Concrete Calculator
A 10 ft × 10 ft slab, 4 in thick, needs 33.3 cubic feet — 1.23 cubic yards. With a 10% waste buffer that is about 1.4 yards of ready-mix, or 62 80-lb bags if you are mixing by bag. Enter your own dimensions for an exact count.
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How it works
Concrete volume for a slab is plain rectangular volume: length × width × height. Thickness is measured in inches on the site, so divide by 12 to convert to feet. With all three dimensions in feet, multiply them to get cubic feet, then divide by 27 to get cubic yards — one cubic yard is exactly 3×3×3, or 27 cubic feet — because ready-mix concrete is sold by the yard. For bag-mix, a standard 80 lb bag yields about 0.6 cubic feet and a 60 lb bag about 0.45 cubic feet, so divide the volume by those figures and round up, since you cannot buy a fraction of a bag. Because ground is rarely perfectly level and some concrete spills or sticks to the wheelbarrow, a waste buffer (10% by default) is added before converting to yards or bag counts.
Worked example
A 10 ft × 10 ft patio slab at 4 inches thick: 4 in ÷ 12 = 0.33 ft. Volume = 10 × 10 × 0.33 = 33.3 cu ft → 33.3 ÷ 27 = 1.23 cu yd. Add 10% waste → about 1.36 cu yd, so order 1.5 yards of ready-mix. For bag-mix: 33.3 × 1.10 = 36.7 cu ft, then 36.7 ÷ 0.6 = about 62 80-lb bags, or 36.7 ÷ 0.45 = about 82 60-lb bags. These are the numbers a supplier or bag calculator will quote you.
Assumptions
- Thickness in inches: You input thickness in inches; the tool converts it to feet by dividing by 12
- Bag yield: Standard mix yields about 0.6 cu ft per 80 lb bag and 0.45 cu ft per 60 lb bag — check your specific bag's working coverage, which varies slightly by mix
- Rectangular shape: Volume assumes a level, rectangular form; sloped or irregular pours need a more detailed takeoff
- Identical sections: The slab count multiplies the same volume for multiple identical forms or patches
- Waste buffer: A 10% buffer covers uneven ground, spillage, and rounded-up bag counts; adjust it up for rocky or sloped sites
What the result means
The cubic-yards figure is what you order from a ready-mix plant — round it up to the next half or whole yard, since a short load still costs a delivery fee. The bag count is what you bring home from the hardware store if you are mixing by hand; it already includes the waste buffer, so it is the total you need, not an average. If the slab supports anything structural — a retaining wall, house foundation, or load-bearing column — the thickness must be engineered: this tool sizes a simple slab, and structural concrete still needs a structural engineer's review.
Frequently asked questions
How many bags of concrete do I need for a 10x10 slab?
A 10×10 slab 4 inches thick needs about 33 cu ft. Add 10% waste and you need about 62 80-lb bags (each yields ~0.6 cu ft) or 82 60-lb bags (each ~0.45 cu ft). Double the bag count for a 6-inch-deep slab of the same footprint.
How do I calculate how much concrete I need?
Multiply length × width × thickness in feet. Convert inches to feet by dividing by 12, multiply the three dimensions to get cubic feet, then divide by 27 for cubic yards. Add about 10% for waste before ordering.
How many cubic yards of concrete do I need for a slab?
Divide your slab's cubic-foot volume by 27. A 10×10 slab 4 inches thick is 33.3 cu ft, or about 1.23 cubic yards — about 1.36 yards with a 10% waste buffer, so you would order 1.5 yards.
How many 80 lb bags of concrete are in a yard?
A cubic yard is 27 cu ft. Since an 80 lb bag yields about 0.6 cu ft, roughly 45 80-lb bags make a cubic yard. A 60 lb bag yields about 0.45 cu ft, so about 60 of those make a yard.
How thick should a concrete slab be?
4 inches is standard for patios and walkways, 5-6 inches for most residential driveways, and 8 inches or more with reinforcement for heavy vehicles or structural loads. Check your local building code, which often sets minimums by use.
How much does a cubic yard of concrete cost?
Ready-mix typically runs $110-$175 per cubic yard depending on region, mix strength, and delivery, often with a minimum-order fee. Bagged mix costs more per yard but needs no minimum and suits small jobs.
Related tools
This calculator gives an estimate based on simple rectangular volume and standard bag yields. It is not engineering advice and is not a substitute for a licensed structural design, especially for load-bearing columns, footings, or structural walls. Confirm ready-mix orders and sack counts with your supplier before placing an order.