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Concrete for a 30x40 Slab
A 30 × 40 ft slab at 4 inches thick needs about 16.3 cubic yards of concrete including 10% waste. Change the thickness or bag size below for your own job.
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Estimate only. Bag yields, material density and waste vary by product and supplier. Confirm quantities with your supplier and check local building codes before you buy or build.
How much concrete for a 30x40 slab?
A 30 × 40 ft slab covers 1200 square feet. At 4 inches thick that is 400 cubic feet, or 14.81 cubic yards before waste. Add 10% for spillage and uneven ground and order about 16.3 cubic yards (12.46 m³).
Bags of concrete for a 30x40 slab
With 10% waste, a 4 inch 30x40 slab takes 734 bags of 80 lb mix, 978 bags of 60 lb or 1467 bags of 40 lb. Above about one cubic yard, ready-mix delivery is usually cheaper and much faster than mixing bags by hand.
30x40 slab at 5 or 6 inches thick
Driveways, garage floors and slabs for heavy loads are usually 5 to 6 inches. At 6 inches the same 30x40 slab needs 24.44 cubic yards with waste, which weighs about 49.5 US tons wet. Check local building codes for your use.
Don't forget the base
Most slabs sit on 4 inches of compacted gravel. For 30x40 that is about 28.7 US tons of crushed base with waste and compaction. Use the gravel calculator to change the depth or material.
Frequently asked questions
How many yards of concrete for a 30x40 slab?
14.81 cubic yards at 4 inches thick before waste, about 16.3 with 10% extra. At 6 inches it is about 24.44 with waste.
How many 80 lb bags of concrete for a 30x40 slab?
About 734 bags at 4 inches thick, including 10% waste. Each 80 lb bag yields about 0.6 cubic feet.
How much does a 30x40 concrete slab weigh?
At 4 inches thick with waste, about 66,000 lb (33 US tons) of wet concrete, at 150 lb per cubic foot.
How accurate is this estimate?
It is a planning estimate, not an order. Bag yields, material density and waste vary by product, supplier and job, so confirm quantities with your supplier and check local building codes before you buy or pour.
Are my measurements saved?
No. Everything is calculated in your browser. Nothing is uploaded or stored.
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Common sizes: 10x10 slab, 12x12 slab, 10x20 slab, 12x20 slab, 16x20 slab, 20x20 slab, 20x30 slab, 24x24 slab, 30x30 slab.