Concrete Calculator
Estimate the cubic yards, weight, premix bags needed, and cost for a concrete slab from its length, width, and thickness.
Results from this calculator are estimates provided for general informational purposes only, based on formulas, rates, and standards commonly accepted as of 2026. Figures may differ slightly from other calculators or professional sources due to rounding methods, differing assumptions, or regional regulations, and rules may change over time. Always consult a qualified professional — such as a financial advisor, healthcare provider, or other relevant specialist — before making decisions based on these results.
Concrete Needed
1.235 cu yd
Example
A 10 × 10 ft slab at 4" thick needs 1.23 cu yd of concrete (33.3 cu ft), weighing about 2.50 tons — that's 56 bags of 80 lb premix. By the cubic yard, that costs about $185.19.
Volume
33.3 cu ft
Weight
2.5 tons
Premix Bags Needed
56 bags
Estimated Cost (Delivered)
$185.19
Bags Needed by Size
40 lb
112
60 lb
75
80 lb
56
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 100 sq ft area, here's how much concrete different thicknesses require.
| Thickness | Cubic Yards | 80 lb Bags |
|---|---|---|
| 4" (yours) | 1.235 | 56 |
| 5" | 1.543 | 70 |
| 6" | 1.852 | 84 |
| 8" | 2.469 | 112 |
| 10" | 3.086 | 139 |
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 10 × 10 ft slab at 4" thick needs 1.23 cu yd of concrete (33.3 cu ft), weighing about 2.50 tons — that's 56 bags of 80 lb premix. By the cubic yard, that costs about $185.19.
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.