Work out how many cubic yards of concrete a slab or footing needs, with a waste allowance, a bag estimate and a 1:2:4 cement/sand/aggregate mix.
Enter the slab dimensions (thickness in the same units). Add a waste allowance – 5-10% is typical – for spillage and uneven ground. “How many” repeats identical pours.
Volume = length x width x thickness, plus your waste allowance. Bag and mix figures are estimates – bag yields and mix ratios vary by product and project, so always check the bag and confirm with your supplier.
How to Calculate Cubic Yards of Concrete for a Slab
Multiply length × width × thickness, all in feet, then divide by 27 because a cubic yard holds 27 cubic feet. A 10 ft × 10 ft slab 4 inches thick is 10 × 10 × 0.333 = 33.3 ft³, or 1.23 cubic yards. Add 5 to 10% for waste before you order.
- Convert the thickness to feet: 4 in ÷ 12 = 0.333 ft.
- Multiply: 10 × 10 × 0.333 = 33.3 cubic feet.
- Divide by 27: 33.3 ÷ 27 = 1.23 cubic yards.
- Add 10% waste: 1.23 × 1.1 ≈ 1.36 yd³, then round up when ordering.
| Slab size (4 in thick) | Cubic feet | Cubic yards | With 10% waste |
|---|---|---|---|
| 10 × 10 ft | 33.3 | 1.23 | 1.36 |
| 12 × 12 ft | 48 | 1.78 | 1.96 |
| 20 × 20 ft | 133.3 | 4.94 | 5.43 |
| 24 × 24 ft | 192 | 7.11 | 7.82 |
| 30 × 40 ft | 400 | 14.81 | 16.30 |
So the yards of concrete for a 12×12 slab come to about 1.78 before waste, and a 20×20 slab needs about 4.94. Thickness matters as much as area: the same 20×20 slab at 6 inches needs 7.41 yd³.
How the Volume Is Worked Out
Formula: Volume = Length × Width × Thickness × Quantity × (1 + Waste% ÷ 100). The wet volume (in m³) converts to cubic yards (× 1.307950619) and cubic feet (× 35.314667). Bags needed = wet volume ÷ 0.011 m³ per 25 kg/60 lb bag, rounded up. For a from-scratch mix, dry volume = wet volume × 1.54, then split 1:2:4 by volume across 7 parts for cement, sand and aggregate, with cement bags = cement volume ÷ 0.0347 m³ per 50 kg bag, rounded up.
- Length: the long side of the slab, footing or pad, in metres or feet
- Width: the short side of the same pour, in the same unit as length
- Thickness: the depth of the pour, entered in the same unit (e.g. 0.1 m for a 10 cm slab)
- How many: the quantity field, for repeating identical pours (e.g. several fence-post footings)
- Waste allowance: the extra percentage added for spillage and uneven ground, typically 5 to 10%
How to Calculate Cubic Yards of Concrete for Footings
Enter one footing’s length, width and depth (in the thickness box), then set How many to the number of identical footings. For example, eight 2 ft × 2 ft × 1 ft pads come to 32 cubic feet, or about 1.19 cubic yards, before waste. Add 5 to 10% for spillage and uneven trenches, and round up when ordering ready-mix or bags.
Example Scenarios
| Length (m) | Width (m) | Thickness (m) | Waste | Concrete needed |
|---|---|---|---|---|
| 5 | 4 | 0.10 | 10% | 2.20 m³ |
| 10 | 8 | 0.15 | 10% | 13.20 m³ |
| 3 | 3 | 0.10 | 5% | 0.95 m³ |
| 8 | 6 | 0.20 | 15% | 11.04 m³ |
| 2 | 2 | 0.05 | 10% | 0.22 m³ |
| 12 | 6 | 0.10 | 10% | 7.92 m³ |
Turning the Estimate into an Order
Measure depth at several points and use the average, because the ground is never perfectly flat and an extra half inch over a large slab adds up.
- Keep units consistent. Mixing inches of thickness with feet of length is the most common error, so convert everything first.
- Round up, not down. Running short midway through a pour can leave a cold joint, a weak line where fresh concrete meets concrete that has already begun to set.
- Ready-mix suppliers often have a minimum load, so for small jobs compare the bag count against the cost of a delivery.
The 1:2:4 mix shown is a common general-purpose ratio, not a structural specification. For footings, retaining walls or anything carrying a building load, follow the engineer’s drawings or your local building code. The concrete article on Wikipedia covers how curing and water content affect strength, which matters as much as the volume.
Common questions about concrete for slabs
How many yards of concrete for a 10×10 slab?
At 4 inches thick, 10 × 10 × 0.333 ft = 33.3 cubic feet, which is 1.23 cubic yards before waste. With a 10% allowance, order about 1.36 yd³.How many yards of concrete for a 20×20 slab?
At 4 inches thick it is 133.3 cubic feet, or 4.94 cubic yards, before waste. At 6 inches thick it rises to 7.41 yd³.How many 60 lb bags of concrete do I need?
The calculator estimates roughly one 25 kg/60 lb bag per 0.011 m³ of concrete, rounded up to a whole bag. Bag yield varies by brand and by how much water is added, so check the yield printed on your bag.What waste allowance should I use for a concrete pour?
5 to 10% is a common starting point for a level, well-formed slab. Uneven or sloped ground, hand-mixing or a first pour can justify the higher end, since spillage and formwork gaps use up material quickly.What does the 1:2:4 mix ratio mean?
It’s a common volume ratio for site-mixed concrete: 1 part cement, 2 parts sand, 4 parts aggregate (gravel or crushed stone). The calculator shows the dry volume of each part so you know roughly how much of each material to have on hand.Does this account for ready-mix truck minimum orders?
No. It only estimates the volume your project needs. Ready-mix suppliers often have minimum order sizes and short-load fees, so confirm those terms with your supplier once you have your volume.Related Calculators
If you’re batching cement and aggregate yourself rather than ordering ready-mix, the concrete/cement calculator can help refine bag counts by mix ratio. Ordering base material for underneath the pour is easier with the gravel calculator, and once the slab is in you can measure the finished floor area with the square footage calculator. For a related estimate, try the Septic Tank Size Calculator.