Concrete Calculator

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

Something wrong or missing? Tell us — we read every report and fix issues fast.

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

Related Calculators

Slope Percentage Calculator — % Grade, Angle, Ratio Calculate slope percentage from rise and run, or convert between percent grade, degrees, and 1:N ratio. Includes ADA ramp, driveway, road, and drainage standards. Acreage Calculator — Land Area in Acres, Hectares Calculate land area in acres from length and width. Converts to sq ft, hectares, sq m, and more. Square Yards Calculator — Carpet, Flooring, Cost Calculate square yardage from length and width. Includes unit conversion and optional cost estimation for carpet and flooring. Square Footage Calculator — Room Area + Paint Estimate Calculate square footage for rooms and spaces. Get paint and flooring estimates from one entry. Paint Calculator — Coats, Doors, Windows, Cost Calculate paint and primer needed with custom door/window dimensions, ceiling toggle, and cost estimates. Flooring Calculator — Multi-Room with Pattern Waste Calculate flooring for multiple rooms with waste presets by pattern (straight/diagonal/herringbone) and box/carton calculator.

On conv.awajis.com

Cubic Yards to Cubic Meters Convert concrete volume to metric.

Frequently Asked Questions

How much concrete do I need for a 10x10 slab 4 inches thick?

A 10 by 10 ft slab at 4 inches thick needs about 1.23 cubic yards of concrete. Convert 4 inches to 0.333 feet, multiply 10 by 10 by 0.333 to get 33.3 cubic feet, then divide by 27. With a 10% waste buffer, order about 1.35 cubic yards. The same method works for any rectangular slab.

How many bags of concrete make a cubic yard?

One cubic yard of concrete takes about 45 bags of 80 lb mix, 60 bags of 60 lb mix, or 90 bags of 40 lb mix. Bags make sense for jobs under 1 cubic yard. For larger pours, ready-mix delivery is faster and gives more consistent mixing than hand-mixing dozens of bags.

How thick should a concrete slab be?

A concrete slab should be 4 inches thick for sidewalks and patios, 6 inches for driveways, and 8 inches or more for heavy vehicles or foundations. Pour it over a 4-inch layer of compacted gravel. Skipping the gravel base is the most common slab mistake, because soft soil settles and leads to surface cracks.

How much does a yard of concrete cost?

Ready-mix concrete costs about $100 to $150 per cubic yard delivered. Bagged concrete costs more per yard but is practical for small projects under 1 cubic yard. Budget for at least 10% extra material, and avoid adding water at the site, because extra water weakens the finished slab even though it makes pouring easier.