Concrete Slab Calculator
Planning a concrete slab for a patio, garage, or foundation? Enter your dimensions and get the exact cubic yards needed plus bag counts for 60lb and 80lb mixes.
How to Calculate Concrete for a Slab
Concrete volume comes down to basic geometry. Multiply length by width to get the square footage of your slab. Then multiply that area by the thickness to get volume. The catch? Thickness is measured in inches while length and width use feet, so you divide thickness by 12 to convert to feet first.
Once you have cubic feet, divide by 27 to convert to cubic yards. Concrete trucks and ready-mix suppliers work in yards, not feet. A typical residential pour ranges from 2 to 10 yards depending on the project.
For small jobs using bagged mix, count on 45 sixty-pound bags or 40 eighty-pound bags per cubic yard. Larger projects make ready-mix delivery cheaper than buying hundreds of individual bags.
Standard Slab Thickness by Project Type
Sidewalks and small patios can get by with 4 inches of concrete. That thickness handles foot traffic and light furniture without cracking under normal conditions. Reinforce with wire mesh or rebar if the slab spans soft ground.
Garage floors need 6 inches minimum. Cars and trucks concentrate weight on small tire contact patches, creating stress that 4-inch slabs cannot handle long-term. Heavier vehicles or workshop equipment justify going to 8 inches.
Driveways see the most abuse, especially at the apron where trucks turn in. Use 6 inches for standard passenger vehicles and 8 inches if delivery trucks or RVs will cross it regularly. Thicker slabs resist cracking from freeze-thaw cycles and heavy loads.
Ready-Mix Delivery vs. Bagged Concrete
Bagged concrete makes sense for small repairs and projects under half a yard. You avoid delivery fees, mix only what you need, and work at your own pace. The trade-off is physical labor mixing batch after batch, plus higher cost per yard.
Ready-mix becomes cost-effective above 1 to 2 cubic yards. A truck delivers consistent, professionally mixed concrete in minutes. You pay a premium for small loads under the truck's minimum, typically 3 to 5 yards depending on the supplier.
For anything over 2 yards, calculate both options. Include your time mixing bags, the cost of a mixer rental if needed, and the convenience of having a crew pour everything in one session. Most contractors switch to ready-mix at 1.5 yards or more.
Frequently Asked Questions
How thick should a concrete slab be?
For residential slabs like patios and sidewalks, 4 inches is standard. Garage slabs typically need 6 inches. Driveways carrying heavy vehicles may require 6 to 8 inches.
How many 60lb bags of concrete per cubic yard?
It takes approximately 45 sixty-pound bags to make one cubic yard of concrete. Eighty-pound bags need about 40 per yard.
Do I need to add extra for waste?
Yes. Add 5 to 10 percent to your calculated volume to account for spills, over-excavation, and uneven subgrade.
What is the difference between cubic feet and cubic yards?
One cubic yard equals 27 cubic feet. Concrete suppliers price by the cubic yard while bags are sold by weight. This calculator shows both for easy comparison.
Can I use this calculator for metric measurements?
Switch to the metric mode and enter dimensions in meters. The calculator will still output cubic yards since that remains the standard unit for concrete ordering.