Concrete Calculator
Work out how much concrete you need for a slab, footing, post hole or round pad – in cubic yards, bags of 80, 60 and 40 lb mix, and ready-mix cost.
Formulas shown on this page · Last reviewed October 2026
How to use the concrete calculator
Pick the shape, enter the dimensions and leave the waste at 10% unless your forms are very precise. The result is rounded up to whole bags, since a partial bag is never enough to finish a pour. For a ready-mix order, use the cubic-yard figure.
slab: cubic feet = length (ft) × width (ft) × thickness (in) ÷ 12
column: cubic feet = π × (diameter in ÷ 24)² × height (ft)
cubic yards = cubic feet ÷ 27
bags = cubic feet × (1 + waste %) ÷ bag yield
Cubic yards for common slab sizes
| Slab size | 4 in thick | 5 in thick | 6 in thick |
|---|---|---|---|
| 10 × 10 ft | 1.23 yd³ | 1.54 yd³ | 1.85 yd³ |
| 12 × 12 ft | 1.78 yd³ | 2.22 yd³ | 2.67 yd³ |
| 20 × 20 ft | 4.94 yd³ | 6.17 yd³ | 7.41 yd³ |
These figures are net volumes. Add about 10% before ordering.
Bag yield reference
| Bag size | Yield | Bags per cubic yard |
|---|---|---|
| 80 lb | 0.60 cu ft | 45 |
| 60 lb | 0.45 cu ft | 60 |
| 40 lb | 0.30 cu ft | 90 |
Practical tips
- Patios and walkways are normally 4 inches thick; driveways and garage floors are 5–6 inches.
- Measure the actual depth of the excavation – a subgrade that is 1 inch too deep adds about 25% to a 4-inch slab.
- Footings and post holes are rarely square-cut. Use the column shape for round holes and add extra for the wider top.
- Order slightly over rather than under: a cold joint from a second delivery weakens the slab.
Concrete for common slab sizes
4x4 slab · 6x6 slab · 8x8 slab · 8x10 slab · 10x10 slab · 10x12 slab · 10x16 slab · 10x20 slab · 12x12 slab · 12x16 slab · 12x20 slab · 12x24 slab · 14x14 slab · 16x16 slab · 16x20 slab · 18x20 slab · 20x20 slab · 20x24 slab · 20x30 slab · 24x24 slab · 24x30 slab · 24x36 slab · 30x30 slab · 30x40 slab · 40x60 slab
Frequently asked questions
How many 80 lb bags of concrete do I need for a 10×10 slab 4 inches thick?
The slab is 10 × 10 × (4 ÷ 12) = 33.3 cubic feet, or 1.23 cubic yards. At 0.60 cu ft per 80 lb bag that is 56 bags. Adding 10% for waste and uneven subgrade raises it to 36.7 cu ft, or 62 bags.
How many bags of concrete make one cubic yard?
One cubic yard is 27 cubic feet. That takes 45 bags of 80 lb mix (0.60 cu ft each), 60 bags of 60 lb mix (0.45 cu ft) or 90 bags of 40 lb mix (0.30 cu ft).
When is ready-mix cheaper than bagged concrete?
Roughly above one cubic yard. A yard of ready-mix runs about $150–$185 delivered, while 45 bags of 80 lb mix cost around $270–$360 at $6–$8 per bag. Many plants add a short-load fee below 3–4 yards, so ask before ordering.
How much does a cubic yard of concrete weigh?
About 4,000 pounds (2 tons). Normal concrete weighs roughly 150 lb per cubic foot, and 150 × 27 = 4,050 lb. Plan truck access and wheelbarrow trips accordingly.