Deck Lumber Calculator

A board count is not an order because a yard does not sell boards. It sells lengths. Walk in with "26 decking boards" and the first question back is which length, and the answer to that changes what you pay, what you carry and what the finished deck looks like.

What the yard actually sells you

This deck needs 26 rows of 5/4 x 6 decking. That much is fixed by the deck. What is not fixed is that those rows arrive as 26 boards at 16 ft from one yard and 52 at 8 ft from another — identical decking, twice the pieces.

Across the whole order that is 44 boards against 87. The footage barely moves; 644 linear feet against 696. Everything that does move is handling, joints and offcut, and none of those appear on a board count.

The offcut nobody prices

Offcut is the length you buy and cannot use, and it is set entirely by how your rows divide into the boards you can get. On the decking line it is stark: a 10 ft row from 12 ft stock leaves 2 ft on every board — 52 ft in total, or 17% of that line thrown away. The same row from 20 ft stock yields two per board and leaves 0%.

Those figures are the decking line alone, and that is the honest way to quote them — joists and posts pull the order-wide number around. On the default deck a 12 ft-only yard puts 0 ft of decking in the offcut pile, which is 0% of that line.

Buying long is not the same as buying well

The rule of thumb is to buy the longest boards you can. On this deck it is the most expensive thing you can do.

A yard stocking only 20 ft lumber returns the same 44 boards — but 868 linear feet against 644, at $1,483 against $1,102 on our survey prices. That is $381 more for the same deck, and 26% of what you paid for ends up in the offcut pile. The 20 ft boards are not wrong. Buying 20 ft boards to cut 16 ft rows is.

Length only helps when your rows divide into it.

Buy the shortest stock length your rows divide into exactly. That is the whole rule, and it puts you in one of three situations.

Your row length is a stock length. Buy it and stop thinking. A 16 ft row from 16 ft stock is one board, no joint, nothing over — which is why the default deck above shows no offcut at all.

Your row divides into a longer board. Buy the multiple. A 10 ft row from 20 ft stock gives you two rows per board and leaves nothing; 13 boards instead of 26, and 260 feet instead of 312.

Neither. Now you are choosing between a joint and an offcut, and the joint is usually the better trade where the numbers are close — lumber is cheap and a second trip is not. Read the joint section below before you decide, because a joint costs work rather than money and the calculator cannot price that for you.

When the yard cannot tell you what is coming in, measure your rows first and ask for lengths second. Most people do it the other way round, get told what is on the rack, and design the order around it.

This calculator works out how many pieces the deck needs, then decides how to buy them.

rows = deck width ÷ (board width + gap)
joists = (deck length ÷ spacing) + 1
order = fewest feet of your stock lengths that yields the pieces

The quantities come from the same function the whole-build material list uses, so the two pages cannot disagree about the deck — 192 sq ft (17.8 m²), 26 rows, 13 joists and 3 posts on both. Joist, beam and post sizes are planning assumptions rather than span-checked figures, marked ASSUMED in the panel where they appear.

The buying decision has two cases. Where a piece fits inside a board, it packs whole pieces per board and takes no joints — the 3 posts come out of a single 8 ft timber with 0.5 ft over. Where a piece is longer than anything you can buy, it joins boards and reports the joints. It sticks to one stock length per item rather than mixing: mixing shaves a foot occasionally and turns a shopping list into a puzzle.

Two limits on the subtotal, both worth knowing before you quote it. Prices are our own survey estimates rather than quotes, and the calculator applies one rate per linear foot whatever length you buy — real yards often charge a premium for long stock. That second limit is load-bearing here: the finding that 44 boards and 74 boards cost the same only holds while price per foot is flat. Where it is not, the shorter order is the cheaper one — which makes it the order you were about to choose, and it still carries 30 joints.

A butt joint is where two boards meet end to end in the same row. You get one whenever a run is longer than anything the yard carries, so the deck decides how many you need and the yard decides whether you can avoid them.

Every joint lands on a joist. Not near one — on one, with both board ends bearing and fastened. A joint between joists has nothing underneath it: the ends cup away from each other as they weather, the gap opens, and it becomes the place on the deck that catches a toe and holds water.

Stagger them between adjacent rows. Two joints side by side read as a seam running across the deck, and they put every board end in the same joist bay. Move each row's joint at least one bay along from its neighbour's.

A joint is cheap in lumber and expensive in everything else. It costs two board ends cut square, two more fasteners at that joist, and a line for water to sit in for the life of the deck. That is why 30 joints from a 12 ft yard is a real cost even though the subtotal does not move: at the default lengths this deck needs 0.

Where you do need them, a doubled joist or a pair of blocks under the joint gives both ends proper bearing. Deciding that at the yard is easier than deciding it with a saw in your hand.

The trade calls both of these waste and they are different things, one of which you can design away and one of which you cannot.

Offcut is length the cut list cannot use. It is deterministic — pick your stock lengths and the offcut is decided before you cut anything. It runs from near nothing at the default lengths to 26% of the order from a 20 ft-only yard. This is the one worth shopping around for.

The miscut allowance is 5% on a straight layout — about 2 spare boards on this deck — for boards you cut short, split at the end, or reject at the rack. It is a contingency, not a quantity, and it is an estimate.

A diagonal layout changes both. Every row meets the joists at an angle and finishes a different length, so there is no cut list to give and the calculator says so where the cut list would be — it buys footage instead, and the miscut allowance rises because every board is now a compound cut with two triangular ends.

Four questions, and the first one is the only one most people skip.

Which lengths are in stock today? Not in the catalogue — today, in the rack. Catalogue lengths are what the mill produces; stock is what you can put on a trailer this afternoon, and the whole order above is built on the difference.

Are long boards a special order, and what is the lead time? Twenty-foot stock is often a bring-in rather than a stock item, and a two-week lead time is worth knowing before you plan a deck around it.

Does the price per foot change with length? It often does, and our subtotal cannot know that — it applies one rate per foot whatever you buy. A long-length premium is the thing most likely to make the cheapest order different from the one with the fewest boards.

Can I pick my own boards? Some yards let you cull the pile, some do not. On pressure-treated pine, where crown and twist vary a lot from board to board, it matters more than the price difference between two yards.

Hand over the order, not the cut list. A desk wants quantity, size and length, one line each — "26 of 5/4 x 6 at 16 foot" is a line somebody can key in. The panel is already in that order, and it stays in it whatever lengths you tick.

The cut list is for you. Keep it for the saw horses, where knowing that one 8 ft timber becomes 3 posts saves you working it out with a tape and a marker.

Check the delivery against the order before the truck leaves. A missing board is a phone call on the day and a second trip a week later, and the yard is far more willing to fix it while the driver is still in the drive.