How to Calculate How Many Decking Boards You Need

Decking & Cladding
product guide
How to Calculate How Many Decking Boards You Need

When constructing a deck in your garden, it’s vital to work out how many decking boards you’ll need before beginning the project. Decking boards can be expensive, so you don’t want to under- or overestimate. This is why calculating the number you’ll need as accurately as possible is so crucial, as you’ll save so much time and money in the long run.

Calculating Your Decking Area

Firstly, you need to work out the area of your decking in square metres. If your decking is square or rectangular, this is very simple. You just need to multiply the length and width of your planned decking area together. However, if your decking will be a more irregular shape, it’s easiest to break it up into a series of smaller rectangles, squares, or triangles to calculate the area as appropriate. Then you just need to add the area of all these together to get your total.

Working Out How Many Metres of Decking Board You’ll Need

For this next step, you’ll need to take the result of the first equation and divide that by the width of the decking boards you plan to use. To accommodate for the small gaps between each decking board, it’s a good idea to add around 5mm to the board width so you take this into consideration. Then, take this measurement and divide it once again by the length of the decking boards you’re going to purchase. You now know how many decking boards you’ll need. We’d recommend purchasing around 10% in excess, to ensure that every part of your new deck is covered efficiently.

An Example Calculation

Now let’s put what we’ve learned into practice with an equation. Imagine you’re planning to build a decking area that’s 7m x 5m. You’re using 110m decking boards with a length of 2m.

  • 7m x 5m = 35m²
  • Adding 5mm to 110mm gives you a board width of 115mm
  • 35m² ÷115mm = 304.35 linear metres. Round this to the nearest whole number
  • Then, add an additional 10% to this figure (304 x 1.1 to equal 334.4)
  • 4 ÷ 2 = 167.2
  • Round this to the nearest whole number = 167

Therefore, you will need 167 decking boards to cover a 7m x 5m area.

Decking Board Spacing Calculator

When calculating the number of boards, you need always remember to take into account their expansion gaps. These are spaces between the boards that allow them to expand and contract with changing temperatures without causing damage to neighbouring boards.

In most cases, this expansion gap will need to be 5 – 8mm. Once you have calculated how many boards you need for your area, you can time that number by 5 for a rough estimate of the total area of your expansion gaps. Taking this away from the total length of your deck may well save you a few boards that would have otherwise ended up in storage.



As you can see, there are a few different ways to calculate how many decking boards you’ll need to carry out your project. It’s up to you if you’d like to sit down and write out the equations for yourself, or even consult a professional. But we’d always recommend using our on-site calculators. They’re simple to use and will give you the full cost of your project along with the calculations, fixings and all.

Ready to buy? Take a look at our extensive decking range, which includes both natural timber decking and modern composite decking.

If you’re still unsure, get in touch with our award-winning customer service team. They’re on hand to help you with any questions you may have, via telephone on 01295 565 565 or the live chat on our website. With over 40 years’ experience in DIY and roofing, they’re sure to help you get your project underway as soon as possible.

Discover More