How do I calculate how much concrete I need?

To calculate concrete volume, multiply the length × width × depth of your project (all in feet), then divide by 27 to convert cubic feet to cubic yards. For example, a 10×10 ft slab at 4 inches thick = (10 × 10 × 0.333) ÷ 27 = 1.23 cubic yards. Always add 10% extra for waste and spillage.

How many bags of concrete do I need per cubic yard?

It takes approximately 68 bags of 60 lb concrete or 51 bags of 80 lb concrete to fill one cubic yard (at standard 150 lb/ft³ density). A 40 lb bag covers about 0.27 cubic feet, a 60 lb bag covers 0.40 cubic feet, and an 80 lb bag covers 0.53 cubic feet.

What is the standard concrete slab thickness?

A standard residential slab is 4 inches thick. Driveways require 6 inches, and heavy-load areas like commercial floors may require 6–8 inches. Footings are typically 8–12 inches deep.

How much does a cubic yard of concrete cost?

Ready-mix concrete typically costs $110–$165 per cubic yard, depending on your location and the concrete mix. Bag concrete costs more per cubic yard but is practical for small jobs under 1 yard.

What is the difference between cubic yards and cubic feet of concrete?

One cubic yard equals 27 cubic feet. Concrete is sold and ordered by the cubic yard (yd³). When calculating, you measure in feet and divide by 27 to get cubic yards.

Should I use Quikrete or Sakrete?

Both Quikrete and Sakrete are high-quality bag concrete brands. Quikrete is more widely available at big-box stores. Both offer the same basic mixes (concrete, mortar, sand mix). Choose based on what is available locally and the best price.

How do I calculate concrete for a round column or sonotube?

For a round column, use the formula: Volume = π × r² × height, where r is the radius (half the diameter). Then divide by 27 to get cubic yards. Our cylinder tab above handles this automatically — just enter the diameter and height.

What PSI concrete do I need for my project?

PSI (pounds per square inch) is the compressive strength of cured concrete at 28 days. Use 3,000 PSI for light-duty residential work (sidewalks, patios, non-structural slabs). Use 3,500–4,000 PSI for driveways, garage floors, and any slab exposed to vehicle loads or freeze-thaw cycles. Use 4,000–5,000 PSI for structural footings, columns, and commercial applications. Most ready-mix suppliers offer these as standard options — specify the PSI when you call.

How long does concrete take to cure?

Concrete reaches approximately 70% of its 28-day design strength after 7 days and effectively full strength at 28 days. You can walk on a slab after 24–48 hours and drive on it after 7 days under normal conditions (above 50°F). Keep slabs moist for the first 7 days by covering with burlap and plastic, or applying a liquid curing compound. Avoid pouring when air temperature is below 40°F — concrete cures very slowly in cold weather and can freeze before reaching adequate strength.

Concrete Calculator — Calculate Cubic Yards, Bags & Cost

Estimates volume and bags for any shape — slabs, columns, walls, tubes, and stairs.

What Is Concrete?

This concrete calculator estimates volume in cubic yards, bags needed, and cost for slabs, walls, columns, footings, and more — concrete is the world's most widely used construction material, a composite of Portland cement, water, fine aggregate (sand), and coarse aggregate (gravel or crushed stone). The cement and water react chemically (hydration) to form a paste that binds the aggregates into a mass with compressive strength comparable to rock. After 28 days, concrete reaches roughly 99% of its design compressive strength, which is specified in PSI (pounds per square inch): 3,000 PSI for general residential work, 4,000 PSI for driveways and structural slabs, and 5,000+ PSI for high-load commercial applications.

Concrete is sold in two forms: ready-mix, delivered by truck from a batch plant in cubic yards ($110–$165/yd³), and bagged dry mix (Quikrete, Sakrete) for small projects under 1 cubic yard. For projects over 1–2 cubic yards, ready-mix is significantly cheaper and less labor-intensive. Use this calculator to find your volume, then decide which delivery method makes sense.

How to Use This Concrete Calculator

This free concrete calculator helps you estimate how much concrete you need for any project — from a small patio slab to a large foundation. Select the shape that matches your project using the tabs above, enter your dimensions, and get instant results in cubic yards, cubic feet, cubic meters, and the number of bags needed.

How to Calculate Concrete (The Formula)

The concrete calculator formula is straightforward: Volume (yd³) = Length (ft) × Width (ft) × Depth (ft) ÷ 27. Divide by 27 because there are 27 cubic feet in one cubic yard, which is the standard unit concrete is ordered in. To calculate how much concrete you need, measure all three dimensions in feet before plugging them in.

For example, to calculate the concrete for a 10-foot × 10-foot patio at 4 inches thick:

Convert 4 inches to feet: 4 ÷ 12 = 0.333 ft
Multiply: 10 × 10 × 0.333 = 33.3 cubic feet
Divide by 27: 33.3 ÷ 27 = 1.23 cubic yards
Add 10% waste: 1.23 × 1.10 = 1.35 cubic yards to order

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How Many Bags of Concrete Do You Need?

The number of bags depends on the bag size. Here's a quick reference:

40 lb bag — covers 0.27 cubic feet (about 101 bags per cubic yard)
50 lb bag — covers 0.33 cubic feet (about 81 bags per cubic yard)
60 lb bag — covers 0.40 cubic feet (about 68 bags per cubic yard)
80 lb bag — covers 0.53 cubic feet (about 51 bags per cubic yard)

For projects larger than 1 cubic yard, consider ordering ready-mix concrete delivered by a truck — it is significantly cheaper per cubic yard and saves labor.

Ready-Mix vs. Bag Concrete: Which Should You Use?

Bag concrete (Quikrete, Sakrete, etc.) is ideal for small projects: fence posts, small slabs under 1 yard, repairs, and patches. It is available at any home improvement store, and you mix it yourself.

Ready-mix concrete is delivered to your site already mixed and is the best choice for large projects like driveways, foundations, or any slab over 1–2 cubic yards. It is more cost-effective and consistent than mixing bags.

Common Concrete Projects and How Much You Need

Here are typical concrete volumes for common residential projects. For detailed slab estimates including cost, see the concrete slab calculator. To find out how many bags you need for a specific project, use the concrete bag calculator. For standalone pad projects like AC pads or equipment bases, try the concrete pad calculator. For roadway and curb work, see the concrete curb and gutter calculator.

10×10 patio (4 in thick) — 1.23 yd³ / about 84 bags (60 lb)
20×20 driveway (6 in thick) — 7.41 yd³ — order ready-mix
Fence post (8 in dia, 2 ft deep) — 0.026 yd³ per post / 2 bags (60 lb)
12 in sonotube, 3 ft deep — 0.087 yd³ / about 6 bags (60 lb)

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Tips for Ordering Concrete

Always add a 10% waste factor to your calculated volume
Ready-mix trucks typically deliver a minimum of 1 cubic yard
Call your local concrete supplier for current prices — they vary by region
Have your site prepared, forms set, and rebar in place before the truck arrives — use the rebar calculator to determine the correct steel quantity
In cold weather, use hot-water mix or additives to prevent freezing before curing

Sources & References

ACI 318-19: Building Code Requirements for Structural Concrete — American Concrete Institute
ASTM C94/C94M: Standard Specification for Ready-Mixed Concrete — ASTM International
PCA: Design and Control of Concrete Mixtures (16th Edition) — Portland Cement Association

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Common Thicknesses