How to calculate decking: boards, joists, clips and cost

In short: enter the length and width of your deck in our decking calculator. It works out the number of Ipe deck boards (with extra for offcuts), joists at 40 cm centres, foundation posts, weed membrane and every clip and screw. A 5 x 4 metre deck came to about £2,740 including VAT on 20 October 2026.

Calculate your decking

Why working it out by hand rarely adds up

Square metres times price per square metre sounds easy. In reality you buy boards by the piece, joists in 3 metre lengths and posts by the row. Add offcuts, an 8 mm gap between boards and the direction you lay them, and most people end up ordering too much or slightly too little. The calculator follows the same rules we use when we put an order together.

Six steps to your decking materials

  1. Length and width. Length is the direction the boards run. Use metres with a decimal, for example 4.35.
  2. Fixing. Hidden with clips or visibly screwed. With hidden fixing the clip sits in the side groove of the board, so no screw heads show.
  3. Board and profile. Smooth planed, comfortable barefoot, or reeded. The board size is 21 x 140 mm, and the ends are tongue and groove.
  4. Joists. Ipe joists of 40 x 60 mm at 40 cm centres. Same timber as the boards, so they move together.
  5. Foundation and membrane. 40 x 40 mm posts every 40 to 50 cm, at least two thirds into firm ground, with weed membrane underneath.
  6. Edging and oil. Edge boards all round or none, plus optional oil: one 0.9 litre tin covers 5 m² if you treat all four sides.

Worked example: a 5 x 4 metre deck (20 m²)

This is what the calculator showed on 20 October 2026 for Ipe deck boards 21 x 140 mm with clips, joists, posts, weed membrane and screws, without edging or oil. UK prices follow the exchange rate, so treat the figures as a guide.

ItemQuantityAmount
Ipe deck boards 21 x 140 mm, 180 cm long (about £21.70 each)84about £1,822
Stainless steel clips and screws, black coated396 clips, 792 screwsincluded
Ipe joists 40 x 60 mm, 3 metres (about £41.30 each)19about £785
Foundation posts 40 x 40 mm, every 40 cm154in the calculation
Weed membrane, 2.15 m wide12about £52
Screws 5 x 80 mm (pack of 200) and countersink bit2 + 1about £83
Total including VATabout £2,740

The calculator lays 28 boards side by side over 14 rows of joists. It deliberately adds boards on top of the net area to cover offcuts.

Work out your own size

Tips for an accurate result

  • Measure in several places and use the largest figure. A few centimetres are easy to trim, a shortfall means a second delivery.
  • Not sure about board direction or the base? Save your calculation and send us a WhatsApp, and we will check it before you order.
  • More on the boards themselves on our Ipe decking page.

Save it and order later

"Save and send" emails you a link. Use it to reopen the calculation, change the size or order in one go. Everything is in stock, with delivery in the UK in 7 to 14 working days.

Decking calculator FAQ

How many deck boards do I need per square metre?

With 140 mm boards and an 8 mm gap you need just over 6.7 linear metres per m², plus offcuts. The calculator counts it per board for you.

How many joists do I need?

Joists go at 40 to 45 cm centres. For 5 x 4 metres the calculator comes to 14 rows and 19 joists of 3 metres.

What does Ipe decking cost per square metre?

In the 20 m² example the boards with clips and screws come to about £91 per m². With joists, membrane and screws it is about £137 per m². The calculator gives the exact price for your size.

Are clips and screws included?

Yes. With the clip fixing boards, all black coated stainless steel clips and screws are in the pack.

Does the calculator work on a phone?

Yes, although a bigger screen makes it easier to see the steps and the summary side by side.

Written by Tom Wegter (team leader IT) and Marc Westerhuis (team leader communications) at Hardwood Discount · last updated: 20 October 2026 · Facebook · Instagram · YouTube · Pinterest