the project. To finish your estimate, pair this tool with the calculators for the parts it does not cover:

• Use the Concrete Calculator or Cubic Yard Calculator to figure the concrete for your footings.
• Use the Square Footage Calculator to total an L-shaped or multi-level deck before estimating boards.
• Use the Gravel Calculator for a base pad under a ground-level or floating deck.
• Use the Paint Calculator to estimate stain or sealer for the finished surface.
• Use the Mulch Calculator for the landscaping beds around the new deck.
• Use the Board Feet Calculator if you are pricing rough lumber for the framing by volume rather than by length.
• Building a perimeter to match? The Fence Calculator and Stair Calculator size the rail run and the steps off your finished deck.

How this deck calculator compares to others

Most online deck estimators do one job each, so it helps to know what this page is built to answer:

• Board count vs. cost: this tool gives you quantities: boards, joists, footings, fasteners. Multiply those quantities by your local unit prices to turn them into a budget, or feed the framing volume into the Board Feet Calculator for rough-lumber pricing.
• Surface vs. structure: this page sizes the deck surface and floor joists and shows the DCA 6 joist size for your span. It does not run beam or post span tables, so it pairs with a code span table rather than replacing one.
• Rectangle vs. complex shapes: for a single rectangle the numbers here are direct. For an L-shape, a wrap-around, or a multi-level deck, size each rectangle with the Square Footage Calculator first, then add the board counts.
• Wood vs. composite: the math is identical; only the board coverage width and required joist spacing change. Enter your product's actual face width and follow the manufacturer's span table.

Estimating cost from the board count

The calculator stops at quantities on purpose, because prices swing by region, species, grade and decking material. To get to a dollar figure, multiply each quantity by a local unit price and add them up. For the 16 × 12 example that means 29 decking boards, 13 joists, the beam and ledger, 3 footings with posts and hardware, and about 700 screws, before railings, stairs and fascia. Because composite and PVC boards are priced very differently from treated wood, the board count matters: shaving the waste factor or choosing a board length that spans the deck in one piece saves real money across a large surface. Get a current quote from your supplier for each line.

Sources

• International Code Council: 2021 International Residential Code, Section R507 (exterior decks), including Table R507.7 decking joist spacing.
• American Wood Council: DCA 6, Prescriptive Residential Wood Deck Construction Guide (Table 2 joist spans, Table 3A beam spans, decking fasteners, footing depth).
• NIST: PS 20-20, American Softwood Lumber Standard (dressed sizes and board measure).
• Always confirm joist spacing, beam sizing, fastening and footing depth with your local building department and the decking manufacturer's installation instructions.