Concrete Needed for a 12 x 24 ft Slab at 6 Inches
Cubic yards, weight, and premix bags required. Use the calculator below to try your own dimensions.
Concrete Needed
5.333 cu yd
Example
A 12 × 24 ft slab at 6" thick needs 5.33 cu yd of concrete (144.0 cu ft), weighing about 10.80 tons — that's 240 bags of 80 lb premix. By the cubic yard, that costs about $800.00.
Volume
144 cu ft
Weight
10.8 tons
Premix Bags Needed
240 bags
Estimated Cost (Delivered)
$800.00
Bags Needed by Size
40 lb
480
60 lb
320
80 lb
240
What Is Concrete Made Of?
Concrete is a composite material made from coarse and fine aggregates (gravel and sand) bound together by cement paste, which hardens through a chemical reaction with water rather than by simply drying out. Fresh concrete must be placed and finished before it hardens — typically within about 90 minutes to 2 hours depending on the mix and temperature.
Concrete gains strength gradually as it cures: it's generally safe to walk on after about 24-48 hours, but reaches around 70% of its final strength after 7 days and doesn't reach over 90% of its full strength until roughly 28 days after pouring.
Concrete Needed by Slab Thickness
For your 288 sq ft area, here's how much concrete different thicknesses require.
| Thickness | Cubic Yards | 80 lb Bags |
|---|---|---|
| 4" | 3.556 | 160 |
| 5" | 4.444 | 201 |
| 6" (yours) | 5.333 | 240 |
| 8" | 7.111 | 321 |
| 10" | 8.889 | 400 |
How Is Concrete Volume Calculated?
Volume is simply length × width × thickness, with thickness converted from inches to feet first. The result in cubic feet is then divided by 27 to convert to cubic yards, the unit ready-mix concrete is typically sold and priced in.
Bagged Concrete vs. Ready-Mix Delivery
Small projects (under about 1 cubic yard) are usually most economical with bagged premix concrete — an 80 lb bag yields about 0.6 cubic feet of concrete once mixed with water. Larger pours are typically cheaper and far less labor-intensive with a ready-mix truck delivery, priced per cubic yard.
Choosing the Right Thickness
4 inches is the standard thickness for patios, walkways, and shed foundations under normal residential loads. Driveways handling vehicle weight, especially larger trucks, typically need 5-6 inches or more, and often benefit from steel reinforcement (rebar or wire mesh) to resist cracking.
Always Buy a Little Extra
Uneven ground, spillage, and small measurement errors mean actual concrete needs often run 5-10% higher than the calculated volume — many contractors round up to the next whole bag count or full cubic yard delivery to avoid a costly mid-pour shortage.
Example — Your Current Inputs
A 12 × 24 ft slab at 6" thick needs 5.33 cu yd of concrete (144.0 cu ft), weighing about 10.80 tons — that's 240 bags of 80 lb premix. By the cubic yard, that costs about $800.00.
Additional Example — A Shed Foundation
An 8 x 10 ft shed foundation poured 4 inches thick needs about 0.99 cubic yards of concrete — roughly 45 bags of 60 lb premix, or one small ready-mix delivery if a supplier's minimum order allows it.
About These Parameters
- Length & Width
- The slab's footprint dimensions in feet. Multiply these together for the total area before applying thickness.
- Thickness
- The depth of the pour in inches. Thicker slabs use volume (and cost) proportionally — doubling thickness exactly doubles the concrete required.
- Premix Bag Size & Price per Cubic Yard
- Bag size only affects the bag-count estimate for DIY mixing. Price per cubic yard is optional, used only if you're pricing out a delivered ready-mix order instead.
Frequently Asked Questions
How many 80 lb bags are in a cubic yard?
About 45 bags of 80 lb premix are needed to make one cubic yard of concrete, since each bag yields roughly 0.6 cubic feet and a cubic yard contains 27 cubic feet.
Should I add extra for waste?
Yes — most contractors add 5-10% extra to account for uneven subgrade, spillage, and measurement variance, then round up to the nearest full bag count or delivery minimum.
When is ready-mix delivery worth it over bags?
Once a project needs more than about 1 cubic yard, ready-mix delivery is usually cheaper per unit of concrete and dramatically less labor than hand-mixing dozens of bags — though many suppliers have delivery minimums, so check locally.