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How to Calculate How Many Decking Boards I Need

Ensuring you get your decking board calculations correct is important. If you want to build a deck using composite decking boards, you will need to know the number of boards required to complete your project. This will help to determine the overall cost of your composite boards and how much you need to budget.

If you don’t know how to calculate how many composite decking boards you will need, reviewing this article will help you.

There Are Two Ways To Calculate

The simplest way to determine how many decking boards you will need is to calculate it yourself, when doing this there are two ways in which we have listed below to support, another option is to use an online decking calculator. However, some other factors need to be considered to determine the exact number of boards needed.

You will need to know the square metreage of your decking area and also consider how the design layout of your decking could affect the number of decking boards you will need. Don’t forget to add 5% to 10% for wastage if your decking design is complex or includes diagonal or curved shapes and multiple cuts. You will also need to know the size of the composite decking boards you plan to use.
You can use a calculator to calculate the number of decking boards you need. The first step to take if you want to calculate the number of composite or wood decking boards for making your deck is to determine your deck size. How large do you want your deck to be? Do you want it small or big? Estimate the dimensions of your deck to know its size.

Use a tape measure to discover the length and width of your deck. Once you know the length and its width, multiply both together to determine the deck area, please note when you use the tape measure calculate in metres and then x the length and width together and this will provide your square metre area.

Decking Board Length

Next, you also need to know the lengths of the decking board you want to use. Most wood and composite decking boards have different board lengths that range from 2.4m, 3m, 3.6m & 4m. The widths of composite decking boards range from 120mm up to 160mm and wider. Once you get the board length you want, choose your joist spacing. Most domestic joist spacings are 30cm (300mm) to 40cm (400mm) in the centre, depending on whether your decking is for residential or commercial use. For commercial use, place joists centres at 20cm (200mm) to accommodate for the additional footfall and weight. This should be checked with the composite decking supplier.

You should always check with your local authority for any building regulations to determine adequate spacing required for your decking joists. This is mostly on commercial projects as there maybe requirements depending on your project

Calculating The Boards By There Sq mm & Sqm Area

Steps to Calculate the Number of Decking

You can calculate the number of decking boards you need for your deck project in three simple steps. You will need a jotter, pen and a calculator.

Step 1 – Estimate the Deck Area

First measure your deck area then, multiply the length and width in metres to calculate the total size of the decking area in square metres. For example: A total deck area of 5m x 4m = 20sqm, Your decking area your looking to install on, length being 5m and width being 4m is 20 sqm.

Step 2 – Calculate the Board Area

Knowing the total area of your deck in this case 20sqm, proceed to find the total area of the decking board you want to use. You have to multiply the boards length together with its width in mm.

But most decking suppliers give the length in metre and the width in millimetres for example our boards are given 3.6m x 140mm x 25mm, most suppliers display the dimension exactly same way. So in this case, you have the width in mm which is great and now you just need to convert the length of the board to mm .

To do this, multiply the length of the board by 1000; for example, if you have a 3.6m board multiply by 1000 = 3600mm. Now all are in mm for example in this case 3600mm x 140mm. You then multiply the length by the width of the board, which in our example is 504,000 sq mm, please note if you come across 504,850 always round up, so our example 505,000 sq mm.

Step 3 – Calculate the Boards

Now you have you sqm area of your decking project, the area you measure in length and width in metres which your installing, our example 20sqm above. But note you have your boards currently in sq mm, in our example 504,000 sq mm, so these need to be converted to sqm first.

To do this you simple divide by 1 million, which will give you the board’s sqm value which in this instance is 0.504sqm. Always round up, even if you do not have a single number.

You can now calculate how many decking boards you will need since you have your deck and board sqm area. To find the number of decking boards you need, divide the total deck area sqm (our example 20sqm) by the board sqm area (our example 0.504sqm). Your result is the number of decking boards you will need to complete your decking project.

To do this based our example, you can calculate by taking the sqm deck area of 20sqm and the board’s sqm value of 0.504 and then dividing.

20/0.504 = 39.68, always round up providing 40 boards; please note as the clips space and dimension aren’t included this will amount for a minor amount of wastage, but it’s always advisable to add wastage of 5 % on standard decking designs and 10 to 15% with designs requiring unique shapes and cuts. In addition to that, it is also good to purchase extra fasteners for your decking.

Calculating The Boards By The Width

The next method of calculating how many decking boards you will require is using the width of the boards. Again convert width to mm if not already provided. With this example, we will use home choice premier again at 140mm width.

Step 1 – Convert The Length To Millimetres

Convert the metres to millimetres by multiplying by 1000. With this method, you need to convert m (metres) to mm (millimetres) where the boards are running the width of the decking; For example, using again our 5m x 4m area, if your boards are running parallel to the 5m area, the boards will run the width of the 4m, so covert this to mm which x 1000 = 4000mm

Step 2 – Don’t Forget The Clip

Now you are almost ready, don’t forget the clips; if you are using 3mm decking clips, add 3mm to each 140mm board width.

Step 3 – Calculate The Amount Of Boards

Now your ready using our run in width 4000mm divided by 143mm = 27.97 always round up = 28

Now covert to linear metres. 28 x the length 5m = 140 linear metre then / (divide by 3.6 boards length) = 38.888 =39 boards

These are the two ways of calculating. In our experience, the first option is always the best unless you have an exact width, like 4m or 5m, etc.


If you want to build a deck, it is good to calculate how many decking boards you will need. Follow the simple steps in this article to calculate the number of decking boards needed for your next project.

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