Deck Board Chart: Boards Needed by Deck Size & Waste

What do 40 boards mean?

It means the order is 40 boards for a 200 sq ft deck: the board coverage is 5.5 inches wide, which is 0.4583 ft, and 200 sq ft divided by 0.4583 ft is 436.36 ft of board laid side by side. The 10% waste allowance adds 43.64 ft for end cuts, bad boards and mistakes, taking the buy length to 480.00 ft, and 480.00 ft divided by 12 ft boards is exactly 40 boards, rounded up to whole boards. The part people get wrong is ordering by the number of boards that cover the deck with no waste: a deck ordered at exactly 37 boards, the no-waste figure, runs short at the last few cuts, and a short order means a second delivery and a possible colour mismatch between batches.

Three things to check before you trust the number

Is the board width the face width or the nominal size? Use the actual face width that covers the deck, not the nominal lumber size: a nominal 6-inch board usually measures 5.5 inches across the face. The 7.25-inch branch shows a 200 sq ft deck needing only 31 boards, which is the kind of gap a wrong width creates.

Is 10% waste enough for this deck? Straight decks with square ends manage on 10%, but diagonal boards, picture framing and many angles push waste toward 15% or more. The waste reverse shows 22% pushing the same deck from 40 boards to 45, so a fancy pattern ordered at 10% will run short.

Do the board lengths suit the deck shape? Twelve-foot boards suit a deck up to 12 ft wide with no joints, but a wider deck needs joints or longer boards. The 8-foot branch shows the same deck needing 60 boards instead of 40, with far more joints to stagger.

Worked example

On the defaults: 200 sq ft deck, 5.5 in boards, 12 ft lengths, 10% waste.

1. Coverage width: 5.5 inches divided by 12 is 0.4583 ft.
2. Board length needed: 200 sq ft divided by 0.4583 ft is 436.36 ft.
3. Buy length: 436.36 ft times 1.10 is 480.00 ft.
4. Boards: 480.00 ft divided by 12 ft is 40 boards.

And the other way: the deck area for 40 boards at 5.5 in boards, 12 ft lengths and 10% waste is 200 sq ft, the smallest whole waste percent giving at least 45 boards is 22%, and the largest deck area (10 sq ft steps) keeping boards at or below 40 is 200 sq ft, while 210 sq ft would need 42 boards.

Every branch, checked

• 150 sq ft deck: 150 sq ft deck: 150.00 sq ft deck: 327.27 ft of board, 360.00 ft to buy, 30 boards.
• 300 sq ft deck: 300 sq ft deck: 300.00 sq ft deck: 654.55 ft of board, 720.00 ft to buy, 60 boards.
• 200 sq ft at 0% waste: 200 sq ft at 0% waste: 200.00 sq ft deck: 436.36 ft of board, 436.36 ft to buy, 37 boards.
• 200 sq ft with 7.25 in boards: 200 sq ft with 7.25 in boards: 200.00 sq ft deck: 331.03 ft of board, 364.14 ft to buy, 31 boards.
• 0 sq ft deck: 0 sq ft deck: 0.00 sq ft deck: 0.00 ft of board, 0.00 ft to buy, 0 boards.
• 200 sq ft with 8 ft boards: 200 sq ft with 8 ft boards: 200.00 sq ft deck: 436.36 ft of board, 480.00 ft to buy, 60 boards.

The reverses, worked backwards

• Deck area for 40 boards at 5.5 in boards, 12 ft lengths, 10% waste: 200 sq ft — at 200 sq ft the figures are 200.00 sq ft deck: 436.36 ft of board, 480.00 ft to buy, 40 boards.; at the lower neighbor the figures are 190.00 sq ft deck: 414.55 ft of board, 456.00 ft to buy, 38 boards.; at the upper neighbor the figures are 210.00 sq ft deck: 458.18 ft of board, 504.00 ft to buy, 42 boards..
• Smallest whole waste percent giving at least 45 boards: 22% — at 22% the figures are 200.00 sq ft deck: 436.36 ft of board, 532.36 ft to buy, 45 boards.; at the lower neighbor the figures are 200.00 sq ft deck: 436.36 ft of board, 528.00 ft to buy, 44 boards.; at the upper neighbor the figures are 200.00 sq ft deck: 436.36 ft of board, 536.73 ft to buy, 45 boards..
• Largest deck area (10 sq ft steps) keeping boards at or below 40: 200 sq ft — at 200 sq ft the figures are 200.00 sq ft deck: 436.36 ft of board, 480.00 ft to buy, 40 boards.; at the lower neighbor the figures are 190.00 sq ft deck: 414.55 ft of board, 456.00 ft to buy, 38 boards.; at the upper neighbor the figures are 210.00 sq ft deck: 458.18 ft of board, 504.00 ft to buy, 42 boards..

What the defaults assume

The defaults are a 200 sq ft deck, 5.5 in boards, 12 ft lengths and 10% waste, so the headline is one ordinary straight deck at the usual board size. The model is deck area divided by board width in feet for the board length, times one plus waste for the buy length, divided by board length and rounded up to whole boards. The area reverse is checked at 10 sq ft either side; the waste reverse is checked at 1% either side, because 1% of 436.36 ft is 4.36 ft, which is about a third of a board. The model does not include the gap between boards, joists, screws, fascia, or railing — those are ordered separately, not by the board count.

Output · formula · meaning

Who uses this

• A homeowner getting a decking quote — 40 boards on a 200 sq ft deck is the number to compare against the contractor's material list.
• A builder ordering for a job — the 300 sq ft branch shows 60 boards, and the 7.25-inch branch shows how a wider board cuts the order to 31.
• A DIYer decking a small patio — the 150 sq ft branch at 10% waste shows 30 boards, a small order that still rounds up.
• A student learning trade arithmetic — one worked example from area to board length to boards, and branches that show how each input moves the answer.

Accuracy & limitations

Figures are the model's own arithmetic on the numbers you type, computed from unrounded values and displayed rounded HALF_UP to 2dp; boards are whole units rounded up, so a computed 40.00 stays 40 while a computed 36.36 becomes 37. The main limitation is measurement, not arithmetic: the deck area must be the finished deck surface, the board width must be the face width, and waste depends on the pattern. Gaps between boards, joists, screws, fascia and railing are not included. This page is a calculation tool, not building advice.

Cite this calculator

A 200 sq ft deck with 5.5 in boards in 12 ft lengths needs 40 boards with 10% waste. Source: Bright Side Kit, https://brightoolbox.com/deck-board-chart/, accessed 28 October 2026.

Frequently asked questions