How Many Deck Boards Do I Need? [Decking Calculator 2021]

One of the best pieces of advice I got before I built my deck was to plan how much lumber I would need down to the very last inch. Measuring the dimensional lumber for the framing was straightforward, but I still wondered how many deck boards I would need.

The simplest way to determine the number of deck boards for your deck is to use a decking calculator . But you can calculate it knowing that deck boards are actually 5 ½” wide. You’ll need a space of ⅛” between each board. For a 10×10 square deck, you would therefore need twenty-two 10’ deck boards.

Dimensions for deck boards can vary depending on the look you want and the deck board type you use. However, the most common is a 5/4” thick by 6” wide deck board of any length. Knowing your board dimensions and typical board spacing allows you just to use pen and paper to determine the number of boards you’ll need.

In this article, we’ll explain how to estimate decking materials, use a deck board calculator and other aspects of installing and cutting deck planking that might also factor into your calculations when determining how much decking you require.

How Many Deck Boards Do I Need

How to Calculate How Many Deck Boards Do I Need?

There are several factors you should consider before calculating the number of decking boards you’ll require. First, what is the design of the deck? What is the shape? What is the total square footage? Are you going to angle your deck planks or are they going to be perpendicular to the joists?

Second, what size wood planks are you going to use? The length of the board will determine how much waste you will have, so you’ll want to plan to make the best use of your deck planks. That may mean getting extra long or shorter lengths than you’d planned to waste as little as possible and save you some money.

Deck Design

The design of your outdoor structure will dictate the number of wood deck boards you’ll need. For instance, if you have a square deck, that is as long as it is wide, then you’ll probably be able to get your boards to run the length of your deck. That way you don’t have any butt joints – when the ends of two decking planks meet.

If your deck is long, butt joints are unavoidable as the longest decking board you’ll find is 20’. If you have a rounded edge on your deck, then you’ll have more waste since once you make your circular cut, you’ll have to remove a larger section of the remaining section to use it.

If your joists are further apart – such as 24” on center – then you’ll have more waste if you are trying to stagger your deck boards. Since butt joints can only meet over a joist, you’ll have less room to play with if your joists are spread far apart. That means more waste.

Deck Board Sizes

The first thing to understand is that decking planks are not actually 6” wide. Just like dimensional lumber, there is a bit taken off each side to “finish” the piece – which means the edges have been taken down with sanding for a smoother finish. Thus, the term “nominal size” refers to the size of lumber before finishing. “Actual size” refers to the literal, measured size of the board as you buy it.

Therefore, whether it is cedar or standard pressure-treated lumber, a wood deck board is actually 5 ½” wide. Similarly, a 5/4” thick deck board is really 1” thick. However, the thickness of the board will not change how many decking planks you’ll need.

Keep in mind that some opt to use 2×6 dimensional lumber as their deck planking. In that case, the actual dimensions are 1 ½” thick by 5 ½” wide. Lengths of 2x6s that you can purchase range from 8 to 16 feet, in even intervals.

A caveat to the actual size would be composite decking. Composites have varying widths, from 5 ½” to 5 ¼”. There are smaller widths available to as low as 4” wide. Thicknesses range from 1” to 2”.

Now that you understand the width and thickness of deck boards, let’s tackle lengths. Common lengths of composite and wood are, in feet, 8, 10, 12, 14, 16, and up to 20’.

If you have a deck that is up to 16’, then it is easy to purchase all 16’ decking and install the planks without any butt joints. However, for decks that are very wide or not a perfect square or rectangle, you’ll have to cut your planks and butt the ends. The key is to avoid too much waste – you should only end up with a part of one plank when you finish.

Let’s say you have a 24×8 deck. You only want to use 12’ lengths, which means you’ll have all your planks butt-jointed along the same joist in the middle. While this might work for some homeowners, the aesthetic isn’t ideal. The long joint in the middle will detract from the rest of your deck and be an eyesore.

Instead, many stagger their decking. Much in the same way you would install laminate or hardwood flooring, you stagger your deck planks so that the butt ends of one row are never next to the butt ends on the following row. A stair-step pattern, in which you stagger each piece so that each joist has a butt end starting with the last joist and working your way back joist by joist as you add rows.

Size of Your Deck

Knowing the square footage of your deck will help you plan how many deck boards you’ll need. This is easy if it is a square or rectangular – length x width. If you have a deck with sections, you should divide it into parts, find each area, and then add them together.

As you plan your deck, you need to know that there are two options for laying your deck boards: overhanging the boards 1 ½” past each joist, including the rim joist or having them sit flush with the joists. An overhang will protect your deck frame from the impacts of moisture and sun more than a flush deck board.

If you choose an overhang, you won’t be able to use 12’ boards on a 12×12 deck. Why? Because that 12×12 deck with an overhang ends up being 12’ 3” x 12’ 1.5” – longer than a 12’ board. If that is the case, you can use 14’ deck boards and stagger the boards.

Deck board calculatorYou can also adjust your framing to accommodate 12’ boards. Let’s say you have 16” on center joists. You can put your final joist on either end 13” from the previous joist. You’ll then have a 12×11’ 9” deck. You can now lay your 12’ boards with an inch and a half overhang to achieve your desired look without using longer decking. This is preferable as you will not have any waste that you would have with 14’ lengths.

Decking calculatorOnce you know the square footage, you can input that number into a decking calculator to determine how many deck planks you’ll want for your project.

Decking Materials

The type of decking material you use when you build a deck will not affect how many boards you need if they are all the same width. Let’s look at each option and see how each might be used as deck planking in your outdoor area.

Pressure Treated Lumber

The standard decking material, pressure-treated wood, is the local framing lumber found in your big box home reno store that has also been pressure treated with preservative. That means it was put into a pressure chamber and infused with chemicals that will make it last longer in the outdoors.

Since pressure-treated lumber is often quite wet when you purchase it, the possibility of shrinkage is very real. Bank on this type of lumber to shrink significantly over the first 6 months after you’ve installed it. Sometimes up to ¼ across the width of the piece. Lumber is far less prone to shrinking lengthwise, so your butt joints will remain fairly snug.

PT lumber comes in a variety of lengths as mentioned above. The standard thickness and width is 5/4×6”. Actual dimensions are 1” x 5 ½”. When using it you need gloves, plus a mask and eye protection when cutting it.


Cedar decking has the same dimensions available as pressure-treated decking. The difference is that the options for cedar can vary depending on where you live. Cedar species may not be available out west but are redwood instead.

Some also opt to use 2×6 cedar lumber as decking. It provides a sturdier feel and the edges are not as rounded as the 5/4” cedar planks.

You would calculate the number of cedar lengths needed for a deck in the same way you would for pressure-treated lumber since the widths of each do not differ.

Composite Decking

Composite decking is sometimes not quite as wide as the other types of decking mentioned above. However, many are 5 ½” wide and must be spaced according to the manufacturer’s instructions.

No two brands of the composite are alike, but these brands try to mimic the appearance and dimensions of real wood, so the finished appearance looks natural at a glance.

Joist Spacing

Another factor in deck board spacing is your joists. Typically they are either 16” or 24” apart, but some people opt for 12” if they intend on sustaining heavy loads on their structure.

Joist spacing comes into play when you stagger your wood planks. Joists spaced further apart – at 24” on center – restrict your flexibility in terms of staggering, as the minimum stagger will always be 24”. You may end up wasting a bit more at the end of your project. Also, the staggering’s overall look might not be as visually appealing, but that depends on the preferences of the homeowner.

How Far Apart Should Deck Boards Be Placed?

You’ll see many different opinions on this topic, but I maintain that ⅛” is the ideal spacing for your deck planking. I wasn’t comfortable butting the widths together and assuming that shrinking and drying would provide an adequate gap over time, as some installers claim. So I figured ⅛” was sufficient.

I used a speed square for my spacing, which is about ⅛” thick. It’s a great tool because the “I” shape of the edge lets you sit the square on the surface of the board while you position the next board into place.

How Many Decking Boards Do I Need for a 12×12 Deck?

How much decking do i need

Let’s take a real-world example to give you a better idea of how you would calculate the proper amount of deck boards for the square footage of a 12×12 deck.

First, you’ll want to know the width of your deck. In this case, it’s easy, both the length and width are 12’. In case you’re confused, the width of a deck is the orientation of the structure that is perpendicular to the joists. For most people, it will be the side of the deck that is against the house unless you have a freestanding deck.

No Overhang or Smaller Deck Frame

Our first option is to go with no overhang or a shorter frame to accommodate the 1.5” overhang. This would mean adjusting a joist to make it 3” closer to the previous joist. That would make the total width 11’9”. Whether we adjust a joist to accommodate 12’ lengths plus an overhang or lay the boards flush to the ends, we will be using 12’ boards.

A 12×12 deck has a perimeter of 48’ and square footage of 144 square feet. And just for kicks, we’ll say that the joists are 16” on center. The joist spacing doesn’t matter too much for this example, except that we can determine the exact amount of fasteners needed (2 per board per joist).

Our planks for this deck are 5/4” x6” x12’. We won’t be staggering these planks as they’ll fit the entire length.

We know the board’s actual width is 5 ½”. Each board will have a spacing of ⅛”. Thus, every board plus one space takes up a total of 5 ⅝”. We need to cover a total of 144”. So all we need to do is divide 144 by 5.625 (⅝).

Our answer is 25.6. What does that mean, though? It means we’ll require 25 decking planks that are 12’ long. Plus we’ll use ⅗ of another plank of the same length, so we’ll need 26 total lengths. The price of these pieces is around $15 per board. Multiplying 26 by 15 tells us that you’ll spend $390 plus tax for your decking.

Your variables could change, however, if you opt to change the desired spacing. If you opt not to space your planks at all, you would divide 144 by 5 ½ – you wouldn’t need to account for the gap.

If you decide that a 45-degree angled deck board looks better, expect to have an extra board or two of waste. Since you are cutting at angles, you’ll have to remove the previous angled cut first before cutting your next piece. That means trimming off several inches or so each time unless you don’t mind which side of the plank you are using.

Deck With Overhang and Actual Size is 12×12

For a deck that is actually 12×12 that still requires an overhang, we’ll just need to purchase the same amount of decking planks as above, but one size larger. Ideally, your home reno store has 14’ lengths. You’ll have to buy 25 lengths, as you’ll require one less since you are using longer boards.

You’ll have to pay more for the 14’ lengths, but in the end, the difference in cost between the two is not drastic and would be worth it to have an overhang to protect your deck frame. Many also feel an overhang looks better.

Decking Board Calculator

While calculating the amount of deck boards you need by hand is relatively straightforward, a calculator makes it much faster and will optimize your plan to tell you the best length of decking to get to optimize your costs.

A calculator is also super useful if you are planning your deck, as you can quickly enter a variety of deck dimensions to determine your total cost for each.

Before you use the calculator, you need to know your dimensions, the total square footage of your deck, your desired type of decking, joist spacing, and the price of each board, if you know it.

How Many Composite Deck Boards Do I Need?

Composite decking calculator

Thankfully most are 5 ½” wide, making it easier to calculate the number of composite pieces you’ll need. In that case, use the above method for calculating the number of composite planks you want for your project.

Be sure you know the spacing first. The fastening system dictates the spacing of composite lumber. There are usually clips that pre-determine the spacing. Most spacing requirements are around ⅛”.

Finally, don’t assume all composite decking is 5 ½” wide. A quick glance online will tell you that there are many types of composite planks that are narrower than 5 ½”, so do your homework before you hit the “buy” button.

How Much Do Deck Boards Cost?

We’ll take a look at 8’ pieces to compare costs between the major types of decking widely available.

First, a 2x6x8 cedar plank weighs in at around $17. You can get a coated cedar board of the same dimensions for around $27.

For standard pressure-treated lumber, a 2x6x8 costs around $11. A 5/4x6x8 costs just under $10. For this type of lumber, it almost always pays to get longer lengths. For instance, a 16’ board of the same type, width, and thickness costs around $14, less than double the 8’ length even though the length is doubled.

A premium composite deck board comes in at around $20 for an 8’ length, although they sometimes are sold in packs of 2. These are 1×5.5×8. Unlike real wood, composite planks are not usually cheaper as they get longer.


When spacing your deck boards consider aesthetics first. What look do you like? Check out your neighbor’s deck, your friends, and see what looks good and what doesn’t. Spacing that is too far apart is a code violation and wouldn’t look good, while spacing too close won’t allow water through it, which will deteriorate your planks faster.

Make sure you know your exact dimensions before you build a deck, and consider that you may have one or two bad pieces of lumber in a batch if you choose to have it delivered and you didn’t get a chance to inspect each piece before purchase.

Decking is the face of your yard. Don’t skimp on materials or staggering just because you didn’t want to go to the store to buy one last length of decking. And remember to use our decking calculator to double-check your calculations – it could save you a buck or two!

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