Concrete needed for a 3 × 4 m slab, by thickness
These figures are for the whole 12 m² slab. The ready-mix column adds 10% for waste and rounds up to the next quarter m³, which is how most suppliers sell it. The cement bags are for mixing it yourself at 1:2:4.
| Thickness | m³ (no waste) | m³ with 10% waste | Ready-mix to order (m³) | Weight (tonnes) | Bags of cement (25 kg) |
|---|---|---|---|---|---|
| 75 mm | 0.90 | 0.99 | 1.00 | 2.4 | 13 |
| 100 mm | 1.20 | 1.32 | 1.50 | 3.2 | 17 |
| 125 mm | 1.50 | 1.65 | 1.75 | 4.0 | 21 |
| 150 mm | 1.80 | 1.98 | 2.00 | 4.8 | 26 |
What a 3 × 4 m slab is used for
A 3 × 4 m slab is a roomy shed or workshop base, or a patio with a table and four chairs. 100 mm for a patio, and 125 to 150 mm for a workshop with heavy tools.
Bags or ready-mix for 3 × 4 m?
At 1.32 m³ you are on the border. Mixing means about 17 bags of 25 kg cement plus 0.93 tonnes of sharp sand and 1.74 tonnes of aggregate, all placed before it starts to set. Many people hire a mixer and a few helpers, but get a ready-mix quote too and compare. Ask about short-load charges.
Don't forget the sub-base and the reinforcement
A slab needs a compacted sub-base, usually 100 mm of MOT Type 1, which for 12 m² is about 2.8 tonnes with 5% for compaction (see the gravel calculator). Vehicles need mesh such as A142 and movement joints. For a patio on top of the slab, the paving calculator gives the slabs and sand. Check for buried services before you dig.
Want to change anything else? The calculator above is the full Concrete Calculator (UK, m³): edit any number and the results update instantly.
Tools & materials for this project
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Frequently asked questions
How much concrete do I need for a 3m x 4m slab?
A 3 × 4 m slab is 12 m². At 100 mm thick that is 1.20 m³, or 1.32 m³ with 10% extra, so order 1.50 m³ of ready-mix. At 150 mm it is 1.80 m³, or 1.98 m³ with 10% extra.
How many bags of cement for a 3m x 4m slab?
For a 1:2:4 mix at 100 mm with 10% extra you need about 17 bags of 25 kg cement, 0.93 tonnes of sharp sand and 1.74 tonnes of 20 mm aggregate. At 150 mm it is 26 bags.
How much does a 3m x 4m concrete slab weigh?
At 100 mm thick it weighs about 2.9 tonnes, using 2.4 tonnes per m³. At 150 mm it is about 4.3 tonnes.
Related sizes and guides
Related sizes
Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.