Deck Calculator
Decking boards, joists, beams, posts, footings and screws for a rectangular deck — with the joist and beam spans checked against the span tables and a full cost breakdown.
Instant answers
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Full framing take-off
Span tables checked
Materials cost
The deck
The ledger runs this way.
The joists run this way.
Decks over 30 in need a guard rail.
3/16 in for dry wood, 1/8 in for wet treated stock that will shrink.
Decking boards to buy
—
Deck area—
Joists—
Joist span (ledger to beam)—
Beam size needed—
Posts—
Footings—
Deck screws—
Linear feet of decking—
Framing plan
Looking down on the frame: the ledger against the house at the top, joists running out, the beam and posts carrying the outer end.
Materials and cost
| Item | Qty | Size | Linear ft | Cost |
|---|
Joist span table
Maximum span between supports for pressure-treated deck joists at 40 psf live plus 10 psf dead load. Your spacing and species are highlighted.
| Joist | 12 in o.c. | 16 in o.c. | 24 in o.c. |
|---|
Span tables are a starting point, not a permit. Deck framing is structural, ledger attachment is the single most common failure point, and most jurisdictions require an inspection. Check your local code before you build.
Instructions
How to use this calculator
Step by step
- Enter the deck width (the side against the house, which the ledger runs along) and the projection (how far it sticks out, which the joists span).
- Set the height above grade. Anything over 30 inches to the walking surface needs a guard rail, and the calculator will tell you so.
- Choose the joist size, spacing and framing species. These three together decide the span, and the span decides whether you need one beam or two.
- Pick the decking board width, the gap between boards, and which way they run. Diagonal boards look good and cost more — both in material and in the joist spacing they force.
- Read the board count at the top. It is whole boards of a stock length that spans your run without a butt joint, which is what you want to buy.
- Check the span notes below the numbers. Green is a pass; red means the framing as configured will be springy or unsafe, and the note says what to change.
- Fill in your local timber prices to get the full materials cost and a per-square-foot figure to sanity-check any quote you are given.
Good to know
- The ledger is where decks fail. Through-bolt it or use structural screws into solid framing, flash it properly, and never attach to brick veneer or a cantilevered floor.
- Buy decking long enough to cross the deck in one piece. Butt joints have to land on a joist, they trap water, and they are the first place a deck starts to rot.
- Wet pressure-treated stock shrinks. Lay it tight with a 1/8 in gap and it will open to about 3/16 in on its own; lay dry or composite boards at 3/16 in from the start.
- Crown every joist — sight down the edge and put the high side up. Gravity and the deck load will flatten them over the first year.
- Dig footings below your local frost line. A shallow footing lifts and drops with the seasons and pulls the deck away from the house.
- Order 10% extra decking, 15% for a diagonal lay. Offcuts from a diagonal deck are triangles, and triangles are rarely reusable.
The maths behind it
- Board rows rows = ceil(run across ÷ (board width + gap)) The gap is part of the pitch each board occupies.
- Linear feet rows × run length × (1 + waste) Then divided by the stock length to get whole boards.
- Joist count ceil(width × 12 ÷ spacing) + 1 The +1 is the joist at the far end of the run.
- Bays needed ceil(projection ÷ maximum joist span) More than one bay means an intermediate beam.
- Beam load w = tributary width × 50 psf 40 psf live load plus 10 psf dead is the residential deck standard.
- Deck area A = width × projection Used for the cost per square foot.
- Screws 2 per board per joist crossing Two fasteners at every intersection is standard for face-screwed decking.
How far can a 2×8 joist span?
About 11 ft 10 in for southern pine at 16 in on centre, under the standard 40 psf live plus 10 psf dead deck load. Hem-fir and SPF manage around 11 ft 1 in, and cedar around 10 ft 2 in. Tightening to 12 in on centre buys you a foot or so; going up to a 2×10 buys you two.
What happens if my deck is deeper than the joists will span?
You add an intermediate beam, splitting the projection into two shorter bays — which also means another line of posts and footings. Going up one joist size is usually cheaper and always simpler, so check that option first. This calculator tells you how many bays your configuration needs.
How deep do the footings go?
Below the frost line, which varies from about 12 inches in the southern United States to 48 inches or more in the north. The 42 inch figure here is a common northern default — look up your own local requirement, because a footing above the frost line will heave and take the deck with it.
Do I need a permit?
Almost certainly. Most jurisdictions require a permit and an inspection for any deck attached to a house, and many require one for freestanding decks above a certain height. The span figures on this page follow the published tables, but they are a design aid, not an approval.
How much gap should I leave between boards?
It depends on how wet the boards are. Freshly treated lumber is saturated and will shrink noticeably, so lay it with about 1/8 in and let it open up. Kiln-dried wood and composite decking will not shrink, so gap them at 3/16 in from the start — you need that space for drainage and debris, or the deck will hold water.
Why does a diagonal deck need closer joists?
Because a board crossing the joists at 45° spans further between them — about 1.4 times the on-centre spacing. Boards on 16 in joists laid diagonally are effectively spanning 22½ in, which is past what most decking is rated for. Drop the joists to 12 in on centre and the diagonal span comes back to about 17 in.
Is the cost estimate realistic?
The material total is, if the prices you enter are your local ones. It does not include labour, railings, stairs, lighting, or the permit fee. As a rough check, professionally built decks typically run two to three times the material cost.