Deck Calculator

Deck boards, fasteners, and total cost for your deck size. Updated for 2026.

Last reviewed July 2026 · Free · Nothing you enter is stored

How this calculator works

Board math uses standard 5.5"-wide decking with a 1/4" gap: 2.09 linear feet of board per square foot of deck, plus 10% waste (15% for diagonal layouts). Hidden fasteners or screws run about 3.5 per square foot. Cost estimates use 2026 installed averages — $35/sq ft pressure-treated, $57/sq ft composite — and a DIY materials-only average around $20/sq ft.

The waste allowance

This calculator adds 10% to the board count, and 15% for a diagonal layout. Trex's own published formula is leaner than that: deck width in inches divided by 5.5, plus 5% waste. The gap between the two figures is deliberate. A manufacturer's 5% covers ordinary offcuts on a simple rectangle where every board runs the full width. It does not cover the second cut on every board around a stair opening, the boards ruined by a crooked rip against an out-of-square house wall, or the two or three lengths that arrive with a bow you will not accept in a finished surface. On a diagonal deck every single board takes two angled cuts and the triangular offcuts are unusable, which is where the extra five points go. Decking is also sold in fixed lengths, so a 12 ft 4 in run means buying 14 ft boards and throwing away the difference whatever the percentage says.

The board count, and why 5.5 inches is the only width that matters

Both the 1-inch and the 2x6 profiles measure 5.5 inches across the face. That is the number the coverage maths runs on, and it is the same number whether the board is nominally called a 1x6 or a 2x6, because the thickness changes and the width does not.

Trex's published method is to take the deck width in inches, divide by 5.5, and add 5% for waste. That gives the number of boards in each run. Multiply by the number of runs, or simply work in linear feet: a square foot of deck surface takes a little over two linear feet of 5.5 inch board once the gap between boards is accounted for.

The gap is why the divisor is not quite the finished coverage. Boards are laid with a deliberate side-to-side space, so each board covers slightly more than its own width in finished deck. That works in your favour by a small margin and is not worth chasing; the waste allowance swamps it.

Why composite needs joists closer together than wood

This is the single most useful thing on this page, and it is the mistake that ruins the most composite decks. Composite decking has a lower modulus of elasticity than solid wood of the same dimension. Modulus of elasticity is simply a measure of stiffness: how much a material deflects under a given load over a given span. A lower figure means more deflection between supports, for the same board thickness and the same person standing on it.

So a joist layout that carries pressure-treated 5/4 boards without any perceptible movement will let composite boards of the same thickness sag visibly between joists and feel springy underfoot. The boards are not defective and they are not undersized; they are simply being asked to span further than their stiffness allows. Heat makes it worse, because composite softens as it warms, and a deck in full afternoon sun is considerably hotter than the air temperature.

The published spacings follow directly from that. For residential use with boards run perpendicular to the joists, 1-inch profiles go at 16 inches on centre and 2x6 profiles at 24 inches. Commercial applications, which assume heavier and more concentrated loading, tighten those to 12 inches and 16 inches respectively. Those are maximum spacings, not targets, and there is no penalty for framing tighter.

The practical consequence is that a composite deck is a framing decision before it is a decking decision. Retrofitting composite onto an existing frame built at 24 inch centres for wood is the classic failure, and the fix at that point is sistering in extra joists across the whole deck.

Diagonal and picture-frame layouts change the framing

Running boards at an angle increases the distance each board spans between joists, because the board no longer crosses the framing at a right angle. The published corrections are specific. At 45 degrees, reduce the maximum joist span by 4 inches. At 60 degrees, reduce it by 2 inches. At 30 degrees, reduce it to half the standard span, which is a substantial change and catches people out.

Applied to the residential figures, a 1-inch board that would sit on 16 inch centres running straight needs 12 inch centres at 45 degrees, and 8 inch centres at 30 degrees. That is not a decking adjustment you can make on the day the boards arrive. It has to be designed into the frame.

Picture-frame borders create the same problem in a smaller area. The border board runs perpendicular to the field boards and needs its own continuous support, usually blocking or a doubled joist under every seam. A border laid over a frame that was never blocked for it will move underfoot at exactly the point where the eye is drawn.

Gapping, and what happens when you leave none

Side to side, the minimum gap is 3/16 inch and the maximum is 1/2 inch. The minimum exists because composite expands and contracts with temperature and needs somewhere to go; the maximum exists because a wider gap becomes a trip and drainage nuisance and starts to look like a mistake rather than a detail.

End to end, the gap depends on the temperature at the time of installation. Above 40°F, leave 1/8 inch between board ends. Below 40°F, leave 3/16 inch, because the boards are installed cold and will grow when the weather turns. Where the decking runs into a house wall or any fixed structure, leave a quarter to half an inch so the board has room to move without pushing against something that cannot move at all.

Butting boards tight is the second classic composite failure. There is nothing wrong with the deck on the day it is built. Some weeks later, on the first genuinely hot afternoon, the boards expand into each other with nowhere to go, and the surface buckles or the fasteners tear out. It is not repairable by adjustment; the affected boards come up.

Fasteners, and what a box actually covers

Face-fastened decking takes two screws per board at every joist crossing. That number drives the count more than the deck area does, because it multiplies the board runs by the joist count, and a deck framed at 12 inch centres for a diagonal layout needs a third more fasteners than the same deck framed at 16.

Hidden fastener systems are sold by coverage rather than by count. A box covers 50 square feet and a bucket covers 500 square feet, which makes ordering straightforward as long as you order against the deck area plus the same waste allowance you used for the boards. Running short of clips halfway across a deck is a particularly annoying way to lose a day.

Under the surface, the frame itself is governed by span tables rather than by the decking manufacturer. In the American Wood Council's DCA6 tables, Southern Yellow Pine 2x8 joists span 13 ft 1 in at 12 inch centres and 11 ft 10 in at 16 inch centres; 2x10 joists span 16 ft 2 in and 14 ft respectively. Douglas Fir-Larch, Hem-Fir, Redwood and Western Cedar all span shorter than Southern Yellow Pine at the same dimension, so a span read off the wrong species column is a real error rather than a conservative one.

Maximum joist spacing by decking profile and layout

These are maximum on-centre spacings for the joists under composite decking. Framing tighter than the figure shown is always acceptable; framing wider than it is what produces sag and bounce.

Spacings and diagonal reductions from the Trex installation guide. Joist span limits for the frame beneath are set separately by the American Wood Council's DCA6 prescriptive deck guide and vary by lumber species.

What goes wrong on composite decks

Wood joist spacing under composite decking. Framing at 24 inch centres because that is what carried the old wood deck gives visible sag between joists and a springy feel underfoot. Composite is less stiff than wood of the same dimension and needs 16 inch centres for 1-inch profiles.

Boards butted tight with no gap. Composite grows with temperature. Without 3/16 inch side to side and 1/8 inch end to end above 40°F, the deck buckles on the first hot day and the fasteners pull.

Diagonal or picture-frame layouts on standard framing. Angled boards span further between joists. A 45 degree layout needs 4 inches taken off the maximum span and a 30 degree layout needs half of it, which has to be built into the frame rather than discovered afterwards.

No allowance where the deck meets the house. A quarter to half an inch is needed where decking abuts a wall. Tight against fixed structure, the board has nowhere to expand and will push or cup.

Reading a span table off the wrong species. DCA6 spans differ by lumber species. Douglas Fir-Larch, Hem-Fir, Redwood and Western Cedar all span shorter than Southern Yellow Pine, so borrowing the SYP figure overspans the joists.

Frequently asked questions

Why your state changes the price (but not the quantity)

How much decking you need is physics — it doesn't care where you live. What it costs does. Labor rates, delivery distance, fuel, and local demand move installed prices by roughly 45 points from end to end: Arkansas, Mississippi, North Carolina sit near the bottom of the range, while Alaska, Hawaii, New York sit at the top. The Price this for selector at the top of the calculator applies a published construction cost index for your state to the 2026 national averages, so the dollar figures move and the material list stays put.

Treat the result as a budgeting range, not a quote. Two metros inside the same state can differ by more than the statewide index suggests, and a single busy contractor can outweigh both. See the full cost index for all 50 states and where the price data comes from.