BCBetter Calculators

Concrete Calculator

Calculate how much concrete you need for a slab in cubic yards and bags.

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Enter your values and click Calculate

How It Works

The volume formula is: cubic feet = length (ft) × width (ft) × depth (in) ÷ 12. Dividing depth by 12 converts inches to feet. To convert cubic feet to cubic yards, divide by 27 (since 1 cubic yard = 3 ft × 3 ft × 3 ft = 27 cubic feet). A 10% waste factor is then applied by multiplying by 1.10 to account for ground irregularities, spillage, and slight overpour — this is standard industry practice. For bag estimates: a 60-lb bag of pre-mixed concrete yields approximately 0.45 cubic feet, and an 80-lb bag yields approximately 0.60 cubic feet. The number of bags is calculated as: bags = ceil(total cubic feet ÷ bag yield). Always round up bags since you cannot use a fraction of a bag. For pours larger than 1 cubic yard, ordering ready-mix concrete from a supplier is more cost-effective and less labor-intensive than mixing bags.

Examples

Standard Patio Slab
A 10×10 ft patio slab, 4 inches thick.
Result: About 1.37 cubic yards (with waste). Requires ~90 bags of 60-lb or ~68 bags of 80-lb mix.
Garden Pathway
A 20 ft × 3 ft walkway, 3 inches thick.
Result: About 0.69 cubic yards. Requires ~46 bags of 60-lb or ~35 bags of 80-lb mix.
Garage Floor
A 20×20 ft garage floor at 4 inches thick.
Result: About 5.49 cubic yards. Order ready-mix rather than bags — over 360 bags of 60-lb would be needed.

Frequently Asked Questions

Why is a 10% waste factor included?
Concrete volume calculations assume a perfectly flat, even subgrade — which rarely exists in practice. Minor variations in ground level, form edges, and spillage during the pour typically consume 5–10% more concrete than the exact calculated volume. Ordering short mid-pour creates a 'cold joint' — a weak seam where fresh concrete meets partially set concrete — which can crack over time. The 10% buffer ensures you have enough material to complete the pour in one continuous operation.
When should I order ready-mix instead of using bags?
For projects requiring more than about 0.5–1 cubic yard, ready-mix concrete from a batch plant is typically more economical, stronger, and less labor-intensive. Mixing 50+ bags by hand or with a small mixer is exhausting and risks inconsistent water ratios that weaken the finished concrete. Ready-mix trucks deliver concrete in precise quantities with a consistent mix design. Most suppliers have a minimum order of 1 cubic yard; smaller orders may incur a short-load fee.
What thickness should a concrete slab be?
Thickness depends on the load the slab will bear. Residential sidewalks and patios typically use 4 inches. Driveways supporting passenger vehicles are usually 4–6 inches. Slabs supporting heavy equipment, forklifts, or RVs typically require 6–8 inches. Foundation footings are sized by a structural engineer based on soil bearing capacity and building loads. Using rebar or wire mesh reinforcement is recommended for any slab subject to vehicle traffic or freeze-thaw cycles.
How much does ready-mix concrete cost?
Ready-mix concrete typically costs $120–$200 per cubic yard depending on your region, mix strength (PSI rating), and additives (fiber, air entrainment, accelerators). Delivery charges, short-load fees, and Saturday/holiday surcharges apply. For a typical residential patio or driveway, expect to pay $500–$1,500 for materials alone, plus labor if you hire a contractor. Bags of pre-mixed concrete run $6–$10 each at home improvement stores, making bag-mix more expensive per cubic yard but accessible for small projects.

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