You want to build a deck approx 8 x 17ft how many deckboards will you need size of each board will be either 28 x 140mm x 2.4m or 28 x 140mm x 3.6m?
The answer to your question depends on what type and size of
deckboards you will be using. For a general formula to help you
figure out how much material you will need do this: Figure the
square footage of your project. In your case, 8X17 = 136 SqFt. Then
consider the material you will be using to cover your deck.
Calculate the sqft of each piece and divide the total project size
by each peice. For example if one peice of your material will cover
1.5 sqft. Then do this math: 136 / 1.5 = 91 pieces of material I
would factor in at least 10% more for error or incidental. So in
the above example, you would want to purchase 100 pieces of
material.