Concrete Calculator

Calculate concrete needed for slabs, patios, and driveways. Get cubic yards, number of bags, and cost estimates.

How to Calculate Concrete

Multiply length × width × depth (all in feet) to get cubic feet, then divide by 27 to convert to cubic yards. For depth in inches, divide by 12 first. This calculator includes 10% extra for waste.

Quick Estimation Shortcut

For a 4-inch slab: divide your total square footage by 81 to get cubic yards. For 6-inch slabs, divide by 54. Example: a 200 sq ft patio at 4 inches = 200 ÷ 81 = 2.47 cubic yards.

Recommended Slab Thickness

4 inches: Sidewalks, patios, garden paths
5-6 inches: Driveways, garage floors
8 inches: Heavy vehicle traffic, RV pads
12+ inches: Foundation footings

Example: 20×20 Patio

Area: 400 sq ft at 4 inches thick
Volume: 400 × 0.333 ÷ 27 = 4.94 cubic yards
With 10% waste: 5.5 cubic yards
Cost at $140/yard: approximately $770

Bags vs Ready-Mix

Bagged Concrete

40 lb bag: 0.30 cubic feet (90 bags per yard)
60 lb bag: 0.45 cubic feet (60 bags per yard)
80 lb bag: 0.60 cubic feet (45 bags per yard)

Best for projects under 1 cubic yard. Costs about $6 per 80-lb bag (~$270 per cubic yard including waste).

Ready-Mix Delivery

Best for projects over 1 cubic yard. Costs $125-165 per cubic yard delivered. Mixing 5+ yards by hand takes a full day with a portable mixer vs 30 minutes from a ready-mix truck. Expect short-load fees ($50-150 per yard under minimum) for orders under 8-10 yards.

Pro Tips

Add 5-10% extra for waste, uneven subgrade, and spillage
Ideal pouring temperature: 50-70°F. Cold slows curing; heat speeds it and can cause cracking
Have all prep done before concrete arrives — it sets quickly and cannot wait
Reinforce slabs with wire mesh or rebar for driveways and areas over 4 inches thick
Cure time: walk on it in 24-48 hours, drive on it after 7 days, full strength at 28 days
Keep the surface moist for the first 7 days of curing for maximum strength

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On conv.awajis.com

Cubic Yards to Cubic Meters Convert concrete volume to metric.

Frequently Asked Questions

How thick should a concrete slab be?

4 inches for sidewalks and patios, 6 inches for driveways, 8+ inches for heavy vehicles or foundations.

How many bags of concrete for 1 cubic yard?

About 45 80-lb bags, 60 60-lb bags, or 90 40-lb bags equal one cubic yard.

When should I order ready-mix vs bags?

Use bags for under 1 cubic yard. Order ready-mix delivery for larger projects—it's faster and ensures consistent mixing.

How much does concrete cost?

Ready-mix concrete costs $100-150 per cubic yard delivered. Bags cost more per yard but work better for small projects.

What is concrete slump and why does it matter?

Slump measures how much fresh concrete settles after the form is lifted from a cone-shaped mold. A 4-inch slump is standard for slabs. Adding extra water raises slump but weakens the finished slab, so avoid thinning ready-mix at the site.

How do I calculate concrete for a 10x10 slab at 4 inches thick?

Convert depth: 4 inches equals 0.333 feet. Multiply: 10 times 10 times 0.333 = 33.3 cubic feet. Divide by 27 to get 1.23 cubic yards. With a 10% waste buffer, order about 1.35 cubic yards.

What is the most common slab mistake?

Skipping the compacted gravel base. A 4-inch layer of compacted gravel under the slab improves drainage and reduces cracking. Pouring directly onto soft or unstable soil leads to settling and surface cracks over time.

Can concrete cure properly in cold weather?

Concrete needs temperatures above 50 degrees F to cure correctly. In cold weather, cover fresh pours with insulating blankets and maintain warmth for at least 3 days. Freezing before the concrete gains strength causes permanent internal damage.