How thick should a concrete slab be?

4 inches is standard for patios, sidewalks, and shed pads. Driveways should be at least 5-6 inches. Garage floors typically need 6 inches. For heavy vehicles or equipment, use 8 inches or more.

How much does concrete cost?

Ready-mix concrete typically costs $120-180 per cubic yard delivered. Bags of premixed concrete (80 lb) cost about $5-8 each and yield about 0.6 cubic feet per bag. For larger projects, ordering a truck is more cost-effective.

Should I order extra concrete?

Yes, always order 5-10% extra to account for spillage, uneven ground, and settling. Running short during a pour is far worse than having a little left over.

Concrete Calculator

Estimate the amount of concrete you need in cubic yards for slabs, sidewalks, footings, and columns. Includes cost estimation and bag counts for small projects.

Last updated: May 29, 2026

About this calculator

Concrete is sold by the cubic yard from ready-mix trucks, or by the 60- or 80-pound bag at hardware stores. Knowing exactly how much you need before ordering is critical: short orders during a pour are very expensive (callback trips, cold joints between batches), while over-ordering wastes money on returned concrete.

This calculator converts a project's length × width × depth into cubic yards, number of 60- and 80-pound bags for small jobs, and total cost at your local ready-mix price. The basic formula is straightforward; the practical issues are choosing the right depth, accounting for waste, and deciding between bags and a truck.

The truck-vs-bag breakpoint is about 1 cubic yard. Below that, bags from a home store usually win on convenience and cost. Above 1 yard, ready-mix delivery is cheaper, faster, and far less back-breaking. Most ready-mix trucks have a minimum delivery charge (~$80–150) that erases the savings on very small loads.

Formula

Volume in cubic yards: Cubic yards = (Length × Width × Depth_inches) / 324 The 324 comes from: 1 yd³ = 27 ft³, and 1 inch = 1/12 ft. Number of 80-lb bags (each yields ~0.6 ft³): Bags = (Cubic yards × 27) / 0.6 = Cubic yards × 45 Number of 60-lb bags (each yields ~0.45 ft³): Bags = (Cubic yards × 27) / 0.45 = Cubic yards × 60 Recommended waste/spillage allowance: 5–10%. Example: 20 × 10 × 4-inch slab Cubic yards: (20 × 10 × 4) / 324 = 2.47 yd³ Add 10% waste: 2.72 yd³ → round to 3 yd³ from ready-mix Cost at $150/yd: $450 (plus delivery + Sunday/short-load fees if any)

How to use this calculator

Measure length and width of the slab in feet.
Choose depth in inches. Standards: patio/sidewalk 4 inches, driveway 5–6 inches, garage floor 6 inches, heavy-equipment pad 8+ inches.
Enter the local ready-mix price (varies $120–180 per cubic yard depending on region and order size).
Round result up to the next ¼ yard when ordering ready-mix; trucks typically deliver in ¼-yard increments.
Add 5–10% for spillage, edges, and uneven ground.
For irregular shapes, compute each rectangular section separately and sum.
For deep footings or columns, use cylindrical or rectangular prism formulas (length × width × depth for rectangular; π × radius² × depth for cylindrical).

Worked examples

Standard patio

12 × 16 patio, 4 inches deep. Volume: (12 × 16 × 4) / 324 = 2.37 cubic yards. Order: 2.5 yards from ready-mix at $150/yd = $375. For DIY bag pour: ~107 80-lb bags ≈ $750. Truck wins handily at this size.

Driveway

Driveway 30 × 12 × 5 inches = (30×12×5)/324 = 5.56 cubic yards. Order 6 cubic yards. At $145/yd + $100 delivery = $970. This is squarely in truck-delivery territory; bags would mean ~250 bags and many hours of mixing.

When to use this calculator

Use this calculator for any pour: patios, walkways, driveways, garage floors, shed pads, footings, post holes, columns, retaining walls. The math is the same; the depth and rebar requirements change.

Key references for depth: - Sidewalks/patios with foot traffic: 4 inches - Driveways: 5–6 inches with rebar grid - Garage floors: 6 inches - Equipment pads / RV pads: 8+ inches - Footings: per local building code (usually 8–12 inches deep, 16–24 inches wide for residential)

For real construction work, get a permit if required by your jurisdiction and follow local building codes for rebar, vapor barriers, and frost depth. Bad concrete work is hard and expensive to fix.

Common mistakes to avoid

Underestimating waste. 5–10% is necessary; many jobs need 12–15% on uneven ground.
Mixing depth units. Plug depth into the formula in inches (the formula expects it), or convert to feet first.
Forgetting delivery and short-load fees. Below ~3 yards, ready-mix often adds $50–150 in fees.
Skipping rebar/wire mesh on slabs >100 sq ft. Without reinforcement, slabs crack within a few freeze-thaw cycles.
Pouring on uncompacted base. Inadequate gravel base under the slab leads to settling and cracks.
Pouring in extreme heat or cold without precautions. Concrete cures predictably between ~50°F and ~85°F; outside that range you need additives, blankets, or hot water.

Frequently Asked Questions

Sources & further reading

Concrete construction guide — Portland Cement Association
Residential concrete slab guidelines — American Concrete Institute

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Concrete Calculator

Estimate the amount of concrete you need in cubic yards for slabs, sidewalks, footings, and columns. Includes cost estimation and bag counts for small projects.

Inputs

Length (ft)

Width (ft)

Depth (inches)

Price per Cubic Yard ($)

Results

Cubic Yards Needed

2.47

With 10% Extra

2.72

Estimated Cost

$407.41

Concrete Breakdown

Measurement	Value
Area	200 sq ft
Depth	4 inches (0.33 ft)
Volume	66.7 cubic feet
Cubic Yards (exact)	2.47
Cubic Yards (+10%)	2.72
40 lb Bags Needed	223
60 lb Bags Needed	149
80 lb Bags Needed	112