Deck Material Calculator

Layout arithmetic, worked

The worked deck: 12 ft of width is 144 in, so at 16 in on-center the frame takes floor(144 ÷ 16) + 1 = 10 joists, each 16 ft, plus 2 rim boards at 12 ft. Decking: each 5.5 in board + 0.25 in gap covers 5.75 in of the 16 ft length — 192 ÷ 5.75 = 33.39 rows, 10% waste → 36.73, rounded up to 37 rows × 1 board from 12 ft stock = 37 deck boards. Computed at build time by the same engine wired to the form.

The counts above are pieces. If you want the joist list or the decking priced by volume instead — the way a bill of framing stock can be quoted — the guide Board feet, explained covers the lumber volume unit, the formula the board foot calculator runs, and the nominal-versus-dressed distinction that decides which cross-section to enter. Read it before you carry these counts to the board foot calculator. The rest of this page explains what the deck tool does with the numbers you give it, stage by stage, and how to read what comes back.

What the calculator computes, stage by stage

Every estimator on this site follows the five-stage pipeline documented on the methodology page. The deck engine runs it twice over, in effect: once for the frame, where every figure is an exact layout count, and once for the decking, where the purchasing stages — coverage, waste, rounding — all apply. It counts material for a layout you have already designed and makes no structural decision at any stage: quantities only.

1. Geometry. Length times width gives the footprint in square feet, reported for reference. The frame is laid out from the width: the width is converted to inches (× 12), divided by the joist spacing, rounded down, and one is added — one joist at each edge plus one per full interval between them. Each joist is as long as the deck, and two rim boards, each as long as the deck width, cap the ends. These are exact counts: no waste is applied to them and nothing is rounded beyond that floor.
2. Coverage. For the decking the "unit" is one row of boards, and what a row covers is its board width plus its gap, in inches of deck length. Net rows = length in inches ÷ (board width + gap), a fractional figure the engine carries unrounded. Independently, the whole stock boards each row takes is ceil(deck width ÷ stock board length), decided by the stock you buy rather than by the deck.
3. Waste. The engine applies rows × (1 + waste ÷ 100) to the net row count only. The default of 10% is a trade convention recorded in the site's materials data for end cuts and culled boards, not a manufacturer figure, and the form lets you replace it. Joists and rim boards get no allowance, because a layout count is exact.
4. Package rounding. The waste-adjusted rows round up to whole rows, the boards per row are already a whole number, and boards to buy is the product of the two. Both ceilings carry the shared engine's exact-fit guard, so a quantity within one part in a billion of a whole number is treated as that whole number rather than gaining a phantom row or board from floating-point noise. These are the only roundings in the chain, and both go up.
5. User-priced cost. A screw rate, if you give one, is multiplied by the boards to buy; a price per board, if you give one, is multiplied by the same count. The site ships no fastening schedule and no price: leave either field blank and its line does not appear.

Why each input matters

Length and width are not interchangeable, even though their product is. Length is the direction the joists run; width is the direction they are spaced across and the direction each deck board runs. Enter the worked deck the other way round — 12 ft of length and 16 ft of width — and the same 192 sq ft produces a different list: 13 joists of 12 ft with rim boards of 16 ft, and 28 rows that each take 2 boards from 12 ft stock, 56 boards in all against 37. Enter the fields the way your plan runs the joists. Both go in as decimal feet: a tape reading of 14 ft 6 in is entered as 14.5, the site's convention for every dimension before any arithmetic runs, and the feet & inches calculator handles that conversion for awkward readings.

Joist spacing comes from your plan, and the menu offers the standard layouts without choosing between them. It moves the joist count and nothing else: the 20 × 14 ft deck in the examples takes 15 joists at 12 in on-center, 11 at 16 in, and 8 at 24 in, while its decking rows stay at 46 in every case. Which spacing is right for the joists you are using is the structural question the note under the form hands to your plan, your building department, or an engineer; the tool counts pieces for the spacing you tell it.

Board width and gap set the row pitch, and the row count moves inversely with their sum. The default width is the dressed size of a nominal 6 in board — a sourced figure, listed below — but it is a starting point, not a measurement of your product: a board you measure at 5.25 in with the same 0.25 in gap needs 39 rows on the worked deck instead of 37, and the default board at a 0.125 in gap needs 38. Measure the face of the board you will actually lay and enter the gap your product or your fastening system calls for; the tool chooses neither.

Stock board length decides how many whole boards each row takes, and it is the one input that can change the purchase without changing a single row. Boards per row is ceil(width ÷ stock length), computed per row with no credit for the offcut: a 14 ft wide deck is 1 board per row from 16 ft stock and 2 from 8 ft stock, so the same rows cost 2 times the board count in the second case, with the offcut from each row left over. Run the tool once per stock length your supplier carries and compare the board line.

Waste applies to the decking rows only, so it moves the board count in whole-row steps. On the worked deck the default takes 34 rows to 37; small changes to the percentage often change nothing, because the round-up absorbs them, and then a further change tips the count by a row. Set it to 0 if your cut list already carries a margin, and the engine buys the bare rows rounded up. What the percentage should be for your own project is the subject of Waste factors, explained.

Screws per board and price per board touch only the last stage. Neither feeds back into the quantity, so you can add or change them after the fact without disturbing the count, and both are multiplied by the boards to buy — the rounded figure — because that is what you purchase.

The units toggle changes what the fields expect, not what is computed. In metric mode, meters are divided by 0.3048 and centimeters by 2.54 — both exact definitions — before the engine sees them. Leave the metric board width blank and the engine falls back to the same 5.5 in dressed figure (13.97 cm, which the field label rounds), and a blank gap falls back to 0.25 in. The result then reports the decking to cover in meters; the joist and rim lengths are printed in feet in both modes.

The figures this page relies on

• Default board width, 5.5 in — the minimum dry dressed width of a nominal 6 in board under Voluntary Product Standard PS 20-20, the American Softwood Lumber Standard published by NIST (Table 3), recorded in the site's materials data with its source; a 5/4×6 deck board dresses to 1 × 5.5 in. A published-standard figure, presented as a labeled, editable default because composite boards deviate. The thickness is held alongside it but plays no part here — this tool counts pieces, not volume.
• Default decking waste, 10% — a common trade convention recorded in the materials data, where it is labeled a convention because no primary source publishes a universal figure; it covers end cuts and culled boards. Adjustable in the form, and applied to the decking rows only.
• Rim boards, 2 — the engine's layout convention: two band boards capping the ends of the joists, each as long as the deck width. Arithmetic on the layout, not a sourced figure.
• Board gap, 0.25 in, and the spacing menu — starting values built into the form, not sourced constants. The gap is yours to change; the spacings offered are the standard layouts, and the choice between them belongs to your plan.
• Inches per foot, 12; meters per foot, 0.3048; centimeters per inch, 2.54 — exact definitions, used to turn the width into inches for the joist layout and, in metric mode, to convert what you type into the engine's feet and inches.
• Fastening schedule and price — none. This page holds no screw rate and no price. Both appear only as your entries, as the methodology page states.

Worked examples

Every figure in this table was produced at build time by the same engine that runs the form above. Unless the label says otherwise, each deck uses the default 5.5 in board, a 0.25 in gap, and the default 10% waste; the metric row was entered as meters and centimeters and converted exactly before the engine ran, which is what the metric toggle does.

The worked deck, with and without an allowance. The geometry is the same in both rows: 33.39 net rows. With the default 10% the rows become 36.73 and round up to 37; with the waste field at 0 they round up to 34. The allowance is therefore 3 rows on this deck — 3 boards, because each row is one board from 12 ft stock — and the round-up alone is what separates 33.39 from 34. The frame does not move at all: 10 joists and 2 rim boards either way.

The small deck. 12 × 10 ft at 16 in on-center: 120 in of width ÷ 16, floored, plus one is 8 joists of 12 ft, plus 2 rim boards of 10 ft. Decking: 144 ÷ 5.75 = 25.04 net rows, 28 after the allowance and the round-up. The stock length equals the deck width, so the boards-per-row division is exactly 1 — the exact-fit guard keeps a binary hair above the boundary from buying a second board on every row — and the order is 28 boards for 250.43 ft of net board run.

The 20 × 14 ft deck, three ways. Its 280 sq ft, 168 in of width, and 41.74 net rows are the same in all three rows of the table. At 12 in on-center the frame is 15 joists; at 24 in it is 8; at 16 in it is 11. The decking is 46 rows in every case, but from 8 ft stock each row takes 2 boards, so the order is 92, while from 16 ft stock each row is one board and the order is 46. Spacing moved the frame; stock length moved the decking; neither touched the other.

A narrower board and a tighter gap. A board measured at 5.25 in with the 0.25 in gap has a row pitch of 5.5 in, so the worked deck's 192 in of length needs 34.91 net rows and 39 after the allowance — 39 boards against 37. The default board at a 0.125 in gap has a pitch of 5.625 in: 34.13 net rows, 38 to buy. The engine sees only the sum of width and gap, so a narrow board with a wide gap and a wide board with a narrow gap that add to the same pitch give the same row count.

The deck measured in feet and inches. The tape says 14 ft 6 in by 11 ft 3 in, so the fields take 14.5 and 11.25. The width is 135 in, which at 16 in on-center gives 9 joists of 14.5 ft and 2 rim boards of 11.25 ft. The length is 174 in, so 30.26 net rows become 34 after the allowance; from 12 ft stock each row is 1 board with 0.75 ft of offcut, and the order is 34 boards.

The priced deck, with a screw rate. The worked deck again, this time with 20 typed into the screws-per-board field as a stand-in for whatever your fastener maker's schedule works out to, and $10.00 typed into the price field as a stand-in for your supplier's quote. The quantity does not change — 37 boards — and the two new lines are 37 × 20 = 740 screws and 37 × $10.00 = $370.00. Both figures are placeholders chosen only to show the arithmetic; the site holds no fastening schedule and no price, and both lines multiply the rounded board count because the rounded count is what you buy.

The metric deck. In metric mode, 4.9 × 3.7 m in 14 cm boards with a 0.6 cm gap from 3.7 m stock reaches the engine as 16.08 × 12.14 ft in 5.51 in boards with a 0.24 in gap from 12.14 ft stock. The width is 145.67 in, so the frame is 10 joists of 16.08 ft and 2 rim boards of 12.14 ft; the length is 192.91 in at a pitch of 5.75 in, so 33.56 net rows become 37. The stock length equals the width, an exact fit of 1 board per row, and the order is 37 boards. The decking-to-cover line reads 124.18 m — the engine's 407.41 ft of board run multiplied by the exact 0.3048 m per foot.

How to read the result

The headline line is the purchase: N deck boards, N joists + 2 rim. Beneath it, Joists at your N in o.c. plan gives the joist count with each joist's length and the two rim boards with theirs — the frame list, exact. Board rows shows the net row count and the percentage that was added, then the whole rows times the boards per row, so you can see how much of the board count is allowance and how much is the stock-length split. Decking to cover is the net run of decking before waste — rows times width — in linear feet, or in meters in metric mode; it is a check on the geometry, not a figure to buy. A screw line appears only when a rate is entered and a cost line only when a price is, and both multiply the boards to buy. Read the joist line against your plan, the row line against your board and gap, and the headline to place the order. The copy button places a short plain-text summary of the result on the clipboard.

Common misreadings

A length in inches. Deck length, width, and stock length go in as feet; board width and gap as inches. Enter the worked deck's 16 ft length as 192 and the engine sees a deck 192 ft long: 10 joists of 192 ft, 400.7 net rows, and 441 boards instead of 37 — 12 times the rows, because there are 12 inches to the foot. A joist length of 192 ft means the length field.

A board width in feet. The same slip in the other direction, and the slab tools' thickness-in-feet error in this tool's clothing: type the 5.5 in board as 0.4583 — its width in feet — and each row covers only 0.71 in, so the worked deck needs 271.06 net rows and 299 boards. The joist count is untouched, which is the tell: a frame that looks right under a board count that looks absurd means the board width or gap field.

A stock length in inches. This one is silent. Type 12 ft stock as 144 and the engine sees stock 144 ft long, so every row fits in one board: the 20 × 14 ft deck drops from 92 boards to 46, with no error, because 1 board per row is a legitimate answer for long stock. If the boards per row reads 1 on a deck wider than the stock you can buy, check the field.

Metric figures in the imperial fields. The metric deck above is 4.9 × 3.7 m in 14 cm boards. Type those same numbers with the toggle still on Imperial and the engine sees a 4.9 × 3.7 ft deck in 14 in boards: 3 joists and 5 boards against the correct 10 and 37. If the list looks implausibly small for the deck, check the toggle.

Waste as a fraction. The waste field takes a percentage. The worked deck at 10% buys 37 boards; typed as 0.1 it is read as 0.1%, and the count drops to 34 — the allowance gone, with no warning, because a fraction of a percent is a valid input.

Rows read as boards, or the net run read as the order. The row count is the number of courses across the deck; the board count is rows times whole stock boards per row, and on a deck wider than its stock the two differ by that factor. Likewise the decking-to-cover figure is the geometry's net run, before waste and before the stock-length split — the worked deck needs 400.7 ft of board run and buys 37 boards × 12 ft = 444 ft of stock. Buy the headline, not the run.

Padding twice. Rounding a dimension up before entering it, or adding a margin to the board count afterwards, repeats work the pipeline already does: the allowance is applied to the rows and the round-up to the allowance, exactly once each, and the stock-length split adds its own offcut on every row. The no-waste row in the table shows what the rounding alone adds; the allowance is meant to be the only other margin.

Where the model stops

The engine models one rectangle with joists running its full length at a single spacing, two rim boards, and decking laid straight across the joists in parallel rows from one stock length. It does not model a picture-frame border, diagonal or patterned decking, a notched or multi-level footprint, blocking, stair stringers or treads, railings, or the beams, posts, ledger, and hardware beneath the frame — none of those are counted, and the waste allowance is the single flat percentage you are given to fold the decking's share of them into. It counts no fasteners beyond the rate you type and holds no price. And, as the note under the form says in bold, it makes no structural decision: it does not size joists, set allowable spans, place beams or footings, or design ledger connections. Those come from code span tables, your building department, or an engineer, and the spacing and dimensions you enter should already have passed through that review. The site estimates material quantities only. The methodology page lists the simplifications every estimator shares.

Framing lumber, the posts below, and the guides

Pricing framing stock by volume? The board foot calculator turns your joist list into board feet at your price, and Board feet, explained covers the unit and the nominal-versus-dressed choice it depends on. The posts under the frame are set with the post hole calculator's concrete math, pier forms with the concrete column calculator, and the fence around the yard with the fence calculator, whose posts, rails, and sections are exact layout counts in the same way the joists here are. The feet & inches calculator turns tape readings into decimal feet, and the construction unit converter moves any figure between feet and meters. For the pipeline itself, see how material estimates are calculated, waste factors, explained, and metric and imperial in the estimators, or browse all guides.

Frequently asked questions

Dimensions never leave your browser. Sources, conventions, and the estimating pipeline are documented on the methodology page.