Deck Calculator
Estimates only. Joist, beam, and footing sizes come from your local code's span tables, not from this calculator.
Deck boards are counted in rows: divide the deck's width by the board width plus the gap, multiply by the deck's length, and add about 10% waste. A 16 × 12 ft deck in 5½-inch composite boards takes 26 rows, or 29 boards at 16 ft, on 13 joists spaced 16 inches apart. Enter your deck size below for boards, joists, beams, posts, footing concrete, fasteners, and cost.
Deck boards by deck size
| Deck size | Area | Board rows | Linear ft | 16 ft boards | Joists |
|---|---|---|---|---|---|
| 10 × 8 ft | 80 sq ft | 17 | 170 | 12 | 9 |
| 10 × 10 ft | 100 sq ft | 22 | 220 | 16 | 9 |
| 12 × 12 ft | 144 sq ft | 26 | 312 | 22 | 10 |
| 16 × 12 ft | 192 sq ft | 26 | 416 | 29 | 13 |
| 16 × 16 ft | 256 sq ft | 34 | 544 | 38 | 13 |
| 20 × 14 ft | 280 sq ft | 30 | 600 | 42 | 16 |
| 24 × 16 ft | 384 sq ft | 34 | 816 | 57 | 19 |
| 20 × 20 ft | 400 sq ft | 43 | 860 | 60 | 16 |
How to calculate deck boards
Deck boards are counted in rows. Each row takes up one board width plus one gap, so the number of rows is the deck's width divided by (board width + gap), rounded up; the last row is ripped to fit. Multiply the rows by the deck's length for the total linear footage, add waste, and divide by the board length:
boards = rows × deck length × (1 + waste) ÷ board length
The calculator works from total footage rather than whole boards per row because an offcut from one row usually starts the next. That keeps the count honest on long decks, where every row needs a butt joint over a joist anyway.
Worked example: a 16 × 12 ft composite deck
The deck runs 16 ft along the house and 12 ft out. Composite boards are 5½ inches wide, and with a 3/16-inch gap each row takes 5.6875 inches, so 144 inches of width needs 26 rows. Twenty-six rows of 16 ft is 416 linear feet; with 10% waste that is 458 ft, or 29 boards at 16 ft. Joists at 16 inches on center along 16 ft take 13 joists, each 12 ft long. With face screws you need 676 screws (two per board at every joist), or 338 hidden clips.
Which way the boards run
The calculator assumes the usual layout: boards parallel to the house, joists running out from it. If your boards run the other way, swap the length and width. Diagonal boards cover the same area but are cut at both ends, so the diagonal setting raises the waste to 15% and tightens the joists to 12 inches, which is what Trex calls for under diagonal decking.
How many deck boards do I need?
For standard 5½-inch boards, plan on about 2.1 linear feet of decking per square foot of deck, then add waste. The table above gives the board count for common deck sizes. The things that change it are the board width, the gap, and the board length:
- Board width. Nominal sizes aren't actual sizes. A 5/4 × 6 and a 2 × 6 are both 5½ inches wide, and most composite boards are 5½ inches too. Enter the actual width.
- Gap. Trex specifies at least 3/16 inch between boards. Kiln-dried wood is usually gapped about ⅛ inch, while wet pressure-treated boards are often laid tight because they shrink as they dry.
- Board length. Pick a length that suits the deck: on a 16 ft deck, 16 ft boards leave no joints at all. If the deck is the same length as the boards, lower the waste, because each row is one whole board.
- Picture-frame borders and stairs. A border board around the edge and the stair treads are extra; add their length on top of the field count.
Joists, beams, posts, and footings
Joists. Joists at s inches on center along a deck L feet long need 12L ÷ s spaces, rounded up, plus one: the two outer joists are included. Trex calls for joists no more than 16 inches on center for composite decking, and 12 inches when the boards run diagonally. Each joist spans the deck's width. The calculator counts joists but does not size them: the size comes from the span tables in the International Residential Code (Table R507.6) or the American Wood Council's DCA 6 guide, for your species, joist spacing, and span. As one example from DCA 6, Southern pine 2 × 10s at 16 inches on center span 14 ft between supports.
Ledger and rim. An attached deck hangs its joists from a ledger bolted to the house and closes the outer end with a rim joist, so it needs two boards the length of the deck, plus one joist hanger per joist at the ledger. DCA 6 requires ledgers and rim joists of at least 2 × 8. A freestanding deck has a beam at each end instead of a ledger.
Beams, posts, and footings. The joists sit on a beam near the outer edge (two beams for a freestanding deck). Each beam gets a post at both ends and one at every post spacing in between, and every post stands on a concrete footing. The IRC requires deck footings to bear at least 12 inches below undisturbed ground, or below the frost line where that is deeper, and sizes them by the area each footing carries and your soil. The defaults above (12-inch diameter, 36 inches deep) are only a starting point; your building department gives the frost depth.
Footing concrete is the volume of a cylinder: π × (diameter ÷ 2)² × depth. A 12 × 36-inch footing holds 2.36 cubic feet, or about four 80 lb bags. The concrete calculator handles footings of other shapes.
Deck screws or hidden fasteners
Face screws: two per board at every joist it crosses, so the count is rows × joists × 2. On 5½-inch boards with joists at 16 inches that is about 3.5 screws per square foot.
Hidden clips: one per board per joist. Trex sells its Hideaway clips in a pack of 90 for 50 square feet, about 1.75 clips per square foot, and the calculator's 16 × 12 ft example comes out at 1.76. Hidden clips only work with grooved-edge boards, and the first board needs separate starter clips.
How much does a deck cost per square foot?
Enter prices for boards, joists, posts, and concrete above and the calculator adds them up and divides by the deck area. That is the lumber and concrete; hardware, railing, stairs, and labor are extra. HomeGuide puts a complete new deck at $25 to $50 per square foot installed, with materials alone at $14 to $28 per square foot for pressure-treated pine.
| Decking | Installed per sq ft |
|---|---|
| Pressure-treated pine | $25–$50 |
| Cedar | $30–$47 |
| Redwood | $28–$54 |
| Composite | $40–$80 |
| Trex composite | $45–$70 |
| Ipe | $50–$70 |
By size, HomeGuide gives $3,600 to $7,200 for a 12 × 12 deck, $6,400 to $12,800 for 16 × 16, and $10,000 to $20,000 for 20 × 20.
Common mistakes
- Dividing area by nominal board width. A "6-inch" board covers 5½ inches plus the gap. Using 6 inches leaves you short by about 5%.
- Counting joists as length ÷ spacing. 16 ft at 16 inches is 12 spaces but 13 joists; forgetting the last one is the classic short order.
- Using wood joist spacing for composite. Wood decking is sometimes run on joists 24 inches apart; composite needs 16 inches, or 12 on the diagonal.
- Forgetting the second beam on a freestanding deck. Without a ledger, the house end needs its own beam, posts, and footings.
- Leaving out stairs, borders, and fascia. They are separate takeoffs on top of the field boards.
What this calculator can't handle
- Structural sizing. Joist, beam, post, and footing sizes depend on span, species, load, snow, and soil. Use IRC R507 or DCA 6, or an engineer, and your permit drawings.
- Odd shapes. L-shaped and multi-level decks: split them into rectangles, run each one, and add the results.
- Cantilevers and blocking. Joists that overhang the beam, mid-span blocking, and doubled joists at board seams are not counted.
- Railings and stairs. Posts, balusters, stringers, and treads are separate; the stair stringer calculator covers deck steps.
About these figures: the 16-inch and 12-inch joist spacing and the 3/16-inch board gap are from Trex's installation guides, and the clip coverage is Trex's Hideaway pack rating. The footing depth rule and joist span tables are in the International Residential Code, section R507; the 2 × 10 span example and the 2 × 8 ledger minimum are from the American Wood Council's DCA 6 as reported by the Journal of Light Construction. Bag yields are QUIKRETE figures and costs are from HomeGuide (2026). The 10% and 15% waste allowances are common practice, not a published standard.
Frequently asked questions
How many deck boards do I need for a 12 × 12 deck?
A 12 × 12 ft deck in 5½-inch boards with a 3/16-inch gap takes 26 rows. With 12 ft boards that is 26 boards, one per row, plus a few spares; with 16 ft boards and 10% waste it is 22 boards. It needs 10 joists at 16 inches on center.
How many deck boards do I need for a 16 × 16 deck?
About 38 boards at 16 ft with 10% waste. The 16 ft width takes 34 rows of 5½-inch boards with 3/16-inch gaps, and since the deck is 16 ft long, each row is one whole board, so 34 boards plus a few spares can be enough. It needs 13 joists at 16 inches on center.
How do you calculate linear feet of decking?
Divide the deck's width in inches by the board width plus the gap to get the rows, round up, and multiply by the deck's length in feet. For 5½-inch boards that works out to about 2.1 linear feet per square foot of deck, before waste.
How far apart should deck joists be?
16 inches on center is the standard for composite and most wood decking. Trex requires 16 inches or less for straight boards and 12 inches when the boards run on a 45° diagonal. Some thicker wood decking can span 24 inches; check the board maker's span chart.
How many joists do I need for a deck?
Divide the deck's length in inches by the joist spacing, round up, and add one. A 16 ft deck at 16 inches on center has 12 spaces and 13 joists; a 12 ft deck has 10.
How many screws do I need for a deck?
Two screws per board at every joist. A 16 × 12 ft deck with 26 rows on 13 joists needs 676 screws, about 3.5 per square foot. Hidden clips take one per board per joist, about 1.75 per square foot at 16-inch spacing.
How deep do deck footings need to be?
The International Residential Code requires deck footings to bear at least 12 inches below undisturbed ground, or below the local frost line if that is deeper. Footing size depends on the load each footing carries and your soil, so check with your building department.
How much concrete do I need for deck footings?
A footing is a cylinder: π × (diameter ÷ 2)² × depth. A 12-inch footing 36 inches deep holds 2.36 cubic feet, about four 80 lb bags. Three footings take 12 bags.
How much does it cost to build a deck per square foot?
HomeGuide puts a new deck at $25 to $50 per square foot installed in 2026, from about $25 to $50 for pressure-treated pine to $40 to $80 for composite. Materials alone run about $14 to $28 per square foot for pressure-treated pine.
Is a composite deck more expensive than wood?
Yes, up front. HomeGuide lists composite at $40 to $80 per square foot installed against $25 to $50 for pressure-treated pine. Composite skips the yearly staining and sealing that pine needs, which narrows the gap over the deck's life.
Related calculators
Sources
- How to Install Deck JoistsTrexJoists no more than 16 in. on center for standard applications, 12 in. for diagonal decking; worked example of rows of 5½-in. boards.
- Trex Refuge Decking Installation GuideTrexMinimum width-to-width gap of 3/16 in. (5 mm) between boards.
- Trex Hideaway 50 sq ft Universal Hidden Deck FastenersThe Home DepotPack of 90 clips rated for 50 sq ft of decking.
- International Residential Code, Chapter 5: Floors (R507 Decks)International Code CouncilDeck footing depth (12 in. below undisturbed ground or the frost line), footing sizing table, and deck joist span table R507.6.
- Highlights from the American Wood Council's DCA 6Journal of Light ConstructionDCA 6 prescriptive deck guide: 2×6 joists added to the span table, ledgers and rim joists at least 2×8, Southern pine 2×10 at 16 in. spanning 14 ft.
- How Much Does It Cost to Build a Deck?HomeGuide$25–$50 per sq ft installed; installed cost by decking material and by deck size; materials $14–$28 per sq ft for pressure-treated pine (2026).