Building materials calculator
Bricks and blocks for a wall, plasterboard and skim for a room, ballast and cement for a slab. Each one draws what you've described and lists what to put on the order.
These are estimates, not a bill of quantities
Every figure comes from standard sizes and trade rules of thumb, listed under each calculator and explained at the bottom of the page. Before you order:
- Check the assumptions match what you are buying: brick and block sizes, bag weights and bulk-bag weights vary by supplier.
- Round up to whole packs, bags and pallets. The calculator rounds bags up but not packs.
- Waste is an allowance, not a measurement. Cut-heavy work (piers, corners, alcoves) needs more than the default.
- For anything structural, get the specification from the engineer or supplier. This page does not check strength, loading or compliance.
Bricks for a wall
- Standard UK brick 215 × 102.5 × 65 mm laid with 10 mm joints: 60 per m² single skin (half brick), 120 per m² one brick thick.
- Mortar 0.025 m³ per m² single skin, 0.055 m³ per m² one brick thick, including frogs and normal waste.
- Mortar mixed 1:5 cement to building sand: 1.5 t of sand and 14 × 25 kg cement per m³ of mortar. A 1:4 mix needs about 20% more cement.
- A bulk bag is taken as 800 kg; suppliers fill by volume and 750–900 kg is normal.
- Openings are deducted at their full size; no allowance for lintels, piers, returns or soldier courses.
Blocks for a wall
- Standard block face 440 × 215 mm laid with 10 mm joints: 10 per m² whatever the thickness.
- Mortar 0.012 m³ per m² at 100 mm thick, 0.017 at 140 mm, 0.026 at 215 mm, including normal waste. Hollow or cellular blocks use slightly less.
- Mortar mixed 1:5: 1.5 t sand and 14 × 25 kg cement per m³. Bulk bag taken as 800 kg.
- Openings deducted at full size; no allowance for lintels, piers or cutting around services.
- Blocks are usually sold by the pallet (72 to 100 depending on thickness); round up to whole pallets or check the split-pack price.
Plasterboard and skim for a room
- Board areas: 1200 × 2400 mm = 2.88 m², 900 × 1800 mm = 1.62 m². Boards are counted whole after dividing the area, then waste is added.
- Deducts 1.9 m² per standard door and 1.4 m² per window. Large windows, patio doors and alcoves change this; adjust the room size or waste.
- Drywall screws at roughly 300 mm centres on 600 mm studs: 11 per m². Joint tape 1.3 m per m²; one roll is 90 m.
- Joint filler 25 kg per 20 m² (taping and two coats). Multi-finish plaster 25 kg per 10 m² at a 2 mm skim; a thicker coat uses proportionally more.
- Waste default 10%; rooms with many cuts, sloping ceilings or bulkheads want 15%.
Concrete for a slab or base
- General-purpose mix of 1 part cement to 5 parts all-in ballast by volume, roughly C20 (paths, shed bases, post holes, garden slabs).
- Per m³ of placed concrete: 14 × 25 kg cement and 1.8 t all-in ballast. Bulk bag taken as 800 kg.
- Ready-mix bags: one 25 kg bag makes about 0.011 m³, so around 90 per m³.
- Finished concrete weighs about 2.4 t per m³, which matters for a floor or a trailer.
- Waste default 10% covers an uneven sub-base and spillage. For anything structural or over about 2 m³, order ready-mixed by strength class.
Where the numbers come from
- Brick and block counts are geometry: a standard brick with its joints occupies 225 × 75 mm, a block 450 × 225 mm.
- Mortar volumes are the joint volume plus the usual allowance for frogs, waste and dropped muck. They match the trade rule of roughly half a cubic metre per thousand bricks.
- Sand and cement per cubic metre of mortar allow for the bulking of damp sand; wet mortar is less volume than the dry parts added together.
- Plasterboard waste depends on how well the board sizes fit the walls: 10% suits most rooms, a room full of alcoves wants 15%.
- Round everything up to whole bags and whole packs when ordering. A pack of bricks is usually 400 to 500; blocks are palletised by 72 to 100 depending on thickness.
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Common questions
60 standard bricks (215 × 102.5 × 65 mm) per square metre of half-brick (single skin) wall with 10 mm joints, because each brick plus its joints takes up 225 × 75 mm. A one-brick-thick wall needs 120. Add 5–10% for cuts and breakages.
10 standard blocks (440 × 215 mm face) per square metre with 10 mm joints, whatever the thickness: a 100 mm and a 140 mm block cover the same face area but use different amounts of mortar.
About 0.5 m³ of mortar, which at a 1:5 mix is roughly three quarters of a tonne of building sand (one bulk bag) and 7 × 25 kg bags of cement. The old rule of thumb of 'a tonne of sand and 8 bags' per thousand bricks is the same figure with waste allowed.
Add up the wall area (perimeter × height), take off doors and windows, add the ceiling if you are boarding it, then divide by the board area: 2.88 m² for a 1200 × 2400 board, 1.62 m² for 900 × 1800. Add 10% for cuts. A 4 × 3 m room with 2.4 m ceilings, one door and one window, walls and ceiling, needs about 16 full-size boards.
Length × width × depth in metres gives cubic metres. A 3 × 2 m shed base 100 mm deep is 0.6 m³. Per cubic metre allow about 1.8 tonnes of all-in ballast (two to three bulk bags) and 14 × 25 kg bags of cement at 1:5, or around 90 × 25 kg bags of ready-mix. Finished concrete weighs about 2.4 tonnes per m³.
A woven one-tonne sack of sand, ballast or gravel, filled by volume rather than weight. Most hold 750–900 kg; this page assumes 800 kg. Two bulk bags are usually cheaper than the same weight in 25 kg bags once you need more than about 20 small bags.