Decking Calculator

Decking calculator for deck boards by row, last-row rip widths, butt seams, screws or hidden clips and cost, for rectangular, L-shaped or known-area decks, with an IRC joist check.

How a Decking Calculator Counts Deck Boards

A decking calculator finds how many board rows cross the deck width, then how many stock boards each row needs along the length. Counting rows instead of dividing area by board size catches the gap between boards and the narrow last row you rip to fit.

$$N_{rows}=\lceil \frac{12W+g}{b+g}\rceil$$

W is the deck width across the boards in feet, b is the actual board face width in inches and g is the gap in inches. A nominal 6 in board measures 5-1/2 in, so with a 1/8 in gap each row covers 5-5/8 in of deck.

Take a 16 × 12 ft deck with boards running the 16 ft way. The 144 in width needs 26 rows, and the last row is ripped to 3-3/8 in. Each row is one 16 ft board, so the deck takes 26 boards and 28 with 5% extra for splits and crooked stock.

That buy is 448 linear feet of decking. A quick area shortcut of 192 sq ft divided by 8 sq ft per board, plus 10%, gives 27 boards, because it treats a 6 in board as 6 in wide and ignores the gap and the rip.

How to Measure a Deck for Decking

For a decking calculator, measure the length in the direction the boards will run and the width across them. Boards usually run square to the joists, so the width is normally the joist length and the length is the distance the rows travel along the rim.

Deck area in square feet is length times width. An L-shaped deck is two rectangles measured separately and added, and a curved or angled deck is easiest to split into rectangles and triangles before you add the areas up.

A 12 × 12 ft deck is 144 sq ft, and a 300 sq ft deck can be 15 × 20 ft, 12 × 25 ft, 10 × 30 ft or about 17 ft 4 in square. The shape changes the decking calculator result even when the area stays the same, as the table below shows.

Deck Boards Needed for Common Deck Sizes

Each row of the chart uses 5-1/2 in boards, a 1/8 in gap, joists at 16 in on center, two screws per joist crossing and 5% extra. The decking calculator picks the stock length that leaves the fewest butt seams, then the least board footage.

The two 300 sq ft decks show why shape matters. A 20 × 15 ft deck fits 20 ft boards with no seams, while a 25 × 12 ft deck needs a 20 ft board plus a 5 ft piece in every row, which adds a butt seam to each of its 26 rows.

For 500 sq ft of decking laid 25 × 20 ft, plan on 57 boards 20 ft long, about 1,140 linear feet, and 1,634 deck screws. Each row joins a 20 ft board to a 5 ft piece, and four of those pieces come out of one 20 ft board.

Last Row Width and Board Gaps

The deck width rarely divides evenly into board rows, so the decking calculator shows the last row ripped down. A rip under about half a board looks thin and can split at the screws, and a small change in the gap often fixes it.

All rows use 5-1/2 in boards. On a 16 ft wide deck, opening the gap from 1/8 in to 3/16 in turns a 3/4 in sliver into a 4-5/16 in row and saves one full row of boards. Ripping the first and last rows equally also keeps both edges even.

Gap size follows the material. Decks.com notes about 3/16 to 1/4 in for hidden fasteners and about 1/8 in that some installers use on pressure-treated wood, which shrinks as it dries. Composite boards list their own gap on the label.

Stock Board Length and Butt Seams

When the deck is longer than the longest board, every row needs a butt seam, and the decking calculator counts each one as a joint that has to land on a joist. A board length that divides evenly by the joist spacing lands its ends on joist centers without trimming.

At 16 in on center, 8, 12, 16 and 20 ft boards break on joists, while 10 and 14 ft boards fall halfway between them, at 7.5 and 10.5 joist spaces. Use those lengths for runs that fit inside one board, or trim each end back to the nearest joist.

Wood decking comes in 8 to 20 ft lengths in 2 ft steps, and composite and PVC are usually stocked at 8, 12, 16 and 20 ft, according to Decks.com. Stagger the seams so no two rows break on the same joist side by side.

For a composite decking calculator estimate, choose 12, 16 or 20 ft stock so the lengths match what the yard carries, then confirm the joist spacing on the board label.

Joist Spacing for Deck Boards

Board thickness sets how far apart the joists can be, and the decking calculator checks your spacing against that limit. The 2021 International Residential Code limits joist spacing for wood decking in Table R507.7, and the limit tightens when boards run at an angle.

Values are maximum on-center spacing, as reproduced in the Anne Arundel County deck guide for the 2021 IRC. Multiple span means a board crosses three or more joists. Short pieces that bridge only one bay fall under the tighter single-span column.

Your local code edition may differ, so check it before you frame. The Deck Post Spacing Calculator lays out the beams and posts under those joists, and the Deck Footing Calculator sizes the footings that carry them.

Joist count is the deck length in inches divided by the spacing, rounded up, plus one end joist. A deck 16 ft long at 16 in on center takes 13 joists, and each joist is as long as the deck is wide, so a 16 × 12 ft deck needs 13 joists 12 ft long.

Deck Screws and Hidden Clips per Board

Wood decking needs at least two 8d threaded nails or two No. 8 wood screws at every supporting member, according to the 2021 IRC as summarized in the same county guide. So the decking calculator counts screws as two times the number of places a board crosses a joist.

$$N_{screws}=2\times N_{rows}\times \left(\lfloor \frac{12L}{s}\rfloor +1\right)$$

L is the deck length in feet and s is the joist spacing in inches. The 16 × 12 ft deck has 26 rows crossing 13 joists, which is 338 crossings, 676 screws or 338 hidden clips at one clip per crossing.

The common rule of 350 screws per 100 sq ft only holds at 16 in spacing with 6 in boards. At 12 in on center it runs about a quarter short, and at 24 in it buys roughly 100 more screws per 100 sq ft than the deck uses.

Stair Treads and Fascia Boards

Stair treads are cut from the same decking, and the IRC sets a 10 in minimum tread depth in R311.7.5.2. Two 5-1/2 in boards with a 1/8 in gap make an 11-1/8 in tread, so each step takes two tread pieces as long as the stair is wide.

Four steps on a 4 ft wide stair need eight 4 ft tread pieces, or 32 linear feet. A 16 ft board yields four of them, so the whole stair comes out of two boards, and the decking calculator adds those boards, their screws and their cost to the deck total.

Fascia covers the rim joists and is usually a wider trim board, so it is counted apart from the decking. On a 16 × 12 ft deck against a house wall, fascia on three sides is 40 linear feet, and wrapping all four sides of a freestanding deck makes it 56 linear feet.

Diagonal and Picture Frame Decking

Boards laid at 45° cover the same area but waste more on the angled end cuts, so the decking calculator counts them from area with extra for those cuts. Decks.com adds 15% for a 45° layout against 10% for boards square to the joists, and up to 20% for chevron or herringbone patterns.

A diagonal board crosses a joist every 22.6 in at 16 in spacing, because the run between joists grows by the square root of 2. The 16 × 12 ft deck laid diagonally takes 30 boards 16 ft long with 15% extra, and about 487 screws.

Diagonal 5/4 decking needs joists at 12 in on center under the 2021 IRC, so a diagonal layout often adds joists as well as boards. Two-inch boards keep their 24 in spacing when they cross three or more joists.

A picture frame border runs around three sides, so its length is the deck length plus two widths. On a 16 × 12 ft deck that is 40 linear feet of border board, with the infill rows set inside it. The border needs blocking under its mitered corners.

Decking Calculator in m² and Linear Meters

Metric decking is often sold by the linear meter, so the decking calculator m² to lm conversion depends only on the board face plus the gap. Divide 1,000 by the board width plus gap in millimeters to get linear meters per square meter.

$$lm=A\times \frac{1000}{b+g}$$

A is the area in square meters, b is the board width in mm and g is the gap in mm. A 140 mm board with a 5 mm gap needs 6.9 lm per m², so a 20 m² deck takes 138 lm before waste.

Readers in the UK, Australia and New Zealand can run the decking calculator in metric units with 3.0 to 5.4 m stock lengths. In England, the Planning Portal says decking stays permitted development only if it sits no more than 30 cm above the ground.

Metric joists usually sit at 400, 450 or 600 mm centers, and the same seam rule applies. At 400 mm, 3.6 and 4.8 m boards end on joists while 3.0, 4.2 and 5.4 m boards do not, and at 600 mm every length from 3.0 to 5.4 m divides evenly.

Store tools from Home Depot, Bunnings or B&Q count their own board stock. Enter that store's board width and lengths here to compare the same deck across suppliers.

The 30 cm rule sits beside a coverage limit. Decking plus other extensions and outbuildings can cover no more than half the garden, and none of it can sit in front of the wall forming the principal elevation of the house.

Decking Material Cost

In the decking calculator, cost is the linear feet of stock bought times the price per foot, plus whole boxes of fasteners. Pricing by the foot puts 12, 16 and 20 ft boards on the same scale, so the stock length choice shows up as wasted footage instead of a confusing per-board price.

For 500 sq ft of decking figured from area with 10% waste, the decking calculator lands on 74 boards 16 ft long. At an example price of $3.00 per linear foot, that is $3,552 of boards, and 1,734 screws in boxes of 350 at $40 add $200, for $7.50 per sq ft.

Boards and fasteners are only the top layer of a full deck material calculator list. Joists, beams, posts, hangers and footings come next, along with railing parts from the Baluster Calculator and Spindle Spacing Calculator.

Plan for finish and drainage too. The Deck Stain Calculator turns the same deck into gallons of stain, and the Deck Slope Calculator sets the fall that keeps water off the boards.

Squaring a Deck With the 3-4-5 Rule

The 3-4-5 rule checks a square corner by measuring 3 ft along one side, 4 ft along the other, and confirming the diagonal between the marks is exactly 5 ft. Scaling to 6-8-10 or 9-12-15 makes the check more accurate on a large deck.

$$D=\sqrt{L^2+W^2}$$

D is the diagonal of the whole frame. A 16 × 12 ft deck is square when both diagonals measure 20 ft, and a 12 × 12 ft deck is square at 16 ft 11-5/8 in. Equal diagonals mean every corner is square.

Building a 12 × 12 ft deck with 16 in joist spacing takes 10 joists 12 ft long, 28 deck boards 12 ft long and 520 screws. Size the posts and footings under it before the decking calculator numbers matter.

Sheet Decking, Balconies and Online Decking Calculator Use

Roof decking is sold by the sheet instead of the board, so its math starts from the 32 sq ft that one full panel covers rather than from rows and gaps.

What is a decking calculator?

A decking calculator turns deck dimensions into board count, linear feet, fasteners and cost. A row-by-row count also shows the rip width of the last row, the butt seams per row and whether your joist spacing suits the board thickness under the IRC.

Is there a free online decking calculator?

A decking calculator that runs in the browser needs no app or sign-up, and works the same on a phone at the lumber yard as on a desktop. Switching to meters and millimeters covers the decking calculator m² searches common in the UK, Australia and New Zealand.

How many square feet is one sheet of decking?

A standard 4 × 8 ft sheet of plywood or OSB roof decking covers 32 sq ft. A roof decking calculator divides the roof area by 32 and adds waste for cuts, so 1,200 sq ft of roof takes 37.5 sheets before waste, which is a different job from counting deck boards.

How do you work out decking for a balcony?

Measure the balcony floor the same way as a deck, with length along the boards and width across them. The Balcony Size Calculator gives the floor area and dimensions to feed into the board count.