Deck Calculator

Enter a deck's length and width to get decking boards, joists, beams, posts, and footing concrete, all in one place. Results update as you type. Need the concrete math on its own, or a fence instead? Try the concrete calculator or the fence calculator.

Deck length and width, beam span, and post spacing use the unit toggle above. Board width, gap, and board length stay in inches/feet, and post hole depth stays in feet, the same way the fence calculator keeps post hole measurements fixed regardless of the toggle.

Deck length (ft)

Deck width (ft)

Decking board width, actual (in)

Gap between boards (in)

Board stock length (ft)

Decking waste / extra (%)

Joist spacing, on-center

Beam span (ft)

Post spacing along beam (ft)

Post hole diameter (in)

Post hole depth (ft)

Concrete waste / extra (%)

Deck area: 0 ft²
Decking rows: 0
Decking needed (with waste): 0 ft
Boards to buy: 0
Joists needed: 0
Beam lines needed: 0
Posts needed: 0
Concrete per footing (with waste): 0 ft³
Total concrete (all footings): 0 ft³
Cubic yards (all footings): 0 yd³
40 lb bags (all footings): 0
60 lb bags (all footings): 0
80 lb bags (all footings): 0

How to plan a deck build

Measure the deck's footprint as a length and a width, in the same units you plan to use throughout. This calculator treats the deck as a simple rectangle; split an L-shaped or multi-level deck into separate rectangular sections and run each one through the calculator on its own.

For decking boards, enter the actual board width, not the nominal size; a nominal 2x6 board is actually 5.5 inches wide, which is the default here. Add the gap you plan to leave between boards, 1/8 inch is a common default for pressure-treated lumber that will shrink slightly as it dries. The calculator divides the deck's width by the board width plus gap to get the number of rows, multiplies by the deck's length to get linear feet, adds your waste factor for cuts and culled boards, then divides by your board's stock length to get a board count to buy.

For joists, 16 inches on-center is a common default residential framing spacing convention, with 12 inches on-center sometimes used for extra stiffness or heavier decking and 24 inches on-center sometimes allowed for certain joist sizes and species. This is a standard framing spacing convention like the kind found in US building-code span tables, not a measured site figure and not a substitute for checking your actual joist size and species against a real span table.

For beams and posts, enter a beam span (how far apart beam lines sit across the deck's width) and a post spacing (how far apart posts sit along each beam line). Both default to 8 feet as a plain planning convention; actual allowable spans depend on lumber size, species, ply count, and your local code, so check a span table or a professional before relying on this number. Posts are counted with this site's fence calculator formula (ceil(run length ÷ spacing) + 1) applied once per beam line, then multiplied by the number of beam lines to build a full post grid.

For footing concrete, this calculator treats each round post hole as its bounding square, the same simplifying shortcut used by this site's concrete calculator and fence calculator, which always gives at least as much material as the true cylinder, never less. Total concrete multiplies the per-hole volume (with waste) by the total post count from the grid above.

This tool is a planning estimate, not an engineered structural plan. In most US jurisdictions, a deck attached to the house, elevated more than a step or two, or over a certain size needs a building permit, and larger or taller decks often need an engineer or architect's stamped plan. Check with your local building department before you dig a single hole.

Worked example

A deck is 16 ft long by 12 ft wide, decked with 5.5 in actual-width boards, a 1/8 in (0.125 in) gap, 12 ft stock boards, and a 10% decking waste factor. Joists are spaced 16 in on-center, beams span 8 ft, posts along each beam are spaced 8 ft apart, post holes are 10 in diameter and 3 ft deep, with a 10% concrete waste factor.

Deck area	16 × 12 = 192 sq ft
Effective board width	(5.5 + 0.125) ÷ 12 = 0.46875 ft
Decking rows across the width	12 ÷ 0.46875 = 25.6, rounded up to 26 rows
Linear feet of decking	26 × 16 = 416 ft
With 10% waste	416 × 1.10 = 457.6 ft
Boards to buy (12 ft stock)	457.6 ÷ 12 = 38.13, rounded up to 39 boards
Joists (16 in o.c.)	(16 × 12) ÷ 16 = 12 spaces, plus 1 = 13 joists
Beam lines (8 ft span)	12 ÷ 8 = 1.5, rounded up to 2 spaces, plus 1 = 3 beam lines
Posts per beam line (8 ft spacing)	16 ÷ 8 = 2 spaces, plus 1 = 3 posts
Total posts	3 posts × 3 beam lines = 9 posts
Hole diameter in feet	10 ÷ 12 = 0.8333 ft
Volume per hole (bounding square)	0.8333 × 0.8333 × 3 = 2.0833 cu ft
With 10% waste	2.0833 × 1.10 = 2.2917 cu ft per hole
Total for all 9 holes	2.2917 × 9 = 20.625 cu ft
Cubic yards	20.625 ÷ 27 = 0.7639 cu yd
40 lb bags (0.30 ft³ each)	20.625 ÷ 0.30 = 68.75, rounded up to 69 bags
60 lb bags (0.45 ft³ each)	20.625 ÷ 0.45 = 45.83, rounded up to 46 bags
80 lb bags (0.60 ft³ each)	20.625 ÷ 0.60 = 34.38, rounded up to 35 bags

These are the exact default values shown by the calculator above, so you can check them against your own screen before trusting it with your own numbers.

Concrete calculator. Cubic yards and 40, 60, or 80 lb bags for slabs, footings, and columns, the same bag yields used above.
Fence calculator. The same post-spacing and bounding-square concrete formulas, applied to a single straight run instead of a grid.
How many bags of concrete for a fence post? The true cylinder math side by side with the bounding-square shortcut used above.
How many deck boards do I need? The decking-row formula above, worked out in full.
Deck joist spacing: 12, 16, or 24 inches on-center? The sourced span-table basis for the three options above.
Square footage calculator. For the footprint area on its own.

Frequently asked questions

How many decking boards do I need for a 16x12 foot deck?

For a 16 by 12 foot deck using standard 2x6 decking (5.5 in actual width) with a 1/8 in gap, the deck's 12 ft width needs 26 rows of boards spaced 5.625 in apart, each row running the full 16 ft length. That is 416 linear feet of decking before waste. With a 10 percent waste factor that is 457.6 linear feet, which needs 39 boards at a 12 ft stock length, rounded up since you cannot buy a partial board.

How far apart should deck joists be spaced?

16 inches on-center is a common default residential framing spacing convention, with 12 inches on-center used for extra stiffness or heavier decking materials and 24 inches on-center sometimes allowed for certain joist sizes and species under specific span tables. This is a standard framing spacing convention, not a measured site figure; check your actual joist size, species, and span against your local building code's span tables before building.

How many support posts does a deck need?

This calculator reuses the same straight-run formula as this site's fence calculator (posts = ceil(run length / post spacing) + 1) once per beam line, then multiplies by the number of beam lines to build a post grid. For a 16 by 12 foot deck with an 8 ft beam span and 8 ft post spacing, that is 3 beam lines times 3 posts per line, for 9 posts total.

How much concrete do I need per deck footing?

For a common 10 inch diameter, 3 foot deep footing hole, using the same bounding-square method as this site's concrete and fence calculators (entering the diameter as both sides of a square, a safe overestimate of the true round volume) and a 10 percent waste factor, one footing needs about 2.29 cubic feet of concrete, or 8 bags of 40 lb mix, 6 bags of 60 lb mix, or 4 bags of 80 lb mix per hole.

Do I need a permit or an engineer to build a deck?

In most US jurisdictions, yes, once a deck is attached to the house, elevated more than a step or two off the ground, or over a certain size, you typically need a building permit, and a deck over a certain height or span often needs an engineer or architect's stamped plan. Requirements vary a lot by city and county. This calculator is a planning estimate only, not an engineered structural plan; check with your local building department before you dig a single hole.

What beam span should I use for a deck?

8 feet is used here as a plain planning convention, not a verified engineering calculation. Actual allowable beam spans depend on the beam's lumber species, size, and ply count, the joist span it carries, and your local code's span tables, and can be shorter or longer than 8 feet. Check a span table for your specific beam size and species, or have a professional size it, before you build.