How the result is calculated
- 01
Cubic feet = length x width x (thickness / 12) - 02
Cubic yards = cubic feet / 27 - 03
80 lb bags = cubic feet / 0.60, rounded up
CONSTRUCTION
Concrete Calculator
Calculate concrete volume in cubic yards and 80 lb bags for slabs, pads and rectangular footings. Enter your measurements below and the estimate updates instantly.
Formula, validation and worked example reviewed November 12, 2026. See our calculation methodology.
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CONSTRUCTION
US UNITSConcrete
YOUR ESTIMATE
2.96 cu yd134 × 80 lb bags2.96 cu yd · 80 cu ft · Before extra waste
Approximate yield 0.6 cu ft/bag for QUIKRETE Concrete Mix No. 1101; verify your product. Whole bags rounded up from unrounded volume.
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Volume and bag estimate
Turn slab dimensions into cubic yards and bags.
This concrete slab and yard calculator converts thickness from inches to feet, calculates volume in cubic feet and cubic yards, and estimates 80 lb bags using an approximate 0.60 cu ft yield per bag.
How do you calculate concrete cubic yards and bags?
Ready-mix concrete is ordered in cubic yards (1 yd³ = 27 ft³), while slab thickness is almost always measured in inches. Convert slab thickness to feet by dividing by 12, multiply by length and width to get cubic feet, and divide by 27:
Vcu ft=L · W · Thickness (in)12
(Slab cubic feet)Vcu yd=Vcu ft27
(Cubic yards)where:
- L, W – Slab length and width in feet (find surface area first with the square footage calculator);
- V_cu yd – Exact geometric concrete volume in cubic yards (check haul weight with the concrete weight calculator); and
- Bags_80 – Estimated 80 lb premixed bags (⌈ V_cu ft ÷ 0.60 ⌉). To compare 40, 50, 60, and 80 lb bag sizes, open the concrete bag calculator.
12 ft by 10 ft slab, 4 in thick
- Four inches becomes exactly 4/12 ft (approximately 0.3333 ft).
- 12 x 10 x (4/12) = 40 cu ft.
- 40 / 27 = 1.48 cu yd.
- 40 / 0.60 = 66.7 bags.
Quick reference chart
Concrete Slab Volume & 80 lb Bag Reference (4 in vs. 6 in Thickness)
1 cubic yard = 27 cubic feet (~45 × 80 lb bags at 0.60 cu ft/bag before waste).
| Slab Size (ft) | 4 in Slab (cu yd) | 4 in Slab (80 lb Bags) | 6 in Slab (cu yd) |
|---|---|---|---|
| 8 × 10 ft (80 sf) | 0.99 cu yd | 45 bags | 1.48 cu yd |
| 10 × 10 ft (100 sf) | 1.23 cu yd | 56 bags | 1.85 cu yd |
| 10 × 12 ft (120 sf) | 1.48 cu yd | 67 bags | 2.22 cu yd |
| 12 × 20 ft (240 sf) | 2.96 cu yd | 134 bags | 4.44 cu yd |
| 20 × 20 ft (400 sf) | 4.94 cu yd | 223 bags | 7.41 cu yd |
| 24 × 24 ft (576 sf) | 7.11 cu yd | 320 bags | 10.67 cu yd |
Common mistakes to avoid
- Entering slab thickness in feet when the field expects inches.
- Assuming the subgrade is perfectly level and the slab is uniformly thick.
- Skipping a reasonable project allowance for spillage, uneven depth, or handling loss.
- Assuming every product has the same bag yield instead of checking its label.
- Using one slab rectangle for complex footings, piers, or stepped shapes.
Questions answered
Calculator FAQ
How do I calculate concrete yards for a slab?+
Multiply slab length by width by thickness in feet to get cubic feet, then divide by 27 to get cubic yards. Add a project-appropriate allowance before ordering.
How many cubic yards of concrete do I need for a 10x10 slab?+
A 10 ft × 10 ft slab that is 4 inches thick requires 1.23 cubic yards of concrete (33.3 cu ft). Ordering with 10% waste means planning for 1.35 to 1.5 yards, or 56 to 62 eighty-pound bags.
How many yards of concrete for a 24x24 garage slab?+
A 24 ft × 24 ft slab at 4 inches thick requires 7.11 cubic yards before waste (plan ~7.8 cu yd). At 6 inches thick for heavy vehicle parking, it requires 10.67 cubic yards (plan ~11.7 cu yd).
Why do you divide cubic feet by 27?+
A cubic yard is 3 ft x 3 ft x 3 ft, which equals 27 cubic feet.
How many 80 lb bags make a cubic yard?+
At an approximate 0.60 cu ft yield per bag, one cubic yard takes about 45 bags. Product yield can vary.
Is waste already included?+
No. The calculator shows exact geometric volume before extra waste so you can choose an allowance suitable for the job.
Can I calculate footings or posts?+
You can calculate rectangular sections one at a time and add them. Use a shape-specific method for round or complex sections.
Are bag yields exact?+
No. Published yields are approximate and product-specific. Check the bag or manufacturer data before buying.
Practical selection guide
Concrete slab cubic yard estimates for common residential projects.
Calculate exact concrete yards needed for driveways, patios, garage slabs and walkways with an automatic 10% spillage and grade-variation buffer.
10x10 patio slab (4" thick)
A 10 × 10 ft patio slab at 4 inches depth equals 33.3 cu ft or 1.23 cubic yards. With a recommended 10% waste buffer, order 1.35 to 1.5 cu yd (or 56 to 62 eighty-pound bags).
12x12 shed or patio slab (4" thick)
A 12 × 12 ft slab at 4 inches requires 48 cu ft or 1.78 cubic yards. With 10% waste allowance, order 1.96 cubic yards (or 80 to 88 eighty-pound bags).
20x20 two-car driveway (4" thick)
A 20 × 20 ft driveway slab requires 133.3 cu ft or 4.94 cubic yards. With 10% waste allowance, order 5.43 to 5.5 cubic yards from ready-mix dispatch.
24x24 garage slab (4" vs 6" thick)
A 24 × 24 ft slab at 4 inches needs 7.11 cu yd (order ~7.8 cu yd). If poured at 6 inches for heavy vehicles or equipment, it requires 10.67 cu yd (order ~11.7 cu yd).
Method references: QUIKRETE concrete estimating guide · QUIKRETE Concrete Mix data sheet