Deck Calculator
Calculate how many deck boards, joists and screws you need for a deck. Works for wood and composite decking with adjustable board width, length and gap.
By CalcBuilt Editorial TeamUpdated September 10, 2026Formulas checked against manufacturer specs, see how we calculate.
Results
- Deck boards to buy
- 29 boards
- 16 ft boards, 26 rows x 1 per row plus 10% waste
- Total decking length
- 458 ft
- 192 sq ft deck surface
- Joists
- 13 joists
- 12 ft long at 16 in on center (2x8 or 2x10). Warning: a 2x8 #2 at 16 in OC spans 11 ft 10 in in SYP and 11 ft 1 in in SPF (DCA 6 Table 2); use 2x10s (SYP 14 ft, SPF 13 ft 7 in)
- Deck screws
- 672 screws
- About 2 x 5 lb (2.3 kg) boxes at 350 per box. Hot-dipped galvanized, polymer-coated or stainless in treated wood; composite takes the maker's screws or hidden clips
- Rim / beam lumber
- 56 ft
- Rim and end joists only. Not included: beam, posts, footings, ledger, hangers and stairs. A 12 ft joist run needs a built-up beam sized from DCA 6 Table 3
Deck formula
Rows = Deck width (in) / (board width + gap), rounded up
Boards = Rows x (Deck length / board length, rounded up) x (1 + waste %)
Joists = Deck length (in) / joist spacing + 1
Screws = about 3.5 per sq ft (2 per board at every joist)Worked example
A 20 x 14 ft deck with 5.5 in composite boards, a 1/8 in gap, 12 ft boards and 16 in joist spacing: 168 / 5.625 = 29.9, so 30 rows. Each row needs 2 boards (20 / 12 rounded up), so 60 boards plus 10% = 66 boards. Joists: 240 / 16 + 1 = 16. Screws: 280 sq ft x 3.5 = 980.
Quick reference
| Deck (length x width) | Rows | Boards to buy |
|---|---|---|
| 10 x 10 ft | 21 | 24 |
| 12 x 12 ft | 26 | 29 |
| 16 x 12 ft | 26 | 29 |
| 20 x 12 ft | 26 | 58 |
| 16 x 16 ft | 34 | 38 |
| 20 x 16 ft | 34 | 75 |
How to calculate deck boards
Boards run the length of the deck. Divide the deck width in inches by the board width plus gap to get the number of rows. Then divide the deck length by the board length you plan to buy and round up. Multiply rows by boards per row and add 10 percent for waste.
A 16 by 12 foot deck with 5.5 inch boards and a 1/4 inch gap needs 26 rows. With 16 foot boards that is one board per row, so 26 boards plus waste, or about 29 boards. Board length drives waste. On a 20 foot deck with 16 foot boards every row is a 16 and a 4. The 12 foot offcuts are scrap unless you stagger butt joints on a joist. 12 foot boards cut that deck with no waste, and 20 foot composite is a special order.
Joists, spacing and fasteners
Joists run perpendicular to the deck boards. Divide the deck length in inches by the spacing and add one for the end joist. Most composites and 5/4 wood are rated 16 inches on center perpendicular and 12 inches on center diagonal; no 5/4 board is rated for 24 inch spacing. Joist size comes from species and span. DCA 6 Table 2 gives a #2 2x8 at 16 inches on center 11 feet 1 inch in SPF and 11 feet 10 inches in southern pine. The 12 foot default therefore needs 2x10s.
Plan on two screws per board at every joist, roughly 350 screws per 100 square feet. ACQ and CA treated lumber corrodes electroplated hardware; use hot-dipped galvanized (G185, ZMAX) or stainless for hangers, hanger nails, bolts and screws, and never mix stainless with galvanized. Composite takes the maker's screws or hidden clips; wood screws mushroom the surface, and the clips set the gap (Trex 1/4 inch, CAMO 3/16 inch).
What this calculator leaves out
The rim row is the perimeter only, not a beam. A 12 foot joist run needs a built-up beam sized from DCA 6 Table 3, typically a triple 2x10 on 6x6 posts about 8 feet apart. The posts stand on footings below frost depth. The ledger takes 1/2 inch lags or through-bolts on the DCA 6 Table 4 schedule and flashing over its top edge. Joist hangers get 10d nails or SD screws in every hole; deck screws in a hanger shear.
Posts, ledgers and any framing within 6 inches of grade are ground-contact UC4A or UC4B, not the UC3B decking grade. Any surface more than 30 inches above grade needs a 36 inch guard (IRC R312.1), and stairs need their own stringers and footing. Nearly every town wants a permit with a footing inspection before the concrete goes in.
Code limits that apply
- AWC DCA 6 Table 2checked
2x8 #2 at 16 in OC: 11 ft 10 in SYP, 11 ft 1 in SPF; 2x10: 14 ft SYP, 13 ft 7 in SPF
If you miss it: A 12 ft deck framed with 2x8 SPF bounces and fails the framing inspection; every joist comes out for 2x10s
- IRC R403.1.4
Footings 12 in min below undisturbed grade and below the local frost line
If you miss it: A shallow footing heaves every winter and the deck pulls off the ledger; the footing inspection comes before the pour
- AWC DCA 6 Table 4
Ledger lag or bolt spacing by joist span, 1/2 in fasteners staggered in two rows
If you miss it: A ledger nailed on or short of the bolt schedule is the classic deck collapse; the inspector counts every bolt
- AWC DCA 6 Table 3
Beam sized by joist span and post spacing; a 12 ft joist run is about a triple 2x10 at 8 ft
If you miss it: A doubled 2x8 beam on 10 ft posts sags at mid-span; the inspector asks for the table and the posts and footings get redone
- IRC R507.2
Fasteners and connectors in treated lumber hot-dipped galvanized, stainless or approved coated
If you miss it: Electroplated hangers on ACQ lumber rust through in a few years; the inspector reads the hanger stamp for G185 or ZMAX
- IRC R312.1
Guard 36 in min on any deck surface more than 30 in above grade; openings stop a 4 in sphere
If you miss it: A 4x4 post cut 36 in above the joists finishes an inch short over the decking; every post gets replaced or capped
Rows marked checked produce a warning in the results when an entry crosses the limit. The others are on the plans or on site and this calculator does not test them. Local amendments can be stricter; the adopted edition and the inspector govern.
Mistakes that cost money
- 2x8 SPF #2 joists at 16 in OC top out at 11 ft 1 in (DCA 6 Table 2); a 12 ft deck needs 2x10s.
- Deck screws in joist hangers shear under load; every hanger hole gets a 10d common nail or a Simpson SD structural screw.
- 16 ft boards on a 20 ft deck leave 12 ft offcuts; buy 12 ft or 20 ft boards and stagger the joints on joists.
- An unflashed ledger rots the rim and the house band joist; flash over the ledger and bolt it on the DCA 6 Table 4 schedule.
- Electroplated hangers and screws on ACQ lumber corrode in a few years; use G185 or ZMAX galvanized or stainless, never mixed.
- Wood screws in composite mushroom the surface; use the maker's composite screws or hidden clips, which also set the gap.
Key facts
- A 5/4 x 6 deck board is actually 1 in thick and 5.5 in wide.
- Most composites and 5/4 wood are rated 16 in on center perpendicular and 12 in diagonal; no 5/4 board is rated for 24 in.
- Plan on about 350 deck screws per 100 sq ft of decking.
- Leave a 1/4 in gap between dry boards; space wet pressure-treated boards with a 16d nail unless they are dripping wet.
Frequently asked questions
How many deck boards do I need for a 12x16 deck?
About 29 boards of 16 foot 5/4x6 decking with a 1/4 inch gap, including 10% waste. That is 26 rows of one board each plus extras for cuts.
How many joists for a 16 foot deck?
13 joists at 16 inches on center: 16 ft times 12 divided by 16 equals 12, plus one for the end.
Can 2x8 joists span 12 feet?
Not at 16 inches on center with #2 lumber. DCA 6 Table 2 allows 11 feet 10 inches for southern pine and 11 feet 1 inch for SPF, hem-fir and Douglas fir. Use 2x10s (14 feet in southern pine, 13 feet 7 inches in SPF), or drop to 12 inch spacing, where a southern pine 2x8 reaches 13 feet 1 inch.
What gap should I leave between deck boards?
1/4 inch for kiln-dried lumber. Wet pressure-treated boards shrink as they dry, so space them with a 16d nail unless they are dripping wet; butted tight they dry to uneven gaps. Composite follows the maker: Trex says 1/4 inch, TimberTech 1/8 to 1/4 inch depending on temperature.
Sources and references
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Results are estimates based on standard formulas and typical product specifications. Confirm quantities with your supplier and local code before ordering.