Cubic metres, bags, reinforcement, and cost for slabs, footings, columns, and stairs. Reviewed by a chartered civil engineer.
Estimator · v3.2 · UK
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Order this much— m³
Litres— L
Cubic yards— cu yd
25 kg bags—
20 kg bags—
Steel (12mm @ 200)— m
Cost—
— How the calculator works
From dimensions to a ready-mix order.
Volume is length × width × thickness, returned in cubic metres. For round columns, the tool uses π × radius² × height. For stairs, each step is treated as a separate rectangular block and added together — the correct method for in-situ poured stairs.
A waste allowance of 5% to 10% is added to the raw volume. Eight percent suits most domestic flatwork — driveways, garage slabs, patios. Bump to ten percent for hand-dug footings or rough ground. Ready-mix lorries deliver to the nearest 0.25 m³, so the figure is rounded up to the next quarter cubic metre.
Bag estimates use the printed yields on UK pre-mixed concrete: a 20 kg bag gives about 0.010 m³, a 25 kg bag about 0.0125 m³. Reinforcement assumes 12 mm bar at 200 mm centres each way — typical for a domestic ground-bearing slab to BS 8500.
— Concrete mix selection
Choosing the right grade.
In the UK, concrete is specified by strength class (C25/30 means 25 N/mm² cylinder strength, 30 N/mm² cube strength at 28 days) or by designated mix code (ST2, GEN1, RC25/30, etc.) under BS 8500-1:2023. Designated mixes are the easier route for small jobs — your supplier knows what's in them.
UK designated concrete mixes (BS 8500)
Designation
Strength Class
Typical Use
Exposure
ST1
C6/8 — C8/10
Kerb bedding, drainage haunching
X0
ST2 / GEN1
C12/15
Mass fill, blinding, oversite
X0
GEN2
C16/20
House foundations, mass concrete
XC1, XC2
GEN3 / RC25/30
C20/25 — C25/30
Reinforced slabs, footings, garage floors
XC2, XC3
RC32/40
C32/40
Driveways, exposed reinforced concrete
XF1, XF3
RC40/50
C40/50
Heavy duty, freeze-thaw with de-icing salts
XF4
Exposure classes drive the durability requirements — minimum cement content, maximum water/cement ratio, and minimum cover. XC covers carbonation, XF covers freeze-thaw, XD and XS handle chlorides. Your designer assigns the class; your supplier produces a mix that satisfies it.
— Slab thickness
How thick should the slab be?
Slab thickness depends on the loads and the ground beneath. These are the typical figures for domestic ground-bearing slabs in the UK, aligning with NHBC Standards and Eurocode 2 design guidance.
100mmPatios, paths, garden slabs. C20/25 mix, A142 mesh or fibres.
125mmDomestic driveways, light car parking. C25/30, A193 mesh.
200mmCommercial and agricultural slabs. Designed reinforcement schedule required.
Foundation depth is governed by ground conditions and proximity to trees. For domestic strip footings on shrinkable clay, the NHBC trench depth tables typically require 900–2,500 mm depending on tree type and distance. Always check with Building Control before excavating.
— Pouring & curing
Placing concrete properly.
Concrete reaches its design strength at 28 days through hydration, not by drying out. The surface must stay damp during early curing. Cover slabs with polythene sheeting or hessian for at least three days; seven for heavily loaded work. Foot traffic at 24 hours, light vehicles at seven days, full strength at 28.
For cold-weather placement, the rule of thumb is keep concrete temperature above 5°C until it reaches around 5 N/mm². Options when air temperatures dip below freezing:
Heated water in the mix (raises batch temperature 5–10°C)
Accelerator admixture (chloride-free for reinforced work)
Insulating blankets or quilts for 24–72 hours
Avoid pouring on frozen ground — strip the topsoil and warm the base
For workability, UK concrete is classified by consistence class per BS EN 206: S1 (10–40 mm slump) is stiff, S3 (100–150 mm) is the typical residential pour, S4 (160–210 mm) is pumpable. Higher classes are easier to place but should be achieved with plasticisers rather than extra water — adding water cuts strength.
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Reviewed by Olivia Bennett, CEng MICEChartered civil engineer · 12 years across UK residential and commercial concrete. Calculator logic and content checked against BS 8500-1:2023, BS EN 206:2013+A2:2021, and Eurocode 2. Last reviewed May 2026.
— Common questions
Quick answers.
How many bags of concrete in a cubic metre?
About 100 bags at 20 kg, or 80 bags at 25 kg. A 20 kg bag of pre-mixed concrete yields roughly 0.010 m³ once mixed. Above 0.5 m³, ready-mix delivery is usually cheaper than bagged.
How much concrete for a 3 by 3 metre slab?
0.90 m³ at 100 mm thick. With 8% waste, order 1.0 m³ — ready-mix lorries deliver in 0.25 m³ increments.
What grade of concrete for a UK driveway?
C25/30 (designated RC25/30) is the typical specification for residential driveways. Upgrade to C32/40 for heavier vehicles or exposure to de-icing salts (class XF3).
What is the minimum cover for reinforcement?
BS 8500-1 Table A.5 gives the cover requirements. Most domestic foundations to class XC2 use 40 mm nominal cover (35 mm minimum plus 5 mm tolerance). Slabs exposed to weather (XC3, XC4) need 35–50 mm.
Bags or ready-mix?
Under 0.5 m³, mixing your own from 25 kg bags is cheaper. Above that, ready-mix wins on both price and labour. Most UK suppliers charge a part-load surcharge below 3 m³.
Do I need a Building Regulations approval?
Yes for most structural concrete — foundations, retaining walls, suspended slabs. Approved Document A covers structure. Garden patios and paths usually fall under Permitted Development. Always check with your local authority.