# Deck Material Calculator

How many deck boards, joists, hangers and screws you need — counted row by row, with the board length that wastes least and a code span check.

- Runs in your browser
- Cut-aware board count
- IRC R507.6 span check
- Free to embed

## Quick answer

A **16 × 12 ft** deck laid straight with 5.5-inch boards and 3/16-inch gaps needs **26 rows** of decking, which is 28 boards of 16 ft with 5% extra, 13 joists at 16 inches on center and about 710 deck screws. The joist size has to suit the span: IRC 2021 Table R507.6 limits it by species and spacing.

## About the Deck Material Calculator

A materials takeoff for a rectangular deck. Give it the width along the house, the depth out from it, your board size and gap, and it lays the decking out row by row, tries every common stock length to find the one that wastes least, and counts joists, joist hangers, rim boards and either face screws or hidden clips. An optional check compares the joist span with IRC Table R507.6 for Southern pine, and optional unit prices turn the list into a material budget you can take to the lumber yard.

## How to use it

Enter the deck's **width along the house** and its **depth out from the house**. The calculator assumes the usual layout for an attached deck: joists run from the ledger on the house out to a beam, and decking boards run parallel to the house across the joists. If your boards run the other way, just swap the two numbers.

Set the board face width and the gap you plan to leave. Most decking — 5/4×6 and 2×6 lumber as well as composite boards — is 5½ inches wide in reality, whatever the label says. Leave _Board length_ on Auto and the calculator tries 8, 10, 12, 16 and 20 ft stock and picks the length that wastes the least; the comparison table shows all five so you can choose based on price or what your yard has in stock.

Pick a joist size to get a quick span check against the residential code table, and choose face screws or hidden clips to get the right fastener count. Prices are optional: add a price per board and per joist and you get a material subtotal you can compare between suppliers.

## Use cases

- **Making a shopping list.** Get board, joist, hanger and fastener counts before you call the yard or load the truck.
- **Picking a board length.** Compare 8, 10, 12, 16 and 20 ft stock for your exact width and see the offcut each one leaves.
- **Screws or hidden clips.** Switch fastener types to see how the count changes, and add prices to compare cost.
- **Checking a quote.** Hold a contractor's material list up against an independent count for the same dimensions.
- **Planning a diagonal deck.** See how many more boards a 45° layout needs and whether your joist spacing suits it.

## How the takeoff works

Most deck calculators divide area by board coverage and add 10–15% for waste. That works on average but hides where the waste goes. This one lays the boards out row by row instead:

rows = ⌈depth in inches ÷ (board width + gap)⌉

Each row has to cover the full deck width. If one board is long enough, every row uses one board and the offcut is left over. If the rows are longer than a board, each row uses as many full boards as fit plus one shorter piece — and those shorter pieces are packed onto as few extra boards as possible. That is a small version of the cutting-stock problem lumber yards solve every day, and it is why a 16 ft deck needs far fewer 16 ft boards than 10 ft boards even though the square footage is the same.

Because routine offcuts are already counted, the **Extra boards** percentage only has to cover warped or damaged boards and the odd miscut. Five percent is a sensible default for a straight layout. Diagonal decks are counted by area instead, since every row is a different length; use around 15% extra for the angled end cuts.

Framing and fasteners follow standard practice:

- **Joists** \= ⌈width ÷ spacing⌉ + 1, which includes both end joists. Joists are listed at the next even stock length.
- **Joist hangers**: one per joist at the ledger.
- **Face screws**: two per board at every joist it crosses, plus 5%. That works out to roughly 350–370 screws per 100 sq ft at 16″ on center.
- **Hidden clips**: one per board per joist crossing, about 1.75 per sq ft — in line with the 90 clips per 50 sq ft that Trex quotes for its Hideaway clips at 16″ spacing.

## Worked example: a 16 × 12 ft deck

A 16 ft wide, 12 ft deep deck with 5½-inch boards, a 3/16-inch gap and joists at 16″ on center.

1. Each row covers 5.5 + 0.1875 = 5.6875 inches. 144 inches of depth ÷ 5.6875 = 25.3, so 26 rows — the last one ripped to about 1.8 inches, or adjusted with a slightly wider gap or overhang.
2. 26 rows × 16 ft = 416 linear feet of decking. With 16 ft boards that is exactly 26 boards and no offcut; 12 ft boards would need 35 and leave short pieces behind.
3. Add 5% for defects: 28 boards.
4. Joists: 192 inches ÷ 16 = 12, plus one = 13 joists, bought as 12 ft lumber, with 13 hangers.
5. Screws: 26 rows × 13 joists × 2 = 676, plus 5% = 710.

## Joist span quick reference

The span check uses IRC 2021 Table R507.6 for Southern pine No. 2 joists carrying 40 psf live load and 10 psf dead load in wet service — the common case for a residential deck without heavy snow. Spans are the maximum distance between supports (ledger to beam, or beam to beam), not counting any cantilever.

| Joist | 12″ o.c. | 16″ o.c. | 24″ o.c. |
| ----- | -------- | -------- | -------- |
| 2×6   | 9′-11″   | 9′-0″    | 7′-7″    |
| 2×8   | 13′-1″   | 11′-10″  | 9′-8″    |
| 2×10  | 16′-2″   | 14′-0″   | 11′-5″   |
| 2×12  | 18′-0″   | 16′-6″   | 13′-6″   |

Other species — Douglas fir-larch, hem-fir, spruce-pine-fir, redwood and cedar — span less, and snow loads above 40 psf shorten every value. Look up your species and load in the code table before you buy.

## Gaps, joist spacing and board choice

Composite boards expand and contract with temperature, so manufacturers specify gaps. Trex, for example, asks for at least 3/16 inch between boards side to side, and end gaps that depend on the installation temperature. Hidden clip systems set the side gap automatically. Wood decking shrinks as it dries: pressure-treated boards are often laid tight and open up on their own, while kiln-dried boards get a gap of about 1/8 inch.

Joist spacing depends on the decking, not just the framing. Most 1-inch composite and 5/4 wood boards are rated for 16″ on center when they run straight across the joists, and 12″ when laid at 45°. Thicker 2× boards can span 24″. Check the installation guide for your exact product — exceeding its span voids most composite warranties.

For budgeting, decking boards alone run roughly $2–5 per square foot for pressure-treated pine, $3–7 for cedar and $5–18 for composite, according to Angi's cost guides (January 2025). Framing, fasteners, railings, stairs and labor typically cost several times more than the boards.

## Limits of this calculator

- It handles a single rectangle. For L-shaped decks, split them into rectangles and add the results.
- It does not size beams, posts, footings, ledger fasteners, stairs or railings, and the span check covers one species and load case.
- Butt joints must land on a joist (usually doubled or blocked). In practice you may buy one or two extra boards to stagger seams nicely.
- Picture-frame borders, breaker boards and herringbone patterns need their own count.

## Worked examples

__Straight layout, 5.5-inch boards, 3/16-inch gaps, joists 16 inches on center, 5% extra boards, screws — computed by this calculator.__
| Deck (width × depth) | Rows of boards | Boards to buy | Joists | Screws |
| -------------------- | -------------- | ------------- | ------ | ------ |
| 10 × 10 ft           | 22             | 12 × 20 ft    | 9      | 416    |
| 12 × 12 ft           | 26             | 28 × 12 ft    | 10     | 546    |
| 16 × 12 ft           | 26             | 28 × 16 ft    | 13     | 710    |
| 20 × 14 ft           | 30             | 32 × 20 ft    | 16     | 1008   |

## Sources

- [International Residential Code 2021 — Chapter 5, Floors (Section R507, Decks) (ICC)](https://codes.iccsafe.org/content/IRC2021P2/chapter-5-floors) — checked 2026-10-10
- [American Wood Council — DCA 6, Prescriptive Residential Wood Deck Construction Guide](https://awc.org/publications/dca/dca6/) — checked 2026-10-10

## Frequently asked questions

How many deck boards do I need for a 12×12 deck?

With 5½-inch boards and a 3/16-inch gap, a 12 × 12 ft deck has 26 rows. Using 12 ft boards that is 26 boards plus about 5% extra — 28 boards. Enter your own size above; the board-length table shows the alternatives.

What is the actual width of a 5/4×6 or 2×6 deck board?

Both are 5½ inches wide. 5/4×6 is about 1 inch thick and a 2×6 is 1½ inches thick. Most composite boards are also 5½ inches wide and just under 1 inch thick.

How much gap should I leave between deck boards?

Follow the manufacturer: Trex specifies at least 3/16 inch between composite boards. Kiln-dried wood is usually gapped about 1/8 inch; wet pressure-treated lumber is often butted tight because it shrinks as it dries.

How many screws do I need per square foot of decking?

About 3.5 to 3.7 face screws per square foot at 16″ joist spacing — two per board at every joist. Hidden clip systems use roughly 1.75 clips per square foot.

Should I buy 12, 16 or 20 foot deck boards?

Pick the length that divides your deck width with the least leftover, or that spans it in one piece. Fewer butt joints look better and need less blocking. The board-length table above compares the options for your exact size.

How far can deck joists span?

Under IRC Table R507.6, Southern pine No. 2 2×8 joists can span 11′-10″ at 16″ on center, 2×10s 14′-0″ and 2×12s 16′-6″ (40 psf live load). Other species and higher snow loads span less.

How much extra decking should I buy for waste?

This calculator already counts offcuts, so 5% for defects is usually enough on a straight layout. For diagonal decking add around 15%, and for herringbone or picture-frame designs up to 20%.

**Disclaimer.** Results are a materials estimate for planning and ordering, not a structural design. Decks must be built to your local code (many jurisdictions follow the IRC and AWC DCA 6) and the decking manufacturer's installation instructions; get a permit and an inspection where required.

Last updated October 11, 2026. Found a mistake? [Tell us](https://17x7.com/contact/) — corrections ship the same week.

---

Source: https://17x7.com/tools/deck-material-calculator/ · Last updated: 2026-10-11