Deck Calculator
Decking is counted in rows, not square feet. This works out how many boards the width takes, how many joists they sit on, and how much concrete the posts need.
Deck board and framing calculator
Board rows across the width
| Board and gap | Rows for this width | Linear feet |
|---|
Framing members this deck needs
| Member | Quantity | Rule used |
|---|
Boards are counted in rows, not square feet
Decking runs one direction, so the number of boards is set by the dimension they cross, not by the area. A 12 foot wide deck takes 26 rows of a 5-1/2 inch board with a 1/8 inch gap: 144 inches divided by 5.625 inches per board-and-gap, rounded up. Each row then runs the 16 foot length, so the job needs 416 linear feet of decking, or 26 boards in 16 foot stock.
The square footage is 192, and dividing that by a board's face area gives a number close to the same answer, but it drifts on narrow decks and on any deck where the boards run the short way. Counting rows is the method that survives a change of direction.
How the count is calculated
Rows equals the deck width in inches divided by the board width plus the gap, rounded up — rounding up because a partial row still needs a ripped board. Linear feet is rows times the length. Boards is linear feet divided by the stock length, rounded up, times the waste allowance.
Stock length matters more than people expect. On a 16 foot deck, 16 foot boards need one board per row; 8 foot boards need two, with a butt joint on every row, which is 26 extra joints to align and 26 more fasteners. Longer stock costs more per board and less per square foot of finished deck.
The structure under the boards
Joists at 16 inches on centre is the standard for wood decking up to 5/4 by 6; composite spans further and can go to 24 inches on centre, but check the manufacturer's table because some products require 12. Joists span between beams, and beams are carried by posts, which are carried by footings.
Post spacing drives the footing count. A common arrangement is beams at 8 foot intervals, and every post needs a footing below the frost line — 36 to 48 inches in much of the northern US, and deeper in cold climates. The concrete here is calculated at 1.5 bags of 50 pound mix per post, which fills roughly an 8 inch by 36 inch hole around a 4 by 4. That is a plan-level estimate; the actual footing diameter is set by your local code.
Gap, moisture and composite
Pressure-treated lumber shrinks as it dries, which is why it is laid with a gap on the wide side — a 1/4 inch gap on green wood closes to about 1/8 inch after a season. Composite does not shrink, so it uses a 1/8 inch gap or the spacer that comes with the hidden fastener system, usually 3/16 inch.
Boards laid tight will cup, trap water and rot from the top down. Boards laid too wide let heels catch and small objects fall through. The gap is a functional dimension, not a cosmetic one, and it is the single most common thing done wrong on a first deck.
What this does not count
The ledger board that ties the deck to the house, and its flashing, which is the detail that keeps the rim joist from rotting. Rim joists, blocking, hangers and hurricane ties. Railing, which is priced per linear foot and can rival the decking in cost. Stairs, which are their own calculation of stringers, treads and risers.
And the permit. Most jurisdictions require one for a deck above 30 inches, and the inspection usually happens at framing, before the boards go down — which means the sequence matters as much as the quantity.
Frequently asked questions
How many deck boards do I need for a 12x16 deck?
About 29 boards of 5-1/2 inch decking in 16 foot lengths, including a 10 percent allowance. Without the allowance the count is 26, one per row across the 12 foot width.
How many deck boards are in a square foot?
About 2.3 linear feet of 5-1/2 inch board per square foot with a 1/8 inch gap. A 16 foot board covers roughly 7 square feet.
What gap should I leave between deck boards?
A quarter inch for pressure-treated lumber, which shrinks as it dries, and an eighth inch or the manufacturer's spacer for composite. Never lay boards tight; they cup and hold water.
How far apart should deck joists be?
16 inches on centre for wood decking, 12 inches for diagonal layouts, and up to 24 inches for some composite products — but only the ones whose span table allows it.
How many posts and footings does a deck need?
It depends on beam spacing, which comes from the joist span table. A common plan puts beams at 8 foot intervals along the length, and every post gets a footing below frost depth.
Do I need a permit to build a deck?
Usually yes above 30 inches, and often below that if it is attached to the house. The inspection typically happens at framing, so the footing and joist layout matter before any boards go down.
Should I use 16 foot boards or 8 foot boards?
Sixteen foot boards wherever they span the whole deck, because each row then has no butt joint. Eight foot stock is cheaper per board and much more expensive per finished square foot once the joints are counted.
Disclaimer: This calculator estimates decking, joists, posts and concrete from plan dimensions. Structural sizing depends on species, grade, span tables and local code, which this tool does not evaluate. Have the framing reviewed before building.
Citing this page? FigureMill, “Deck Calculator” (last reviewed October 31, 2026). Decking board rows, linear feet, joists and post footings from deck dimensions, board width, gap and joist spacing. https://figuremill.com/materials/deck-calculator
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