What Is a Concrete Calculator?
A concrete calculator estimates how much concrete you need for a slab, footing, or pour, based on length, width, and thickness — giving you the volume in cubic yards, cubic feet, the number of pre-mixed bags needed, and an estimated cost.
How to Use the Concrete Calculator
- Enter the length and width of your slab in feet.
- Enter the thickness in inches.
- Optionally enter the cost per cubic yard for ready-mix concrete.
- Review the volume needed in cubic yards, cubic feet, and bag count.
Concrete Volume Formula
Worked Example
Volume: 10 × 10 × (4÷12) = 33.3 cubic feet
Cubic yards: 33.3 ÷ 27 ≈ 1.24 yd³
80 lb bags needed: ≈ 56 bags (each bag yields about 0.6 cubic feet)
At $150/yard: estimated cost ≈ $185.19
Understanding Your Results
Cubic Yards is the standard unit ready-mix concrete trucks use for delivery. 80lb Bags estimates how many pre-mixed bags you'd need if hand-mixing instead of ordering a truck — always add 5-10% extra to account for spillage, uneven ground, and form imperfections.
Tips for Concrete Projects
- Order slightly more than calculated (5–10% extra) to account for waste and uneven excavation.
- For pours under 1 cubic yard, bagged concrete is often more practical than ordering a ready-mix truck (many suppliers have delivery minimums).
- Standard slab thickness for driveways and patios is typically 4 inches; heavier loads (like vehicle parking) may call for greater thickness — check local building codes.
Common Mistakes to Avoid
- Forgetting to convert thickness from inches to feet before calculating volume.
- Not accounting for waste, which can leave you short mid-pour.
- Using average bag yield estimates without checking the specific product's yield on the packaging, which can vary slightly.
Frequently Asked Questions
At 4 inches thick, a 10x10 ft slab needs about 1.24 cubic yards of concrete — this calculator adjusts automatically for any thickness you enter.
A standard 80 lb bag yields about 0.6 cubic feet, so roughly 45 bags are needed to make one cubic yard (27 cubic feet), though this varies slightly by brand.
Yes — most contractors recommend ordering 5-10% more than the calculated volume to account for spillage, uneven subgrade, and form imperfections.
4 inches is common for patios, walkways, and sheds; driveways or slabs supporting vehicles often use 5-6 inches or more — check local building codes for your specific project.
For small projects (under about 1 cubic yard), bagged concrete is often more practical and cost-effective; larger pours are usually cheaper and faster with a ready-mix delivery.