Concrete Slab Calculator
✓ Your estimate
--
Cubic Yards
--
Cubic Feet
--
80 lb Bags
⚠ Add 5-10% extra for waste, spillage, and uneven ground. Confirm quantities with your supplier before ordering.
What the Result Means
Concrete is sold by the cubic yard when ordering ready-mix delivery, and by the bag for smaller DIY jobs. One cubic yard equals 27 cubic feet.
For a standard 10 ft x 10 ft slab at 4 inches thick, you need roughly 1.23 cubic yards or about 56 bags of 80 lb concrete. If you are also laying a paver border or need a compacted gravel base underneath, see the paver calculator and gravel calculator for those material estimates. For fence post holes, the post hole concrete calculator handles bag counts by hole size. For a full list of outdoor tools, visit the outdoor calculators page.
For slabs over 1.5 cubic yards, a ready-mix delivery is usually faster, cheaper per unit, and gives a more consistent result than hand-mixing bags.
How the Calculation Works
Cubic Feet = Length (ft) x Width (ft) x Thickness (ft)
Thickness in inches divided by 12 converts to feet
Cubic Yards = Cubic Feet / 27
Bags Needed = Cubic Feet / Yield per bag
80 lb = 0.60 cu ft per bag | 60 lb = 0.45 cu ft | 40 lb = 0.30 cu ft
Bag yields are standard manufacturer estimates (Quikrete, Sakrete). Actual yield can vary by water ratio and how the concrete is consolidated.
Worked Example
Example: 12 ft x 16 ft driveway apron, 6 inches thick
1
Convert thickness: 6 in / 12 = 0.5 feet
2
Cubic feet: 12 x 16 x 0.5 = 96 cubic feet
3
Cubic yards: 96 / 27 = 3.56 cubic yards
4
80 lb bags: 96 / 0.60 = 160 bags -- at this volume, ready-mix is the better call
5
Add 10% waste: order 3.9 cubic yards -- round up to 4 yards from the truck
Ordering and Waste Allowance
Assumptions Used by This Calculator
- Length and width are entered in feet and multiplied to get square footage.
- Thickness is entered in inches and divided by 12 to convert to feet.
- Volume is calculated as: length x width x thickness (in feet).
- Cubic feet are divided by 27 to get cubic yards.
- Bag counts use standard yields: 80 lb = 0.60 cu ft, 60 lb = 0.45 cu ft, 40 lb = 0.30 cu ft.
- Actual yield varies by water ratio, mix type, and consolidation method.
- Waste and spillage are not included in the base calculation -- add 5-15% manually.
Frequently Asked Questions
How much concrete do I need for a 10x10 slab?
At 4 inches thick, a 10x10 slab needs about 1.23 cubic yards or roughly 56 bags of 80 lb concrete. At 6 inches thick, that rises to about 1.85 cubic yards or 84 bags. Always add 10% for waste.
How many 80 lb bags make one cubic yard of concrete?
An 80 lb bag yields about 0.60 cubic feet. One cubic yard equals 27 cubic feet, so you need approximately 45 bags of 80 lb concrete per cubic yard.
How thick should a concrete slab be?
For a standard patio or walkway, 4 inches is typical. For a garage floor or driveway, 5-6 inches is recommended. For heavy vehicle traffic, 6 inches or more with rebar. Check local building codes for any permitted structure.
When should I use ready-mix instead of bags?
If your job is over 1 cubic yard (about 45 bags of 80 lb), a ready-mix truck is faster, cheaper per unit, and more consistent. Under 1 yard, bags are fine for most DIYers. The break-even depends on local pricing and short-load fees.
How much extra concrete should I order?
Add 10% for DIY bag work. Add 5-8% for ready-mix and round up to the nearest half yard. For uneven or rocky ground, add 10-15%. Running short mid-pour is one of the most expensive mistakes on a slab job.
Do I need rebar in a concrete slab?
For patios and shed bases, rebar is optional but recommended for slabs over 10 feet in any direction. For driveways and garage floors bearing vehicle weight, rebar or wire mesh is standard. Check local code before any permitted pour.
What is the difference between concrete and cement?
Cement is the binding powder ingredient in concrete. Concrete is the finished mix of cement, sand, aggregate, and water. A "bag of concrete" contains all dry ingredients pre-blended -- you just add water.
Can I pour concrete in cold weather?
Do not pour when temperatures are at or below 40 degrees F without cold-weather precautions. Freezing before curing permanently damages the slab. Keep temps above 50 degrees F for at least 7 days after the pour.
Disclaimer: This calculator provides estimates only. Actual quantities depend on site conditions, mix design, waste, and installation method. Verify all amounts with your concrete supplier or contractor before ordering. See all outdoor material calculators.