How much to order

Put in your sizes and get the quantity in the units merchants sell: cubic metres, bags, bulk bags, tonnes, sheets, panels and boards. Every factor comes from a maker's or trade body's own page, numbered and listed at the end.

These are order quantities, not a design. Depths, mixes, post sizes and joist spans come from your drawings or whoever designed the job. Where makers publish different figures we show both. Check the final order with the merchant.

Concrete: ready-mix, bags or site-mixed

Figures used

Ready-mix is sold by the cubic metre¹. One maker recommends adding 5 to 10% to the calculated volume²; another says to allow "a small contingency" without giving a figure³.
Ready-mix trucks: the same maker delivers in six and eight cubic metre trucks and charges for part loads when they can't be avoided¹. Ask the plant about its own minimum and part-load charge.
25 kg bag (Hanson MaxiPack25, 40N): about 0.01 m³ of concrete per bag⁴.
20 kg bag (Hanson MaxiPack20, 40N): about 0.008 m³ of concrete per bag⁴.
20mm ballast with cement: one bulk bag makes concrete for about 12.6 m² at 50 mm, and one small pack about 0.35 m² at 50 mm⁵. We turn that into volume: 12.6 × 0.05 = 0.63 m³ a bulk bag.
Mix ratios differ between the maker's pages: 1 part cement to 6 parts 20mm ballast, for coarse concrete⁶; 1 part cement to 5 parts ballast, for a shed base⁷. We don't count cement bags: no maker page we could read gives bags of cement per cubic metre.
Weight: normal concrete is between 2,000 and 2,600 kg a cubic metre⁸. The same maker's FAQ says about 2.4 tonnes⁹, and another maker says around 2.5 tons¹. We show the full range.

Sand, gravel and sub-base: tonnes and bulk bags

Figures used

Tonnes: one maker says to multiply the volume by about 1.5 to 1.7 tonnes a cubic metre, "but it's best practice to ask your supplier"¹⁰. It doesn't split that by material, so we use the same range for each.
Pea gravel, as a top dressing: one bulk bag covers approx 14.4 m² at 50 mm⁶.
20mm gravel, as a top dressing: one bulk bag covers 12 m² at 50 mm⁶.
10mm gravel, as a top dressing: one bulk bag covers 12 m² at 50 mm⁶.
Silver sand: one bulk bag covers 10 m² at 50 mm¹¹.
14mm dust limestone chippings, compacted as blinding: one bulk bag covers 0.5 m³ compacted⁶.
MOT Type 1: the maker's sheet lists it as bulk only and gives no coverage per bag⁶, so we give tonnes and volume, not bags.
Coverage figures are for the maker's own bags. Bag sizes differ between makers and merchants, so check the bag you're buying.

Bricks and blocks per wall

Figures used

Brick: 215 x 102.5 x 65 mm, laid with a nominal 10 mm joint¹². With the joint each brick takes 225 x 75 mm of wall face, so 1 ÷ (0.225 × 0.075) = 59.3 bricks a square metre for one skin.
Block: face 440 x 215 mm¹³, or 450 x 225 mm nominal with the joint¹⁴, so 1 ÷ (0.45 × 0.225) = 9.88 blocks a square metre.
Mortar: one tonne of cement, lime and sand mortar lays about 1,000 bricks or 600 blocks¹⁴; the brick trade body gives the same 1,000 bricks a tonne¹⁵.
Bagged mortar: one maker's page says a 25 kg bag lays 19 bricks in one place and 25 bricks in another¹⁶. We show both ends.
No maker or trade body page we read gives a waste allowance for bricks or blocks, so the extra percentage is yours to choose.

Plasterboard sheets

Figures used

Sheet sizes as one maker sells its standard 12.5 mm board: 1200 x 2400, 900 x 1800 and 1200 x 3000 mm. Its 15 mm board is listed at 1200 x 2400¹⁷.
Area per sheet is length × width. Count a wall with a door as the full wall less the door, then add for cuts: no maker page we read gives a waste figure.

Fence panels, posts and gravel boards

Figures used

Panels: 1.83 m (6ft) wide is the standard width, in heights of 0.91, 1.22, 1.52 and 1.83 m¹⁸. We count posts between panels, so each panel takes 1.83 m plus one post's width.
Posts: 100 x 100 mm is one maker's standard slotted post¹⁸; another sells 75 x 75 mm posts²⁰.
Post length: one maker says to add at least 600 mm to the height of the fence, and sells posts at 1.5, 1.8, 2.1, 2.4 and 2.7 m¹⁹. Another sells an 8ft (2.4 m) post for 6ft panels, "allowing 2ft for setting in ground"²⁰. For a 6ft panel the first rule gives 2.43 m, so the two makers' advice differs; the tool follows the 600 mm rule.
Gravel boards: timber gravel boards are 140 mm high¹⁸, and add to the fence height.
Post fixing: "in general you will need at least one bag of Postcrete per post"²¹. The real amount depends on the hole.

Deck boards and joists

Figures used

Boards are supplied from 1.8 m to 6.0 m long²². Leave a gap of no less than 5 mm and no more than 8 mm between boards²², so we show the board count for both gaps.
Joist spacing: the decking trade body's code of practice gives the maximum support centres for each board size and strength class²³. It gives no softwood figure for 22 mm boards, so the tool gives none.
Joists run across the boards, one at each end and the rest at no more than those centres. Joist size and span depend on the deck's height and structure: see the code of practice, and for a raised deck the person who designed it.
Treatment: deck boards to at least Use Class 3, and joists, beams and posts to Use Class 4 whether or not they touch the ground²⁴.

Maximum support centres in mm for softwood boards, from Table 5 of the code of practice²³. A dash means not given.
Board	C16	C22	C24
95 x 22 mm	-	-	-
95 x 34 mm	450	500	550
120 x 22 mm	-	-	-
120 x 27 mm	400	425	450
120 x 34 mm	500	550	600
145 x 22 mm	-	-	-
145 x 27 mm	400	425	450
145 x 34 mm	500	550	600

Sources

Ready-mixed concrete pricing guide Heidelberg Materials UK (heidelbergmaterials.co.uk). Checked 14/10/2026.
Ready-mix concrete volume calculator Heidelberg Materials UK (heidelbergmaterials.co.uk). Checked 14/10/2026.
Concrete calculator Cemex UK (cemex.co.uk). Checked 14/10/2026.
40N concrete: technical data sheet (MaxiPack25 and MaxiPack20) Heidelberg Materials UK (Hanson) (heidelbergmaterials.co.uk). Checked 14/10/2026.
20mm ballast: technical data sheet Heidelberg Materials UK (Hanson) (packedproducts.heidelbergmaterials.co.uk). Checked 14/10/2026.
Construction aggregates: technical data sheets (combined) Heidelberg Materials UK (Hanson) (heidelbergmaterials.co.uk). Checked 14/10/2026.
How to lay a shed base Heidelberg Materials UK (heidelbergmaterials.co.uk). Checked 14/10/2026.
How much does a m3 of concrete weigh? Cemex UK (cemex.co.uk). Checked 14/10/2026.
Ready-mix concrete FAQs Cemex UK (cemex.co.uk). Checked 14/10/2026.
Aggregates: how much aggregate do I need? Heidelberg Materials UK (heidelbergmaterials.co.uk). Checked 14/10/2026.
Silver sand: technical data sheet Heidelberg Materials UK (Hanson) (packedproducts.heidelbergmaterials.co.uk). Checked 14/10/2026.
Does brick size matter? Wienerberger UK (wienerberger.co.uk). Checked 14/10/2026.
Thermalite aircrete Hi-Strength 7: product page Forterra (forterra.co.uk). Checked 14/10/2026.
Mortar data sheet 01 Mineral Products Association (MPA Mortar) (mortar.org.uk). Checked 14/10/2026.
Mortar for brickwork: technical guide 2023 Brick Development Association (brick.org.uk). Checked 14/10/2026.
General purpose mortar (Rugby bagged) Cemex UK (cemex.co.uk). Checked 14/10/2026.
Plasterboard portfolio leaflet Siniat (media.siniat.co.uk). Checked 14/10/2026.
Fence panel heights and widths Jacksons Fencing (jacksons-fencing.co.uk). Checked 14/10/2026.
How deep to dig fence posts Jacksons Fencing (jacksons-fencing.co.uk). Checked 14/10/2026.
Green incised fence post 8ft (240 x 7.5 x 7.5cm) Forest Garden (forestgarden.co.uk). Checked 14/10/2026.
The benefits of using Postcrete Jacksons Fencing (jacksons-fencing.co.uk). Checked 14/10/2026.
Timber deck board options Timber Decking and Cladding Association (tdca.org.uk). Checked 14/10/2026.
Code of practice TDCA/RD0801E6: raised timber deck structures on new homes (May 2018) Timber Decking and Cladding Association (tdca.org.uk). Checked 14/10/2026.
Specification for timber decking Timber Decking and Cladding Association (tdca.org.uk). Checked 14/10/2026.