Deck Calculator & Material Planner
Enter three dimensions. Get the boards to buy by length, the framing, footings, concrete, hardware, railings and stairs. Every number comes with the rule behind it.
3 FramingSouthern · 16″ o.c. · 2x10
Not sure? The last option is the most conservative.
Joist spacing (on center)
in
Beyond the beamBeam
4 Footings1,500 psf · 36″ deep
Footing shape
in
Bottom below grade; use your frost line5 Railings & stairsrail · stair
in
Deck area
192sq ft
16′ × 12′
Deck boards
28
28×20′
Joists
13
2x10 @ 16″ o.c.
Footings
3
Ø19″ · 13 bags
Allowing staggered butt seams would buy 11% less decking (464 ft vs 520 ft).
Layout waste 19.4% · 520 linear ft bought, 419 ft installed · 26 rows · last row ripped to 2.56″
Plan view
boardalternate piecejoints are cut on joists and staggered
Code & layout checks
2021 IRC tablesThese checks are not a permit approval. A ✓ means the item is within the 2021 IRC prescriptive table for your inputs. Your jurisdiction may amend the code, and only your building department can approve a design.
- ✓Joists: 2x10 @ 16″ o.c.Within 2021 IRC Table R507.6 (Southern pine, 40 psf live load)11′ 10 1/2″ of 14′ max
- ✓Beam: (2) 2x8Within 2021 IRC Tables R507.5(1)–(5), 8 ft column (effective joist span 7.8 ft (× 0.66))7′ of 7′ 7″ max
- ✓Posts: 6x6Within 2021 IRC Table R507.4 for 42 sq ft tributary area1′ 4 1/2″ of 14′ max
- ✓Footing depthAt least 12″ below undisturbed soil, and below your local frost line (IRC R403.1.4)36″
- iDecking joist spacingComposite, PVC and hardwood boards follow the manufacturer’s spacing (IRC R507.2.2). Confirm on the product’s spec sheet or code report.16″
- iLedger attachmentBolt or lag to the house’s solid band joist with flashing (R507.9). Never attach to brick or stone veneer, an overhang, a bay window, or open-web floor trusses.
- ✓GuardsRequired above 30″; 36″ minimum height; openings must not pass a 4″ sphere (IRC R312)included
- ✓Stairs: riser heightIRC R311.7.5.1: 7¾ in. maximum
- ✓Stairs: tread depthIRC R311.7.5.2: 10 in. minimum, nosing to nosing
- ✓Stairs: stair widthIRC R311.7.1: 36 in. minimum clear width
- ✓Stairs: stringer spacingAWC DCA 6: 18 in. maximum for cut stringers; many tread products require 12 in. or less
- ✓Stairs: stringer throatAWC DCA 6: at least 5 in. left after notching
- !Stair handrail5 risers: a graspable handrail 34–38″ high is required on at least one side (IRC R311.7.8)
Shopping list
| Item | Qty | Your price | Total |
|---|---|---|---|
| Decking | |||
| Composite deck boards, 20 ft5.5″ wide | 28 boards | — | |
| Fasteners | |||
| Hidden deck clipsWith screws, usually in the same box | 342 clips | — | |
| Starter clipsFirst row against the house | 13 clips | — | |
| Color-matched face screwsLast row and any border boards | 26 screws | — | |
| Deck screws for treads2 per board per stringer | 84 screws | — | |
| Framing lumber | |||
| Joists: 2x10 × 12 ft | 13 pcs | — | |
| Rim joist: 2x10 × 16 ft | 1 pcs | — | |
| Ledger: 2x10 × 16 ft | 1 pcs | — | |
| Beam: 2x8 × 16 ft | 2 pcs | — | |
| Posts: 6x6 × 8 ft | 3 pcs | — | |
| Hardware | |||
| Joist tape (butyl)Tops of joists, rim and beam | 185 ft | — | |
| Joist hangers (2x10)At the ledger | 13 pcs | — | |
| Joist-to-beam tiesPositive connection at each joist bearing on a dropped beam | 13 pcs | — | |
| Post bases (6x6) | 3 pcs | — | |
| Post-to-beam caps (6x6) | 3 pcs | — | |
| ½ in. lag screws + washersStaggered, about 15 in. on center | 14 pcs | — | |
| Lateral load hold-downs (1,500 lb)IRC R507.9.2: 2 × 1,500 lb within 24 in. of each end, or 4 × 750 lb | 2 pcs | — | |
| Ledger flashingLinear feet | 17 ft | — | |
| Footings & concrete | |||
| Footing form tubes, 10″3 footings, Ø19″ × 7″ pad | 3 tubes | — | |
| Concrete mix, 80 lb bags7.7 ft³ total (0.28 yd³), no spillage allowance | 13 bags | — | |
| Railings | |||
| Guard posts, 4x4 × 8 ft10 posts at about 4′ 1/4″, bolted to the framing | 10 pcs | — | |
| Balusters (2x2)Clear gap ≤ 3.67″ | 76 pcs | — | |
| Top & bottom rails, 2x4 × 12 ft | 6 pcs | — | |
| Cap rail, 2x6 × 12 ft | 3 pcs | — | |
| ½″ through-bolts for guard posts2 per post | 20 pcs | — | |
| Stairs | |||
| Composite tread boards, 20 ft8 pieces at 4′ 2″ | 2 boards | — | |
| 2x12 stringers, 8 ft5 cut stringers at 12″ o.c. | 5 pcs | — | |
| Total of priced lines | 0 of 27 priced | — | |
Printed from Deckwright. Material estimates and IRC prescriptive-table checks only. This is not an engineered design or a permit approval.
Enter the prices from your local store. They stay on this device and fill in automatically next time. We don’t publish prices because they change by region and by week. For fasteners sold in boxes, enter the price per piece (box price ÷ count).
Framing details
| Joist span (ledger to beam) | 11′ 10 1/2″ |
| Joist cut length | 11′ 9″ |
| Beam location | 12′ from house |
| Beam end overhang past posts | 1′ |
| Post length (approx.) | 1′ 4 1/2″ |
| Largest footing tributary area | 41.6 sq ft → 2,078 lb |
| Required bearing area at 1,500 psf | 1.39 sq ft |
| Footing | Ø19″ × 7″ thick |
| Concrete | 7.7 ft³ · 13×80 lb or 18×60 lb or 26×40 lb |
| Ledger fasteners | 14 × ½″ lags @ 15″ o.c., staggered |
Compare beam options
| Beam | Max span | Posts | Spacing |
|---|---|---|---|
| (2) 2x6 | 5′ 11″ | 4 | 4′ 8″ |
| (2) 2x8chosen | 7′ 7″ | 3 | 7′ |
| (2) 2x10 | 9′ | 3 | 7′ |
| (2) 2x12 | 10′ 7″ | 3 | 7′ |
| (3) 2x6 | 7′ 5″ | 3 | 7′ |
| (3) 2x8 | 9′ 6″ | 3 | 7′ |
| (3) 2x10 | 11′ 2″ | 3 | 7′ |
| (3) 2x12 | 13′ 3″ | 3 | 7′ |
Assumptions behind these numbers
- Boards overhang the framing by 0.75″ on the three open sides. Each piece gets a 1″ trim allowance, offcuts shorter than 12″ are scrap, and extra boards are 5%.
- Seams land on joists and never share a joist with the previous two rows. Each seam joist is doubled (0 added).
- Spans come from the 2021 IRC Tables R507.4–R507.6 for your species and load case (10 psf dead load, No. 2 grade, wet service); 2021 covers 40 psf live load and 50–70 psf ground snow load. Hot tubs, roofs and other concentrated loads aren’t covered.
- Where a table allows interpolation we use the next more restrictive column instead. Beams use the 2021 effective joist span (span × the Table R507.5(5) factor for your cantilever).
- Footings are sized as tributary area × 50 psf ÷ soil bearing (the basis of IRC Table R507.3.1). They are at least 12″ square or 14″ round, and at least as thick as their projection beyond the post.
- Concrete: 80 lb bag = 0.60 ft³, 60 lb = 0.45 ft³, 40 lb = 0.30 ft³. The pier extends 2″ above grade. No spillage allowance is included.
- Railings: posts at most 6′ on center, 2x2 balusters at 1½″ with gaps ≤ 3¾″. Stairs: risers ≤ 7¾″, cut 2x12 stringers.
A deck calculator that plans the way you build
Lumberyards don’t sell square feet. They sell 12-, 16- and 20-foot boards, and every joint between two boards has to fall on a joist. That is why a quick “area ÷ board size” estimate so often leaves you a few boards short, or with a pile of offcuts. This planner works row by row:
- Lay out the courses. It covers the deck with rows (or 45° courses, or a picture-frame border with a field inside) at your board width and gap.
- Place the seams. When a row is longer than your longest board, each joint goes on a joist. Joints stay off the joists used by the two rows before, which gives the staggered pattern inspectors and manufacturers expect.
- Buy the right lengths. All pieces are packed into the lengths you said you can buy, and offcuts are reused where they fit. The result is a list like “14 × 16 ft and 8 × 12 ft”, along with the true waste percentage.
- Frame it. Joist size, beam size, post spacing and footing size come from the IRC prescriptive tables for your lumber species, spacing and soil.
How to measure for the planner
- Width along the house is the length of the ledger: the side that attaches to the house (or the long side of a freestanding deck).
- Depth out from the house runs from the house wall to the outside face of the rim joist. The joists run in this direction.
- Height above grade runs from the top of the decking to the ground. It sets post lengths, stair risers, and whether guards are required (more than 30″).
Boards run parallel to the house in the straight layout. For an L-shaped or multi-level deck, plan each rectangle separately and add the lists together.
What the planner will not do
The structural numbers come from the prescriptive tables in the International Residential Code. Those tables assume a 40 psf live load (or, in the 2021 edition, a ground snow load up to 70 psf), a 10 psf dead load, No. 2 lumber, and simple rectangular framing. They don’t cover hot tubs, roofs, heavier snow, stairs over 6 feet of span, or decks attached to brick veneer or cantilevered floors. Your jurisdiction may have adopted different tables. Read the methodology before relying on any number, and check the final design with your building department.
Questions people ask
How do I use the deck calculator?
Enter the deck’s width along the house, its depth out from the house and its height above the ground. Then choose your decking and the lumber species printed on the grade stamp. The planner lays out every board, sizes the joists, beam, posts and footings from the IRC tables, and builds a shopping list you can price, print or export.
Why does this calculator give a different board count than others?
Most calculators divide the deck area by the area of one board. That ignores the lengths you can actually buy, the overhang, and the rule that every butt joint must land on a joist. Deckwright lays out each row, places the seams, and packs the pieces into real stock lengths. You see exactly which boards to buy and how much is waste.
Is this a permit-ready deck design?
No. It is an estimating and planning aid based on the prescriptive tables in the International Residential Code. You choose the edition (2021 by default, or 2018), and each is computed from its own tables. Many jurisdictions amend those tables, and decks with hot tubs, roofs, heavy snow, steep sites or unusual shapes need a designer or engineer. Take the plan to your building department as a starting point.
Should I choose “fewest seams” or “least waste”?
Fewest seams uses one board per row wherever a single stock length can span the deck. It looks cleaner but often wastes more. Least waste allows staggered butt joints over doubled joists when that buys meaningfully less board. The planner tells you how much the other option would save.
Can I save or share my plan?
Yes. Every input is stored in the page address, so “Copy link” shares the exact plan. The prices you enter are kept in your browser on this device.
Related calculators
All calculators →Deck Board Calculator
Boards by stock length with seams placed on joists, plus screws or hidden clips.
Joist Span Calculator
Maximum joist span and cantilever from IRC Table R507.6, and the joist count.
Beam & Post Spacing
Beam size against post spacing from IRC Table R507.5, compared side by side.
Footing Calculator
Footing size from tributary area and soil, plus concrete bags per hole.
Deck Stair Calculator
Risers, treads, stringer length and throat, with a drawing to scale.
Railing & Baluster Calculator
Posts, balusters and rails with every gap kept under the 4-inch sphere rule.