Blog /
Deck Calculator: Boards, Joists, Beams, and Footings
Start With the Decking
Decking is the simplest calculation on the whole project, and it’s the one people get wrong most often because they forget about waste.
Surface area first. A 12x16 deck is 192 square feet. Each 5/4x6 board is 5.5 inches wide (the actual dimension, not the nominal). A 16-foot board covers 5.5/12 x 16 = 7.33 square feet.
192 / 7.33 = 26.2 boards
Add 10-15% for waste. Cuts, bad boards, that one you split when the screw hit a knot. Call it 15% because composite and pressure-treated lumber both have their share of rejects at the lumberyard.
26.2 x 1.15 = 30.1 boards. Buy 31.
If you’re running boards perpendicular to the joists on a 12-foot span instead, you’d use 12-foot boards and the math changes: 5.5/12 x 12 = 5.5 sq ft per board, so 192 / 5.5 = 34.9, plus waste = 40 boards. More boards, more cuts, more seams. Running them the long way looks better and goes faster.
5/4 Decking vs 2x6
5/4x6 decking (which actually measures about 1” x 5.5”) is the standard for residential decks. It’s lighter, cheaper per board, and radiuses on the edges mean it sheds water and dries faster. Around $8-14 per 16-foot board in pressure-treated, depending on grade and region.
2x6 decking (actual 1.5” x 5.5”) costs more and weighs more, but it spans farther. IRC Table R507.7 lets 5/4 wood decking (1-1/4” thick) span 16” on center when it runs perpendicular and continuous over at least two spans, and only 12” on a diagonal. 2x decking goes to 24” perpendicular. At 16” OC, 5/4 works for straight runs.
Composite decking costs several times more than pressure-treated and has its own span limits in the manufacturer’s instructions (IRC R507.2.2). The board count stays the same either way.
Joist Calculation
Joists run the 12-foot dimension on our example deck, supported at each end by a ledger board (house side) and a beam (outer edge).
Number of joists = (deck length in inches / spacing) + 1
At 16” on center across 16 feet:
192” / 16” + 1 = 13 joists
At 12” on center:
192” / 12” + 1 = 17 joists
Joist Size and Spacing
Deck joist spans come from IRC Table R507.6 (No. 2 lumber, wet service). At a 40 psf live load:
| Joist | 12” OC | 16” OC | 24” OC |
|---|---|---|---|
| 2x8 Southern pine | 13’-1” | 11’-10” | 9’-8” |
| 2x10 Southern pine | 16’-2” | 14’-0” | 11’-5” |
| 2x8 Douglas fir-larch, hem-fir, SPF | 12’-6” | 11’-1” | 9’-1” |
| 2x10 Douglas fir-larch, hem-fir, SPF | 15’-8” | 13’-7” | 11’-1” |
Our joists span 12 feet. A 2x8 at 16” OC doesn’t make it in any species (11’-10” at best). You need 2x10s at 16” OC, or 2x8s at 12” OC. This example uses 2x10s at 16”.
Twelve-inch spacing makes a stiffer deck, and some composite decking requires it, especially on diagonals. Check the decking manufacturer’s instructions.
You’ll also need a rim joist along the outer edge and a ledger on the house side, both 16 feet and the same size as the joists, plus end joists on the two 12-foot sides (counted in the 13 above).
Beam Sizing
The beam supports the outer ends of all those joists. For a 12x16 deck where the joists span the full 12 feet to the beam, you need a beam that can carry the tributary load across 16 feet.
IRC Tables R507.5(1)-(4) size deck beams by species, the beam span between posts, and an effective joist span. With no joist cantilever past the beam, Table R507.5(5) puts the effective joist span at 0.66 x 12 = 7.9 feet. At 40 psf, a 2-2x10 beam then spans about 9’-0” in Southern pine and 8’-7” in Douglas fir-larch, hem-fir, or SPF. Posts at 8 feet work.
That means you need 2 pieces of 2x10 at 16 feet for the beam. If you can’t get 16-footers or don’t want to wrestle them into place, you can splice two shorter pieces over a post location. Just stagger the splices so they don’t land on the same post.
For header sizing on the ledger connection, the same principles apply as sizing a header per IRC R602.7.
Footing Calculation
Every post needs a footing. On our 12x16 deck with a doubled 2x10 beam, you’d place posts at roughly 8-foot intervals along the beam. That’s posts at 0’, 8’, and 16’, which gives you 3 footings minimum.
Footing diameter depends on soil bearing capacity and the load each footing carries. Most residential decks use 12” or 16” diameter Sonotube footings poured to frost depth.
For a 12” diameter footing, 42 inches deep (typical frost depth in climate zones 5-6):
Pi x (0.5)^2 x 3.5 = 2.75 cubic feet per footing
3 footings x 2.75 = 8.25 cubic feet = 0.31 cubic yards
That’s about 14 bags of concrete at 0.6 cubic feet per 80-pound bag. Five bags per footing, basically. Small enough that bags beat a ready-mix truck. (For bigger pours, see how to calculate concrete for a slab where we cover the bags-vs-truck breakpoint.)
If you’re in a frost-free zone and local code allows it, precast pier blocks on compacted gravel work for freestanding decks. No digging, no concrete, no waiting for cure time. Check your jurisdiction first. Either way, you’ll need gravel for the base under your footings or pier blocks. The gravel calculator covers that volume math including compaction factor.
Material List: 12x16 Deck at 16” OC
| Item | Size | Quantity | Notes |
|---|---|---|---|
| Decking boards | 5/4x6x16’ PT | 31 | Includes 15% waste |
| Joists | 2x10x12’ | 13 | 16” OC |
| Rim joist | 2x10x16’ | 1 | Outer edge |
| Ledger board | 2x10x16’ | 1 | House side |
| Beam members | 2x10x16’ | 2 | Doubled beam |
| Posts | 6x6 | 3 | Cut to height |
| Post bases | Simpson PBS66 | 3 | Or equivalent |
| Sonotube forms | 12” x 4’ | 3 | To frost depth |
| Concrete (80lb bags) | Quikrete | 15 | 5 per footing |
| Joist hangers | Simpson LUS210 or equal | 26 | Ledger and beam side, if hung |
| Ledger fasteners | 1/2” lag screws | about 14 | 15” OC staggered for a 12 ft joist span, IRC Table R507.9.1.3(1) |
| Deck screws | #8 x 2.5” | ~10 lbs | Stainless or coated |
The Fastener Math Nobody Does
IRC R507.7 requires at least two 8d threaded nails or two No. 8 screws per board at each joist. Our 31 boards cross 13 joists each: 31 x 13 x 2 = 806 screws. A pound of #8 x 2.5” deck screws contains about 90 screws, so you need 9 pounds. Buy 10 because you’ll drop a handful in the grass and never find them.
Joist-to-beam connections need hurricane ties or through-bolts. Post-to-beam connections need bolted post caps. Connectors and fasteners add up faster than most people expect. All of them must be hot-dipped galvanized, stainless, or otherwise rated for treated lumber (IRC R507.2.3).
What People Forget
Stairs. Any set of steps off the deck has to meet the IRC stair rules (R311.7): 7-3/4” max riser, 10” min tread, handrails on four or more risers. Stringers, treads, and a landing pad add material and concrete to your list. A set of 4 stairs needs 2-3 stringers (2x12), 4 treads (2x6 or 5/4x6, doubled), and a small concrete landing pad.
Guards. A deck surface more than 30 inches above the grade below, measured anywhere within 36 inches horizontally of the edge, needs a guard at least 36 inches high (IRC R312.1, R507.10). Openings must not pass a 4-inch sphere, and the top has to resist a 200-pound load (Table R301.5). This isn’t decorative. It’s structural.
Fencing. Most deck projects eventually lead to a fence around the yard. If you’re ordering lumber and concrete for the deck, getting the fence materials on the same delivery saves a trip.
Flashing at the ledger board. The IRC requires corrosion-resistant flashing at the ledger (R507.2.4). Skipping it is how decks rot the rim joist of the house they’re attached to.
Running the Numbers
Trussly has calculators for every piece of this: decking boards, deck joists and beams from the IRC R507 tables, concrete for footings, and stud framing for any walls involved in the project. Punch in your dimensions, pick your spacing, and it spits out the quantities. The material list stays in the app, so you can hand it to your lumber supplier instead of reading off a napkin.