How many deck boards do I need for a 12 × 16 deck?

Twenty-six 16 ft (4.9 m) boards, laid along the 16 ft side, one board to a row. That assumes standard 5/4x6 lumber, 5½ in (140 mm) wide with a ⅛ in (3 mm) gap, over joists at 16 in (406 mm) centres. The count holds no spares, no ripped last row and no stair treads, so culled boards and the treads for a stair are added on top.

How many footings does a 12 × 16 deck need?

Ledgered to the house, three: one at each end of the beam and one midway, 8 ft (2.44 m) apart. Freestanding, a second beam line under the house side adds three more. The middle post's deck share, 2,800 lb (12.5 kN), needs about 1.9 sq ft (0.17 m²) of bearing on 1,500 psf (72 kPa) soil: a 20 in (508 mm) round footing or a flared base under a narrower tube. The stair's top reaction and the pier's weight come on top, so that footing is the one to enlarge.

What size joists does a 12 × 16 deck need?

With the beam set 18 in (457 mm) in from the outer edge, the joists clear-span 10 ft (3.05 m). At 16 in (406 mm) centres and 50 psf (2.39 kPa), the joist check asks for 6.36 in (161.5 mm) of depth, so a 2x8 at 7¼ in (184 mm) passes — on the calculator's default design values. Thirteen of them cover the 16 ft (4.88 m) ledger. The check is worth re-running with the adjusted values for the grade stamped on the lumber before ordering.

Worked example · Decks and fences

A 16 × 12 ft (4.9 × 3.7 m) pressure-treated deck, 3 ft (0.9 m) off grade and ledgered to the house, with one stair and a railing on three sides

An attached 16 × 12 ft deck taken off in the order its load travels — joist check and joist count, ledger screws, footing count and size, concrete, boards, screws, stair, balusters and stain — with every figure computed by the calculator at its step or by the arithmetic shown beside it, the gaps this site cannot fill named where they fall, and the places the estimate is most likely to be wrong.

11Steps, in order
10Figures computed
US, imperial firstModelled for

Carpenter fitting kitchen cabinets in a room with freshly taped drywall

The job

Modelled, not recorded. Every figure on this page is re-run through the calculator it names whenever the site is built; the estimate is worked, but no job was carried out, so the last section gives the mechanism of each likely error rather than a measured overrun.

An attached deck is two structures sharing one load. The joists run out from a ledger bolted to the house and land on a beam near the outer edge, so every square foot of the deck is divided between the house's rim board and a few holes in the yard. This walkthrough follows that division: the joist check first, because its clear span decides how much each side carries, and the joist count it sets; then the ledger fastening, then the footings under the beam and the concrete in them, and only then the surface — boards, screws, stair, guards and stain — which is the part everyone sees and the part the frame underneath has already decided.

The job: a 16 × 12 ft (4.9 × 3.7 m) deck in pressure-treated lumber, ledgered along its 16 ft side to the house rim, with the finished surface 36 in (914 mm) above grade. The joists run 12 ft out from the house onto a dropped beam set in from the outer edge, the beam stands on posts, one stair leaves the middle of the outer edge, and the three open sides carry a railing — a guard, in the code's word. At this height it is not a choice: IRC R312.1.1 calls for one wherever a walking surface sits more than 30 in (762 mm) above the grade below.

THE JOIST SPAN DECIDES WHO CARRIES WHAT. With a 10 ft (3.05 m) clear span from the ledger to the beam and 18 in (457 mm) of cantilever beyond it, the ledger takes half the span and the beam takes the rest. Move the beam out a foot and the ledger's share, the beam's share, the footing size and the concrete order all move together — which is why the joist check opens this page rather than sitting between the boards and the footings as a formality.

WHAT THIS PAGE CANNOT DO. Every quantity here comes from the calculator at its step or from the arithmetic shown beside it, and the page is modelled rather than a record of a deck that was built. Several things the deck needs are named where they fall instead of being filled with a figure. The beam's size: the beam bending check on this site tests a section it is given and does not choose one. The rest of the frame lumber: step 2 counts the joists, but the rim joist, the ledger board, the beam plies, the three structural posts, the eight guard posts, the guard rails and the stair stringers are not taken off, so this is the surface, the foundation and the joists rather than a full lumber order. The joist hangers and structural hardware. The frost depth, which is a local number. And the post lengths, which are cut once the footings are poured and the levels shot.

What was measured, and how

Deck outline, from the sheathing face at the ledger to the outer face of the rim joist
16 ft (4.88 m) along the house × 12 ft (3.66 m) out, diagonals equal
Taken from the sheathing face the ledger will bear on, not from the face of the siding, which is cut back along the ledger line; then squared by equal diagonals before a hole is marked. Laid out from the siding instead, the beam line and every footing on it land farther from the house than the frame will put them, by the siding's thickness.
Finished deck height above grade, at the foot of the stair
36 in (914 mm) at the stair foot
Measured from the finished board surface, not the top of the joists, down to the grade where the stair will land rather than at the house wall, where the ground usually stands higher. It sets the riser count and whether a guard is required, so a figure taken to the framing or at the wrong end of a sloping yard is wrong twice.
Joist layout: clear span, cantilever and centres
10 ft (3.05 m) clear span, 18 in (457 mm) cantilever, 2x8 joists at 16 in (406 mm) centres
Clear span runs from the face of the ledger to the face of the beam, not centre to centre of the supports, and the cantilever from the beam's outer face to the joist ends. Centre-to-centre figures overstate the span and push a marginal joist a size deeper than it needs; a cantilever taken from the beam's inner face is overstated by the beam's width, which can fail the quarter-span check on a layout that passes it.
Design load, soil and frost depth
50 psf (2.39 kPa) total; 1,500 psf (72 kPa) soil; 36 in (914 mm) frost depth, taken for this run only
The load is the residential deck figure the ledger calculator names, 40 psf (1.9 kPa) live over 10 psf (0.5 kPa) dead. Soil that no test has identified is given the lowest presumptive bearing value IRC Table R401.4.1 lists. Frost depth comes from the building department under IRC R403.1.4. Guessing any of the three sizes a footing for somebody else's site.
Guard layout and the stair opening
Six bays at 6 ft (1.83 m) centres, 68½ in (1.74 m) clear between 4x4 posts; eight posts; a 4 ft (1.22 m) stair opening centred on the outer edge
Posts laid out along the three open sides before any spindle is counted, and each bay measured as the CLEAR opening between post faces. Taking post centres instead adds a post's width to every bay, and the count then includes spindles there is no room to fit.

The figures

Each step’s computed figure for this job
Step	Calculator	Figure
Check the joist size against the span	Deck Joist Span and Size Calculator	6.36 in (161.5 mm) of depth required; a 2x8 at 7¼ in (184 mm) passes
Count the joists	Ceiling Joist Spanning Lineal Lumber Aggregator	156 ft (47.55 m) of joist: 13 joists of 12 ft (3.66 m)
Count the ledger lag screws	Ledger Lag Screw Shear Count Calculator	10 lag screws at the stand-in capacity of 405 lb (1.8 kN) each
Size the footing under the middle post	Isolated Spread Footing Sizing Calculator	1.37 ft (16.4 in, 0.416 m) square side, 1.87 sq ft (0.173 m²) of bearing
Quantify the footing concrete	Concrete Bag Calculator	35 bags of 80 lb (42 bags of 30 kg), 20.7 ft³ (0.59 m³)
Count the deck boards	Deck Board Calculator	26 boards of 16 ft (4.9 m), one to a row
Count the deck screws	Deck Screws Calculator	3 boxes of 300 (672 screws)
Lay out the stair	Stair Rise & Run Calculator	5 risers of 7.2 in (183 mm), 4 treads 11 in (279 mm) deep
Count the balusters	Deck Baluster Calculator	12 balusters a bay at 3.88 in (99 mm) gaps
Quantify the stain	Deck Stain Calculator	2 gallons (7.6 L) for two coats

The waste factors, and why these ends of the ranges

The waste factor applied to each material, and why
Material	Applied	Why
Deck boards	A few spare boards picked at the yard, with the stair treads added as their own line	The calculator adds nothing for culls, splits or the ripped last row, and pressure-treated 5/4 stock arrives wet, with some of every bundle crowned or cupped. With 16 ft stock on a 16 ft frame there is no length to trim a split end away, so a damaged board is a replaced board rather than a shorter one.
Footing concrete	Whole bags, with spares held back against holes augered oversize	The count treats each hole as a perfect cylinder and rounds up once for the whole job. An auger in anything but stiff ground leaves a hole wider than its bit, and a batch mixed for one hole usually leaves a residue that is tipped out rather than carried to the next, which comes out of that single rounding. Widening or flaring the middle hole is a change to the order, not waste, and belongs in it once its size is settled.
Deck screws	The published rate rounded to whole boxes, with stair and fascia fasteners bought separately	Rounding 672 screws up to three boxes of 300 already covers the stripped and snapped heads of a face-screwed deck. The screws this rate never counts — stair treads, fascia, guard posts — are a different length or a different fastener, so padding the deck figure does not supply them.
Balusters	The per-bay count, plus a spare or two in each run of guard	The calculator works one bay at a time and adds nothing, and a square pressure-treated spindle is likeliest to crack where a screw goes in close to its end. A spare in each run costs less than a gap in the guard while a replacement is fetched.
Stain	Whole gallons for the walking surface, with the guard and stair quantified as a second area	The calculator covers the deck surface at the bottom of its published coverage range. A three-sided guard with seventy-two balusters, its rails and posts can add nearly as much surface again as the boards, and folding it into one generous allowance hides which half of the finish it belongs to.

Where this estimate is most likely to be wrong

Ledger fastener count
usually under
The run divides the ledger's share of the load by the calculator's default capacity for one screw. The value that applies falls wherever a drainage spacer sits behind the ledger, sheathing is sandwiched behind it, or the rim is a softer species than the stand-in capacity assumes — and the count rises in proportion with nothing about the deck changing.
Narrow it by: Reading the per-screw capacity from the screw maker's published table for this rim, sheathing and spacer, then re-running the count before any hole is bored.
Footing concrete
usually under
Augered holes in loose or wet ground come out wider than the bit, a hand-cleaned bottom is rarely flat, and the collar above grade tends to finish taller than planned. Each adds concrete the cylinder does not contain; a frost depth deeper than the one taken for this run deepens all three holes at once, and a middle hole widened or flared for the loads step 5 leaves out adds more again.
Narrow it by: Confirming the frost depth with the building department before digging, settling the middle footing's size with the stair and pier loads added, forming the top of each hole with tube, and measuring the dug holes before the first bag is mixed.
Deck boards
usually under
The count is exact rows at exact stock length, so every cull, every split end and the ripped last row come out of it. A 16 ft board on a 16 ft frame leaves nothing for squaring the ends or overhanging the fascia, and the stair treads are not in the figure at all.
Narrow it by: Measuring a few boards from the actual bundle for true length and width, and adding the treads as their own line before the order goes in.
Stain
usually under
The walking surface is only part of what gets stained. The balusters, rails, posts, stair and fascia add surface the boards do not, and pressure-treated lumber left to weather for a season before its first coat can drink more than even the low coverage figure the calculator fixes.
Narrow it by: Measuring the guard and stair surfaces as a second area, and checking the coverage printed on the can against the condition of the lumber when it is coated.
Joist and beam sizes
either way
The joist check ran on the calculator's default design values rather than the adjusted values for the grade stamped on the lumber that arrives, and the beam was never sized, because no calculator here chooses one. Adjusted values below the defaults deepen the joist the span needs; values above them leave the 2x8 more margin.
Narrow it by: Re-running the joist check with the NDS Supplement values for the stamped species and grade, and sizing the beam from AWC DCA 6 or an engineer before ordering lumber.

Tools this job needs

Frequently asked questions

How many deck boards do I need for a 12 × 16 deck?: Twenty-six 16 ft (4.9 m) boards, laid along the 16 ft side, one board to a row. That assumes standard 5/4x6 lumber, 5½ in (140 mm) wide with a ⅛ in (3 mm) gap, over joists at 16 in (406 mm) centres. The count holds no spares, no ripped last row and no stair treads, so culled boards and the treads for a stair are added on top.
How many footings does a 12 × 16 deck need?: Ledgered to the house, three: one at each end of the beam and one midway, 8 ft (2.44 m) apart. Freestanding, a second beam line under the house side adds three more. The middle post's deck share, 2,800 lb (12.5 kN), needs about 1.9 sq ft (0.17 m²) of bearing on 1,500 psf (72 kPa) soil: a 20 in (508 mm) round footing or a flared base under a narrower tube. The stair's top reaction and the pier's weight come on top, so that footing is the one to enlarge.
What size joists does a 12 × 16 deck need?: With the beam set 18 in (457 mm) in from the outer edge, the joists clear-span 10 ft (3.05 m). At 16 in (406 mm) centres and 50 psf (2.39 kPa), the joist check asks for 6.36 in (161.5 mm) of depth, so a 2x8 at 7¼ in (184 mm) passes — on the calculator's default design values. Thirteen of them cover the 16 ft (4.88 m) ledger. The check is worth re-running with the adjusted values for the grade stamped on the lumber before ordering.

A 16 × 12 ft (4.9 × 3.7 m) pressure-treated deck, 3 ft (0.9 m) off grade and ledgered to the house, with one stair and a railing on three sides

The job

What was measured, and how

Deck outline, from the sheathing face at the ledger to the outer face of the rim joist

Finished deck height above grade, at the foot of the stair

Joist layout: clear span, cantilever and centres

Design load, soil and frost depth

Guard layout and the stair opening

The takeoff, in order

Check the joist size against the span

Count the joists

Count the ledger lag screws

Count the footings on the beam line

Size the footing under the middle post

Quantify the footing concrete

Count the deck boards

Count the deck screws

Lay out the stair

Count the balusters

Quantify the stain

The figures

The waste factors, and why these ends of the ranges

Where this estimate is most likely to be wrong

Ledger fastener count

Footing concrete

Deck boards

Stain

Joist and beam sizes

Tools this job needs

Frequently asked questions