Deck Calculator
Enter your deck size to get the number of wood or composite boards and the framing: joists, hangers, beam, posts and footing concrete. You also get screws or hidden clips and a decking cost, with waste included.
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Rows of boards = deck width ÷ (board width + gap), and linear feet = rows × deck length. A 16 × 12 ft deck with 5.5 in boards and 1/8 in gaps needs 26 rows, or 29 sixteen-foot boards with 10% waste. It also needs 13 joists at 16 in on center and 3 footings under the beam.
How to use this deck calculator
- Choose wood or composite. The gap, typical price and fasteners change with the material.
- Enter length along the house and width out from it. Boards run parallel to the house and joists run out from it, the most common layout.
- Pick board length. Choose the length that matches your deck length, or divides evenly into it, to cut waste and joints.
- Set joist spacing: 16 in for most decks, 12 in for diagonal boards or many composites.
- Read the list: boards and cost, joists, hangers, beam, posts, footing concrete and fasteners.
How many deck boards do I need?
| Deck size | Area | Rows (5.5 in + 1/8 in) | Linear feet | 16 ft boards (no waste) |
|---|---|---|---|---|
| 10 × 10 ft | 100 sq ft | 22 | 220 | 14 |
| 16 × 12 ft | 192 sq ft | 26 | 416 | 26 |
| 12 × 12 ft | 144 sq ft | 26 | 312 | 20 |
| 20 × 16 ft | 320 sq ft | 35 | 700 | 44 |
| 20 × 20 ft | 400 sq ft | 43 | 860 | 54 |
A useful shortcut: 5.5 in boards with a small gap work out to about 2.2 linear feet per square foot of deck. Add 10% for waste on straight layouts and 15% for diagonal or picture-frame borders.
Board sizes and gaps
| Decking | Actual width | Side gap | Max joist spacing (straight / diagonal) |
|---|---|---|---|
| Pressure-treated 5/4×6 | 5.5 in | Tight if wet, 1/8 in if dry | 16 in / 12 in |
| Pressure-treated 2×6 | 5.5 in | Tight if wet, 1/8 in if dry | 24 in / 16 in |
| Composite / PVC | ~5.5 in | ~3/16 in (per manufacturer) | 16 in / 12 in typical |
| Hardwood (ipe etc.) | 3.5–5.5 in | 1/8–3/16 in | 16 in / 12 in |
Composite boards also need end gaps that depend on temperature at installation. Follow the brand’s guide, because warranties depend on it.
Deck framing: joists, beams and footings
Joists = deck length (in) ÷ spacing + 1. Each joist hangs from the ledger on a joist hanger and sits on the beam at the far end. Typical maximum joist spans for No. 2 southern pine, per the AWC DCA 6 deck guide:
| Joist size | 12 in o.c. | 16 in o.c. | 24 in o.c. |
|---|---|---|---|
| 2×6 | ~9 ft 11 in | ~9 ft 0 in | ~7 ft 7 in |
| 2×8 | ~13 ft 1 in | ~11 ft 10 in | ~9 ft 8 in |
| 2×10 | ~16 ft 2 in | ~14 ft 0 in | ~11 ft 5 in |
| 2×12 | ~18 ft 0 in | ~16 ft 6 in | ~13 ft 6 in |
Spans differ by species and grade, so confirm with your local code. Footings usually sit 6–8 ft apart along the beam and must reach below the frost line. The calculator assumes 12 in × 3 ft holes (about 4 bags of 80 lb concrete each). Size yours with the deck footing calculator and the joist and beam calculator.
Screws, clips and hardware
- Face screwsTwo per board at every joist, about 350 per 100 sq ft at 16 in spacing. Use coated or stainless deck screws rated for treated lumber.
- Hidden clipsOne clip per board per joist for most composite systems. Starter and end clips are extra.
- Joist hangersOne per joist at the ledger, plus hanger nails or structural screws, not deck screws.
- Ledger fastenersThrough-bolts or structural screws in a staggered pattern, with flashing over the ledger to keep water out of the house wall.
Worked example
- Deck 16 ft along the house × 12 ft out
- 192 sq ft
- Rows: 144 in ÷ 5.625 in
- 26 rows
- 16 ft boards (+10%)
- 26 × 1.1 = 29 boards
- Joists at 16 in o.c.
- 192 ÷ 16 + 1 = 13 × 12 ft
- Posts / footings (≤ 8 ft apart)
- 3
- Screws
- 26 × 13 × 2 = 676
All 6 decking calculators
Detailed calculators for each part of a deck build.
Frequently asked questions
How many deck boards do I need?
Rows = deck width ÷ (board width + gap). Multiply by deck length for linear feet, divide by board length and add 10%. A 16 × 12 ft deck needs about 29 sixteen-foot boards.
How many joists do I need for a deck?
Deck length in inches ÷ joist spacing + 1. A 16 ft deck at 16 in on center needs 13 joists.
What joist spacing do I need for composite decking?
Usually 16 in on center for straight layouts and 12 in for diagonal. Follow the manufacturer’s installation guide.
How far can deck joists span?
For No. 2 southern pine at 16 in on center, roughly 9′ for 2×6, 11′ 10″ for 2×8, 14′ for 2×10 and 16′ 6″ for 2×12 (AWC DCA 6). Check species, grade and local code.
How many screws do I need for a deck?
Two per board at every joist, about 350 per 100 sq ft at 16 in spacing.
How far apart should deck footings be?
Typically 6–8 ft along each beam, depending on beam size and load, and always below the frost line.
Deck footing and pier guides
What this calculator assumes
- A rectangular, ledger-attached deck with boards parallel to the house and one beam at the outer edge.
- Boards are bought at one length. Rows longer than the board are joined over joists, and waste is applied to the board count.
- Posts every 8 ft or less along the beam, set in 12 in × 3 ft footings (80 lb bags at 0.60 ft³).
- Span figures are general guidance only. Deck framing must meet your local building code and may need a permit.