Home › Deck Calculator
Deck Calculator
Deck Calculator
How to use it
- Measure the deck's length and width in feet (the length is the direction the boards will run).
- Pick your board — 2×6, 2×4, 5/4 deck board or composite — and the longest board length you can buy and transport.
- Choose joist spacing: 16 in on centre is standard for wood; 12 in for most composite warranties.
- Read the list: boards (with 10% waste), joists, screws and footing bags. Round up at the yard, not down.
How the deck math works
Boards are counted by coverage, not by area. A 2×6 is really 5.5 in wide, and with the standard 1/8 in drainage gap each row covers 5.625 in of deck width. Divide the deck width (in inches) by 5.625 and round up — that is your row count. If the deck is longer than the boards you can buy, each row needs more than one board, and every joint should land on a joist and be staggered from its neighbours.
Joists run under the boards at right angles. At 16 in spacing, a 16 ft deck needs a joist every 16 in along its length — 13 joists including both ends. Screws are the easy one to under-buy: every board gets 2 screws at every joist it crosses, so screws = rows × joists × 2. Footings assume posts every 8 ft along two beams and two 80 lb bags of concrete per footing — adjust if your frost line or soil demands deeper holes.
Worked example: a 16 × 12 ft deck, 2×6 boards
Width 12 ft = 144 in. Each row covers 5.5 + 0.125 = 5.625 in, so rows = 144 ÷ 5.625 = 25.6 → 26 boards, each 16 ft long (one board per row, no joints). With 10% waste: 26 × 1.10 = 28.6 → 29 boards to buy. Joists at 16 in on centre along the 16 ft length: 192 ÷ 16 = 12 spaces → 13 joists. Screws: 26 rows × 13 joists × 2 = 676 screws. Footings: posts every 8 ft on two beams = 3 × 2 = 6 footings, two 80 lb bags each = 12 bags.
What each number means at the yard
| Item | What to know |
|---|---|
| Deck boards | Buy the count shown, all in the chosen length. Longer decks with joints need the extra boards the calculator adds — joints waste board ends. |
| Joists | 2×8 is the common choice up to a 12 ft span; 2×10 or 2×12 for longer spans. Check a span table for your wood species. |
| Screws | Use 2½–3 in exterior deck screws. The count assumes 2 per board per joist — hidden fastener systems replace this entirely. |
| Concrete (80 lb bags) | Two bags per footing is a shallow-bell estimate. Cold climates: dig below the frost line and expect 3–4 bags. |
Deck mistakes that cost money
- Counting boards by square footage. Area math ignores board width and gaps and under-buys by 10–20% on narrow boards.
- Forgetting that a 2×6 is 5.5 in wide, not 6. Every row planned at 6 in loses half an inch; over 26 rows the deck comes up a board short.
- 16 in joist spacing under composite decking. Most composite warranties require 12 in — check before you frame, not after it feels spongy.
- No waste allowance on a deck with diagonal or picture-frame layouts. Diagonals need 15–20% extra, not 10%.
Deck Calculator — common questions
How many deck boards do I need for a 12×12 deck?
With 2×6 boards (5.5 in wide) and a 1/8 in gap: 144 in ÷ 5.625 in = 25.6, so 26 boards — buy 29 with 10% waste, in 12 ft lengths so no row needs a joint.
How many screws does a deck take?
Two screws where every board crosses every joist. A 16 × 12 ft deck on 16 in joist spacing works out to 26 × 13 × 2 = 676 screws — roughly a 5 lb box plus a spare.
Can I use 24 in joist spacing?
For 2×6 wood decking laid flat it is allowed in many codes but feels springy; it is not acceptable for 5/4 boards or most composites. 16 in is the safe default, 12 in for diagonal layouts.
How much concrete does each deck footing need?
Plan on two 80 lb bags for a typical 12 in diameter, frost-depth footing, and more for wider bells or deeper holes. Freeze depth decides the hole, not the deck size.
Related calculators
Fence Calculator
Free fence calculator for wood fences: posts, rails, pickets and concrete bags from your fence length, height …
Open calculator →
Interior · Tile & stoneTile Calculator
Free tile calculator: tiles and boxes needed by tile size, plus thinset and grout bags, with cuts and breakage…
Open calculator →
Interior · Walls & ceilingsDrywall Calculator
Free drywall calculator: sheets needed for walls and ceiling, plus screws, joint compound and tape. Deducts do…
Open calculator →