🪵 Deck Board Calculator

How to Calculate Deck Boards

Deck boards are sold by the piece but they cover by the row, so the whole estimate hinges on one number: how much width one board plus its gap actually covers.

A 5/4x6 board is 5.5 inches wide, not 6. Add a 1/4 in gap and each row covers 5.75 in, which is 0.479 sq ft per linear foot. That is why a 192 sq ft deck needs about 416 linear feet of board — roughly 2.2 linear feet of decking per square foot of deck, a useful number to keep in your head.

Get these three inputs right

• Actual board width, not nominal. A 2x6 and a 5/4x6 are both 5.5 in. Composite planks are often 5.25 or 5.375 in, and that difference adds a row across a wide deck.
• The gap you will actually install. Not the gap you hope for. See the gapping table below — it is different for wet treated lumber than for composite.
• The direction of the run. Boards run across the joists, so the deck dimension parallel to the house sets the board length and the dimension out from the house sets the row count.

Working out the deck size first? The square footage calculator handles L-shapes and odd angles, and the full deck calculator adds framing, posts and footings to this same takeoff.

Deck Board Sizes and Coverage

These are the profiles you will actually find on the rack. Coverage assumes a 1/4 in gap; tighten the gap and coverage drops slightly, meaning more rows.

5/4x6 is the default wood deck board and the right one for most projects: it is thinner and cheaper than a 2x6, it has a rounded edge that is kinder to bare feet, and it is rated for 16 in on center. Use 2x6 where you want the extra stiffness or where you already framed at 24 in with rated decking.

2x4 decking costs more, not less. You need over 50% more linear feet to cover the same area and you nearly double the fastener count. It is a look, not a saving.

Nominal versus actual sizes trip people up constantly — the lumber calculator has the full conversion table and covers board feet, grades and pressure-treatment ratings.

How Much Gap Between Deck Boards

Gap is not a style choice. It is drainage, and it changes as the board dries.

The classic mistake is gapping wet treated lumber a quarter inch. It dries, shrinks, and you end up with 3/8 to 1/2 in canyons that drop keys, phones and cutlery into the dirt below. Set wet PT nearly tight and let it open itself.

Composite runs the other way. It moves with temperature, so installing on a hot day means the boards are already expanded and end gaps must be generous, while a cold-day install needs less. Every composite manufacturer publishes a temperature-based gap chart — follow theirs, not a generic number.

Below the surface, a gap only works if water can get away. Boards laid over a solid ledger flashing detail or over a dry-space ceiling system need a drainage plan, or the gap just moves the water somewhere worse.

Fasteners: Face Screws vs Hidden Clips

Hidden fasteners look better and cost real money. Budget three to five times the fastener spend and add a day of labour on an average deck. They also require a grooved board, which rules out most solid wood decking.

A practical middle ground on a wood deck: hidden or plugged fasteners on the field, face screws on the perimeter and the picture frame, where you need the holding power and nobody is looking closely.

Non-negotiables for fasteners

• Hot-dipped galvanized to G185 or stainless steel. Modern ACQ and CA treatments corrode plain and electroplated steel within a few years.
• Pre-drill near board ends in cedar, redwood, and any hardwood decking. Tropical hardwoods like ipe must be pre-drilled everywhere.
• Hit the joist, not the gap. Snap chalk lines over the joist centres before you start screwing — every missed screw is a hole you cannot hide.
• Never use drywall screws. They are brittle, they are not corrosion-rated, and the heads snap under a deck board that wants to move.

Laying the Boards — the Order That Works

1. Check the frame is flat and square before a single board goes down. A twisted joist telegraphs through the decking permanently. Sight down the frame and plane or shim high joists.
2. Decide the butt joint strategy. Every butt joint needs a joist under it, and staggering joints across the deck reads far better than lining them up. Buying longer boards to eliminate joints is usually worth the premium.
3. Sort the boards. Crown side up, worst boards toward the house or under furniture, straightest boards for the outside edge where the eye lands.
4. Start at the house and work out, leaving a small gap at the ledger so debris and water can escape rather than pooling against the wall.
5. Snap chalk lines over every joist so screw lines stay straight down the deck. This is the single detail that separates a professional-looking deck from an amateur one.
6. Use a spacing jig, not eyeballing. A consistent gap is more visible than a perfect one.
7. Straighten bowed boards with a bar or a wedge as you fasten. Almost every wood board has some bow; fight it one joist at a time rather than at the end.
8. Run boards long and cut the edge in one pass. Let every board overhang the rim, snap a line, and cut the whole edge with a circular saw. Far straighter than cutting each board to length first.
9. Fit the picture frame last if you are running one, with blocking underneath to land the mitres on.

Mistakes that cost boards

• Ordering exactly what the math says. One split board and you are back at the store hoping the colour lot matches.
• Buying short boards to save money. Two 8 ft boards cost more than one 16 ft board and give you a butt joint you did not want.
• Mixing composite production runs. Like flooring, composite colour varies between batches. Buy the whole deck in one order.
• Framing at 16 in and then choosing composite. Many composite boards need 12 in on center, and diagonal layouts almost always do. Choose the board before you frame.

Frequently Asked Questions

How many deck boards do I need?

Divide the deck width by the coverage of one board plus its gap, round up for the row count, then multiply by the deck length for linear feet. A 16 × 12 ft deck with 5.5 in boards at a 1/4 in gap needs 26 rows and 416 linear feet, or 29 boards at 16 ft once you add 10% waste.

How many linear feet of decking per square foot?

About 2.2 linear feet per square foot for a 5.5 in board with a 1/4 in gap, which is 209 linear feet per 100 square feet. A 3.5 in board needs about 3.2 linear feet per square foot. Wider boards need fewer linear feet but cost more per foot, so the material bill lands in a similar place.

How wide is a 5/4x6 deck board?

5.5 inches wide and about 1 inch thick, not 6 by 1.25. Like all softwood lumber, 5/4x6 is sold at a nominal size and dressed smaller. Always calculate with the actual width — assuming 6 inches will leave you two rows short on a wide deck.

How much gap should I leave between deck boards?

It depends on how wet the board is. Wet pressure-treated lumber should go down nearly tight, because it will shrink and open to roughly 1/4 in on its own. Kiln-dried, cedar and composite boards get 1/8 to 1/4 in as installed. Composite manufacturers publish temperature-based gap charts — follow theirs.

How many screws do I need for decking?

Two per board at every joist crossing, so roughly 350 screws per 100 square feet of 5/4x6 at 16 in on center. Use three per crossing on 2x6 and wider boards to control cupping. Drop to 12 in joist spacing and the count rises by about a third.

How many hidden fasteners do I need per square foot?

One clip per board at each joist crossing, which is about 175 clips per 100 square feet at 16 in on center — roughly half the count of face screws. Hidden systems cost three to five times as much in fasteners and take noticeably longer to install.

Are hidden deck fasteners worth it?

For a composite deck with grooved boards, usually yes — the clean surface is the whole point of composite and the clips are designed into the board. For wood, it is a judgment call: budget three to five times the fastener cost plus a day of labour. A good compromise is hidden fasteners in the field and face screws around the perimeter.

How much waste should I add for deck boards?

10% for a plain rectangle, 15% if you are running a picture-frame border or the deck is long enough that most rows need a butt joint, and 20% for a 45 degree diagonal layout. Rounding up to whole boards already absorbs some of this, but the offcuts on a diagonal deck are genuinely large.

Which direction should deck boards run?

Perpendicular to the joists, always — that is what carries the load. Beyond that, running boards parallel to the house is the standard and usually the best-looking choice. A 45 degree diagonal reads well on a large deck but needs 12 in joist spacing and about 20% more material.

Can I use 2x6 instead of 5/4x6 for decking?

Yes, and it is stiffer — a 2x6 is 1.5 in thick against roughly 1 in for 5/4. That extra thickness is what lets rated 2x decking span 24 in on center where 5/4 cannot. The trade-offs are cost, weight, and a square edge rather than the rounded profile of 5/4 radius-edge board.

Should deck boards be installed bark side up or down?

Modern guidance is to install crown side up — that is, the face that is already slightly convex faces the sky, so water sheds instead of pooling. The old bark-side-up rule was about cupping direction, but on properly dried and fastened decking the crown is the more reliable thing to sight for.

Do I need to pre-drill deck boards?

Near the ends, yes, in cedar, redwood and any board that splits easily. Tropical hardwoods such as ipe must be pre-drilled everywhere — they are too dense to take a screw otherwise. Pressure-treated pine generally does not need it except within a couple of inches of a board end.

How long do deck boards last?

Pressure-treated pine typically gives 15 to 25 years with regular cleaning and sealing, cedar 15 to 25 depending on exposure, composite 25 to 30 with a long warranty, and dense tropical hardwood 40 or more. The framing usually outlives the decking, which is why replacing just the boards is a common mid-life project.

Can I put new deck boards on an old frame?

Only after you have inspected the frame properly. Probe the joist tops, the ledger, the rim and every post base with a screwdriver — soft wood means replacement, not resurfacing. Check the ledger flashing and the hardware for corrosion. New boards on a failing frame is the most dangerous thing you can do to a deck, because it hides the problem.