Decking Calculator

Decking is bought by the board, so the useful question is not only how big the deck is, but how many boards go on the van, how many fixings they need and what the pair will cost. This calculator turns a deck size and a board size into exactly those numbers, and it adds the cutting allowance for you instead of leaving you to guess at it.

It works from either end. If a plan quotes the surface area, enter the area and the missing side comes back; if you only have a tape, enter the two sides and the area follows. Lengths can be given in centimetres, metres, inches or feet, board widths also in millimetres, and the money in any of twenty currencies.

The people who use it most are the ones buying the wearing surface: a householder pricing a first deck, a fitter checking a quantity before quoting, or anyone deciding whether a narrow board or a wide one makes a cheaper deck.

Five decisions carry the whole calculation: the deck size, the board width, the stock length, the fixing and the prices. Everything else on the page is a result.

If your result looks too high or too low, check the two things that are easiest to mix up: the board length is the stock you buy rather than the run of deck, and the board width is the face of the board rather than the gap between joists.

The deck surface is the two sides multiplied. Both are converted to metres before they are multiplied, so a deck taped in feet and a board measured in millimetres can be used in the same calculation without converting anything by hand.

Because it is one statement, an area you already know can drive the sides instead:

One board covers its width times its second side. With flat boards the second side is the stock length; with square sections it is the width again.

Dividing the deck by that footprint gives the number of boards, and the ten per cent allowance plus the round-up finish the job:

The rounding is not decoration. A deck whose arithmetic lands exactly on a whole board needs no extra piece; one that lands on 71.5 needs 72, because that half board is the allowance doing its job rather than a rounding artefact.

Fixings are counted from the deck area in square feet, not from the board count. That is why changing the board length or width leaves the fixing count alone:

Costs are two multiplications and one addition — the price of a board times the count, plus whatever the fastener pack costs:

With square sections no fastener estimate is made, so the total is the board spend on its own:

Each cost box carries its own currency, converted with a reference exchange rate, so the boards can be priced in one currency and the fastener box in another. The rate is applied to the finished amount, never to the order of the arithmetic.

The board and fixing counts are results only. A rounded count cannot honestly be turned back into an exact deck area or stock length, so the calculator does not offer that direction — the price and total are readouts in the same way.

Two choices move the shopping list far more than the deck does: how wide the board is and how long a piece you buy.

Width changes the joints, and the price

A wider board covers more deck per piece, so you buy fewer boards and have fewer joints to keep straight. Boards are usually priced up with their width, so compare the price per square metre of decking rather than the price on the ticket — the cheaper board can be the dearer deck.

Stock length changes the piece count

Longer stock covers more per board, so the count falls almost in proportion. A 20 m² deck in 140 mm boards takes 66 pieces of 2.4 m, or 33 pieces of 4.8 m — the same cover, half the pieces. Whether that saves money depends on how the merchant prices the longer board, which is why the price-per-square-metre comparison above is the one to make.

What the ten per cent is for

It is one allowance for the offcuts at the ends of runs, mitred corners, and the occasional board that splits or is delivered damaged. A simple run of boards in one direction, ending near a multiple of the stock length, can need less. A diagonal or herringbone pattern cuts far more boards at an angle and usually needs more.

The calculator cannot see your layout, so treat the count as the number to order, and add a board or two when the pattern is complicated or the deck is large.

Why square sections multiply the count

A square section is counted on width × width. The same 140 mm that gives a flat board 0.336 m² of cover — 140 mm by 2.4 m — gives a square piece only 0.0196 m². The deck has not changed, but each piece covers about a seventeenth as much of it, so the count runs into the thousands.

Boards are spaced, the count is flush

The count assumes boards laid edge to edge. Real decking is spaced for drainage and for the movement of the boards, so each board covers a little less than its full width allows and the result behaves as a minimum. On a normal deck the ten per cent allowance absorbs it; on a very wide surface it is one more reason to order a spare.

The first example uses the square-section mode; the second shows what to do with a deck that is not a plain rectangle.

It is also worth keeping one board back after the job. Colour and grain vary between production batches, and a spare from the same batch is the only way to repair a damaged board later without it standing out.