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Decking calculator

Enter the size of the deck and get the boards, joists, beams, posts, footings, steps, railing and fixings to buy — drawn in 3D so you can see what the numbers describe. Free, no sign-up, and nothing is priced until you add your own rates.

Example numbers, until you enter your size

Decking boards

50 at 3.6 m

Rows packed into stock lengths, 5% waste included

Joists

14 at 400 mm centres

Spacing from UK timber decking practice (BS 8579)

Posts and footings

16 and 16

Posts at most 1.8 m apart

Deck area

17.8 sq m

Finished deck, measured to the outside of the frame

The board count already includes 5% for cuts and offcuts, and never falls below the number the cut list needs once the rows are packed into the lengths a yard sells. Every rule behind these numbers is written out on the methodology page.

How the quantities are worked out

1. 1. Measure the area

   Take the width and the length of the finished deck, not the garden. Work to the outside face of the frame; the calculator adds the board gap for you.

2. 2. Pick the board and the direction

   Board width and the gap between boards change how many boards you buy. Composite and softwood come in different widths, so the count changes with the material.

3. 3. Let the frame follow the span

   Joist and beam spacing follow the building practice of your country. Composite boards usually need closer joists than softwood, which is where most hand-made lists go wrong.

4. 4. Buy in stock lengths

   Boards are sold in fixed lengths. The cut list packs your runs into stock lengths and shows the offcuts, so you order what a merchant actually sells.

Which board length wastes least

Same deck, same rows, different stock lengths. Take a row that runs 4.8 m: one 4.8 m board covers it with nothing left over, while 3.6 m needs a joint and throws a third of the second board away.

UK and Irish yards sell 3.0 m, 3.6 m, 4.2 m and 4.8 m. In the US and Canada the same deck is packed into 8, 10, 12 and 16 ft. Pick the country and the cut list follows it.

Board estimator, joist calculator, decking planner

People call this a few different things. It is one job, so it is one calculator, and the answer changes with what you type rather than with what you called it.

Decking board calculator

Board count for the width you typed, at the board width and gap you pick, packed into stock lengths.

Deck board estimator

Same count, plus the waste allowance and the offcuts, so the estimate survives contact with the merchant list.

Composite decking calculator

Composite boards are usually narrower and need closer joists. Switch the material and the frame changes with it.

Decking cost calculator

Enter your own supplier rates and the same take-off turns into money. No averages, no made-up prices.

Decking planner

The full design: frame, posts, footings, steps, railing and fixings in 3D, with a free summary and paid order-ready files.

Deck joist calculator

Joists, beams and spacing from the practice of the country you pick, with the rule printed beside each number.

What about the cost of the decking?

A merchant calculator gives you boards priced from one stock list. This one gives you the whole take-off and leaves the money to you: put in the rate you actually pay per board, per joist, per bag of concrete, and the total is yours, not an average from somewhere else. Most online decking costs come in low because they quietly leave out the frame, the footings and the fixings.

Using a retailer calculator? See what a store decking calculator leaves out.

Common questions