Deck Calculator

What does a deck calculator count?

The whole material order, not just the walking surface. From two dimensions — the run along the house and the projection out from it — the card counts deck boards in four stock lengths, the joists and rim under them, hangers, support posts with a concrete bag allowance, deck screws by the box, a 36 in railing with posts and balusters, and the stain to finish it.

Four material presets sit at the top: treated pine, cedar/redwood, composite and ipe. The case this page follows throughout is a 20 × 16 ft treated-pine deck on a ledger, standard 5/4×6 boards laid parallel, 16 in joist spacing and the default 10% cutting buffer: 47 boards in 16 ft lengths, 749 linear feet of decking, 16 joists, 4 posts and about 1,120 screws for 320 sq ft of deck.

How does the calculator work?

Type the two dimensions and every line on the right is that one rectangle read a different way. The headline is the board count in the length picked in the Board Length menu; under it sit the buffered linear footage and the deck area. One quirk to know before it bites: the headline label always reads “(16ft Length)” whatever length you pick — the *number* follows your menu, and the three boxes underneath restate the footage as 12, 16 and 20 ft counts so you can sanity-check it.

There is no quantity box and no second-deck box. Two decks means two runs and adding the lists yourself — or one run per level of a multi-level deck.

How do you calculate deck boards?

Boards are counted by rows, not by area. Each row covers the board’s face width plus the 3/16 in gap the card leaves between boards:

On the 20 × 16: 34 rows × 20 ft is 680 linear feet bare. Buffer and round-up print 749 ft — a foot over the paper product, because the card rounds buffered footage up to the next whole foot and orders from that. 749 ÷ 16 ft, rounded up, is the 47 in the headline.

3 worked examples

How many boards for a 12 × 16 deck?

29 boards in 16 ft lengths — 38 in 12 ft, 23 in 20 ft — for the standard build: 5.5 in boards, parallel, 10% buffer. The sizes around it, same settings:

Every row: 5.5 in face, parallel layout, 10% buffer, buffer rounded up to whole feet. Note the 12 × 16 and 16 × 12 rows: same area, same board count, 9 ft apart in footage — rows are counted across the projection, so orientation moves the footage even when it spares the count.

What does a 5.5 inch board actually cover?

5.6875 inches of deck per row — face plus the 3/16 in gap. Skip the gap in your own math and you under-buy every deck by about 3%: face-only math squeezes 35 rows into this projection where the card lays 34 and covers, because the gaps are part of the coverage:

16 ft lengths, 10% buffer. Narrow boards nearly double the piece count and every screw row with it; the screw line does not follow — it stays at ~3.5 a sq ft whatever face you pick, so a 2×4 deck wants a second look at the fastener math.

Parallel, perpendicular or diagonal?

Parallel boards run along the house and the joists span the projection — the default, and the layout behind every number above. Perpendicular flips both: boards run out from the house, rows are counted along the house run, and the joists span the long way. Diagonal keeps the parallel row count but the card floors the waste at 15%, whatever the buffer menu says, because nearly every edge board ends in a 45° cut.

5.5 in boards, 16 in spacing, 10% buffer menu. The diagonal row runs at the forced 15% — set the buffer to 0% and a diagonal deck still prints 782 ft. The card does not tighten joist spacing for diagonal; most codes and brands want 12 in under a 45° layout, so set that menu yourself.

Why does the order carry a cutting buffer?

Because boards end. End trims, a split board, the row that lands 2 in short against the house — the buffer menu (0/5/10/15, opening at 10%) is the difference between the footage on paper and the footage on the truck:

The buffer multiplies decking footage only. Joists carry their own built-in 5% on linear feet, screws and stain ride on area, and none of them move when this menu does — watch the joist line hold at 269 ft across the whole ladder.

Joists, rims and hangers: the frame under the boards

Joists are counted across the framing run: ceil(run ÷ spacing) + 1, the +1 closing the far end, with 5% added to their linear feet for cuts. On the 20 ft run:

20 × 16 ft deck, parallel, so each joist spans the 16 ft projection. Rim and ledger reads 76 linear ft at every spacing — the full 72 ft perimeter plus 5%, the house side included, because the ledger is a board too. Freestanding doubles the hangers (both ends bear on beams): 32 on this deck.

The card counts joists; it does not size them. Whether a 16 ft span wants a 2×8, a 2×10 or a mid-span beam is a span-table question — size picked, the lumber converts to board feet on the board foot calculator. The count settles the shopping list: pieces, footage, one hanger per end that hangs.

Posts, footings and concrete bags

Posts are a planning grid, not an engineered layout: a beam line about every 8 ft of joist span, posts about every 8 ft along the beam, and on a ledger deck the house-side line drops away because the ledger carries it. Each post carries an allowance of two 60 lb bags of concrete.

The freestanding version of the same deck triples the posts — both beam lines stand on their own footings and the count floors at four. Two bags a post is a register allowance for a typical tube footing; where a diameter and depth are specified, the pour itself is figured on the concrete calculator.

A tall deck, a hot tub corner or a roof load leaves this grid behind. It keeps you from forgetting footings; it does not stamp the layout.

What does the cost line do?

It multiplies the deck’s area by the rate you type, then adds the delivery fee: area × $/sq ft + fee, printed to the cent with no thousands separator. Both boxes open empty and the line reads $0.00 — the material presets name themselves and set spacing, but no preset carries a price.

Every dollar on the rate is $320 on this deck, so bring a rate that matches the quote you are testing: a boards-only rate against a boards-only question, an all-in installed rate against the whole job. The card cannot tell which you typed.

What the cost line does not include

• Your labor or a crew’s. If the typed rate was materials-only, the build itself is still unpriced.
• Stairs. No stringers, no treads, no stair railing anywhere on the card — a deck two steps up and a deck eight steps up read identical here.
• Hidden fasteners. Clip systems replace the screw count with a clip per joist crossing; that box is not on this list.
• Ledger flashing, tension ties and the permit. Small lines, real money, none of them counted.
• Footings beyond the allowance. Deep frost or a specified diameter outgrows two bags a post fast.
• Demolition and haul-off. The old deck coming out is a dumpster the card never orders.

Treated pine, cedar, composite or ipe?

On the counts, three of the four presets are the same deck. Treated pine, cedar/redwood and ipe return identical numbers everywhere — the preset records a name for the copied summary. Composite is the one that reaches into the math:

Picking composite forces the spacing menu to 12 in — most manufacturers require it — and picking a wood again restores the spacing you had. If you set 12 in by hand for a diagonal wood deck, the card remembers and hands it back. Stain at a gallon per 200 sq ft is one coat on the deck face; rails, fascia and a second coat are yours to add.

How do you enter metres?

Tap Metric: dimensions in metres, converted to feet inside, priced by the square metre. A 6 × 5 m deck reads 48 boards, 232 m · 30 m², posts 8 (12 bags 40kg), stain ~6.1 L; at $270/m², $8100.00.

Two things stay imperial: framing lines keep their feet — this deck shows joists 16 pcs (276 ft), railing 53 ft — and the toggle converts nothing typed: leave 20 × 16 in the boxes and the card reads a 320 m² terrace needing 501 boards. Retype after switching. Metric stain converts from the rounded gallon figure, so it can sit a tenth of a litre off a from-scratch calc.

How much should you order?

The full list for the common backyard sizes — attached, 5.5 in boards in 16 ft lengths, parallel, 16 in spacing, railing on, 10% buffer:

Boards are the buffered count; the buffer is your cutting allowance, not a spare-parts shelf. Add one extra board of the longest length on top as insurance against the split you find on the last row — on odd-shaped decks, measure each rectangle with the square footage calculator and run the card per rectangle.

What are the common mistakes on a deck order?

• Swapping length and width. Length is along the house. Swap a 12 × 16 and boards hold at 29 but joists jump 10 → 13 — the frame ordered no longer matches the deck drawn.
• Rows at face width, no gap. The 3/16 in gap is coverage; face-only math under-buys by about a row in thirty.
• Buffer at 0% on a diagonal. The card refuses quietly: diagonal floors waste at 15% — 782 ft here, not 680.
• Reading the label, not the menu. It always says 16ft; pick 8 ft boards and the headline 94 is an 8 ft count. Cross-check the three length boxes.
• Composite at 16 in spacing anywhere else. This card forces 12 in and restores yours after; a diagonal in any material wants 12 in set by hand.
• Screws for a hidden-clip deck. The ~3.5/sq ft line is face screws; clips are never counted here.
• Forgetting the stairs. Stringers, treads, stair railing: absent from every line.
• The post grid as engineering. Eight-foot planning spacing is a shopping count; height, snow and hot tubs need a drawing.
• Metric with feet still typed. Nothing converts: 20 × 16 becomes 501 boards and 60 posts.
• Reset keeping your material. It resets everything — treated pine, 16 in spacing — keeping only the unit.

How this calculator counts

Every rule, as the card applies it:

• Rows/footage: pitch = face (3.5 / 5.5 / 7.25 in) + 3/16 in gap. Rows = ceil(width ÷ pitch); perpendicular counts across the length instead. Linear feet = rows × run × (1 + buffer), rounded up whole. Buffer 0/5/10/15% (10 default); diagonal floors it at 1.15.
• Boards: ceil(linear ÷ picked length 8/10/12/16/20 ft); the headline label always says 16ft. The same footage restates at 12/16/20 ft below.
• Frame: joists = ceil(run ÷ spacing) + 1 at 12/16/24 in; joist and rim footage +5%. Rim = full perimeter. Hangers = joist count attached, double freestanding.
• Posts/bags: beam line per 8 ft of span, post per 8 ft along it; attached drops the house line (min 2), freestanding counts all (min 4). Two 60 lb bags a post; metric 1.5 × 40 kg bags, rounded up.
• Screws/railing/stain: screws ceil(area × 3.5), 350 a box. Railing = open sides (length + 2 widths attached, perimeter freestanding), post per 6 ft + 1, balusters 3 a foot; off reads “None (Platform)”. Stain = area ÷ 200 gal, one decimal; “None (Composite)” on composite.
• Materials: treated, cedar and ipe change nothing numeric. Composite forces 12 in spacing (yours is remembered, then restored) and clears stain.
• Cost: area × typed rate + fee, to the cent, no commas; metric rates the m². Fee alone prints the fee; no dimensions print zeros regardless.
• Units/reset: metres convert at ×3.28084; the toggle converts nothing typed; frame and railing lines print feet in both modes. Reset restores every menu and treated pine, keeping only the unit.

Frequently asked questions