Concrete Calculator

What Does This Concrete Calculator Do?

This concrete calculator estimates how much concrete you need for a slab, patio, walkway, or footing project. Enter the length and width of your project area in feet and the depth (thickness) in inches, and the calculator instantly returns the volume in cubic feet and cubic yards — plus the number of premixed 60 lb or 80 lb bags you'd need if you're not ordering ready-mix concrete by the truckload.

The Concrete Volume Formula

Concrete volume is calculated the same way you'd calculate the volume of any rectangular box: length times width times depth. The only tricky part is that slab depth is almost always specified in inches while length and width are measured in feet, so the depth needs to be converted to feet first by dividing by 12.

Cubic yards is the standard unit ready-mix concrete suppliers use for quoting and delivering concrete, since there are 27 cubic feet in a cubic yard (3 ft x 3 ft x 3 ft).

Worked Example: 10 ft x 10 ft Slab at 4 Inches

Say you're pouring a 10 ft by 10 ft patio slab at the standard 4-inch depth:

• Length = 10 ft, Width = 10 ft, Depth = 4 inches
• Cubic Feet = 10 x 10 x (4 / 12) = 10 x 10 x 0.333 = 33.3 cubic feet
• Cubic Yards = 33.3 / 27 = 1.23 cubic yards
• 60 lb bags needed = 33.3 / 0.45 = 74.1, rounded up to 75 bags
• 80 lb bags needed = 33.3 / 0.6 = 55.5, rounded up to 56 bags

How Many Bags of Concrete Do You Need?

Premixed concrete bags list a "yield" — the volume of finished concrete that one bag produces once mixed with water. A standard 60 lb bag yields about 0.45 cubic feet, and an 80 lb bag yields about 0.6 cubic feet. To find bag count, divide the total cubic feet needed by the bag's yield and round up, since you can't buy a partial bag.

• 60 lb bags: Easier to lift and mix by hand, good for solo DIYers
• 80 lb bags: Fewer bags needed for the same volume, more cost-efficient, but heavier

Tips for DIY Concrete Pours

1. Order extra: Add 5-10% extra concrete to cover spillage, an uneven subgrade, or forms that settle slightly deeper than planned.
2. Check your depth code requirements: Walkways are typically 4 inches, driveways 5-6 inches, and structural footings often need more — check local building codes.
3. Prep the base: Compact a gravel base and use proper forms before pouring to prevent cracking and uneven settling.
4. Consider ready-mix for larger pours: If you need more than about 1-2 cubic yards, a ready-mix truck delivery is often more economical and less labor-intensive than mixing dozens of bags by hand.
5. Work in sections: For larger slabs, pour and finish in manageable sections so the concrete doesn't start setting before you can screed and float it.
6. Mind the weather: Avoid pouring concrete in freezing temperatures or direct hot sun without proper curing precautions, as both can weaken the final cure.

Use the calculator directory to find more construction and DIY planning tools, including our mulch calculator for landscaping projects.