Concrete Calculator
Use this free concrete calculator to find out how much concrete you need for a slab, patio, footing, or pad. Enter your dimensions to get cubic yards and the exact number of bags to buy.
Advanced options
How to Use the Concrete Calculator
- Measure the pour area: Enter the length and width in feet.
- Enter the thickness: This is entered in inches since most slabs are only a few inches thick.
- Choose a bag size: Pick the bag weight you plan to buy — 40, 60, or 80 lb.
- Read your results: The calculator shows total cubic yards and exactly how many bags to buy, with a waste allowance included.
What Is This Concrete Calculator?
This concrete calculator estimates how much concrete you need for a slab, patio, walkway, or footing. It gives you both the volume in cubic yards (how ready-mix trucks measure concrete) and the number of pre-mixed bags needed (for smaller DIY pours).
Why Use a Concrete Calculator?
Concrete is unforgiving once it starts setting — running short mid-pour can ruin a slab, and ordering too much ready-mix wastes money on a minimum delivery charge. Getting the volume right before you order or buy bags avoids both problems.
How Does This Calculator Work?
The calculator converts your slab's thickness from inches to feet, multiplies by length and width to get cubic feet, then converts to cubic yards:
Cubic Yards = (Length × Width × (Thickness ÷ 12)) ÷ 27
For bagged concrete, it divides the total cubic feet (with your waste allowance included) by the yield of your chosen bag size — about 0.30 cu ft for a 40 lb bag, 0.45 cu ft for 60 lb, and 0.60 cu ft for 80 lb.
Worked Example: 10x10 Patio Slab
A patio slab is 10 feet by 10 feet, poured 4 inches thick, using 60 lb bags:
- Thickness in feet: 4 ÷ 12 = 0.333 ft
- Volume: 10 × 10 × 0.333 = 33.3 cu ft
- Add 10% waste: 33.3 × 1.10 = 36.7 cu ft
- In cubic yards: 36.7 ÷ 27 ≈ 1.36 yd³
- In 60 lb bags: 36.7 ÷ 0.45 ≈ 81.5 → buy 82 bags
Load this example in the calculator above.
Concrete Pouring Tips
- Order ready-mix for large pours. Once you need more than about 1 cubic yard, a delivered truck is usually cheaper and far less physical labor than mixing dozens of bags.
- Always add a waste allowance. Uneven ground, spillage, and over-excavation mean you almost always need slightly more than the exact math.
- Check the thickness requirement for your specific project — fence post footings, driveways, and patios all have different recommended depths.
- Work in sections if mixing bags by hand; concrete begins setting in as little as 30–60 minutes depending on temperature.
Concrete Calculator FAQs
Multiply the slab's length by its width by its thickness (converted from inches to feet), then divide by 27 to convert cubic feet to cubic yards. For example, a 10x10 foot slab poured 4 inches thick needs about 1.23 cubic yards of concrete.
It depends on the bag size. A 60 lb bag yields about 0.45 cubic feet of concrete, and an 80 lb bag yields about 0.60 cubic feet. Divide the total cubic feet you need by the yield per bag to get the number of bags. This calculator does that automatically for any bag size.
A typical patio or walkway slab is 4 inches thick. Driveways for cars are usually 4 to 6 inches, and areas that carry heavier vehicles may need 6 inches or more. Check local building codes for your specific project.
Ready-mix delivered by truck is usually more cost-effective and less labor-intensive for pours larger than about 1 cubic yard. Bagged concrete mixed on site is more practical for small jobs like fence post footings or small patches.
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