Free Concrete Calculator

Calculate exactly how much concrete you need for any project

Dimensions

Length

Width

Thickness

Quantity

You need

1.23 yd³

33.33 ft³ • 0.94 m³

Add 10% for waste and spillage

Most contractors recommend 10% extra

Bags Needed

113

40 lb bags

60 lb bags

Popular

80 lb bags

Based on standard yield: 40lb = 0.011 yd³, 60lb = 0.017 yd³, 80lb = 0.022 yd³

Estimated Cost

Price per 80lb bag:

$370.50

Prices based on average US retail. Edit to match your local prices.

Total weight: 5,000 lbs (2,268 kg)

Based on standard concrete density of 150 lbs/ft³

Need to calculate multiple sections?

Pro Tips

Standard residential slab thickness is 4 inches. Use 5-6 inches for driveways and garage floors.

For slabs over 1 cubic yard, ordering ready-mix is more cost-effective than mixing bags.

Add wire mesh or rebar reinforcement for slabs larger than 8×8 feet.

Concrete reaches 90% of its strength in 28 days. Keep it moist for the first 7 days for best results.

Always compact and level your sub-base before pouring. A 4-inch gravel base is standard.

How to Calculate Concrete for Your Project

Calculating the right amount of concrete is essential for any construction project. Too little means a second trip to the store or an additional delivery; too much means wasted money and material. The basic formula is simple: multiply length by width by thickness to get volume, then convert to cubic yards (the standard ordering unit in the US).

Common Project Examples

10×10 patio slab (4 inches thick): 10 × 10 × 0.333 = 33.3 cubic feet = 1.23 cubic yards. You would need about 56 bags of 80lb concrete mix, or roughly one cubic yard of ready-mix.

Fence post holes (8 posts, 12″ diameter, 3 feet deep): Each post needs about 0.087 cubic yards. Total for 8 posts: 0.7 cubic yards, or about 32 bags of 80lb mix.

Sidewalk (4 feet wide, 30 feet long, 4 inches thick): 4 × 30 × 0.333 = 40 cubic feet = 1.48 cubic yards. This is right at the threshold where ready-mix becomes more practical than bags.

Bags vs. Ready-Mix: When to Use Each

Use bags for small projects under 1 cubic yard: fence posts, small pads, repairs, and patches. Bags are convenient because you can mix only what you need and work at your own pace.

Use ready-mix for anything over 1-2 cubic yards: driveways, patios, foundation walls, and large slabs. Ready-mix is delivered pre-mixed in a truck, saves hours of manual mixing, and produces more consistent concrete. Most suppliers have a minimum order (typically 1 yard) and charge extra for small loads.

Common Mistakes to Avoid

Not adding waste factor: Always order 5-10% more than your calculated amount. Uneven ground, form imperfections, and spillage add up fast.
Mixing up units: Thickness is usually specified in inches (like a “4-inch slab”) but needs to be converted to feet for the volume formula. 4 inches = 0.333 feet.
Forgetting sub-base preparation: Concrete volume calculations assume level ground. If your ground is uneven, you may need significantly more concrete to fill low spots.
Not accounting for displacement: If you are placing rebar, wire mesh, or other reinforcement, the concrete volume needed does not decrease — these elements are embedded within the concrete.

Frequently Asked Questions

How many bags of concrete do I need for a 10×10 slab?

For a standard 4-inch thick 10×10 foot slab, you need approximately 1.23 cubic yards of concrete, which equals about 56 bags of 80lb mix, 73 bags of 60lb mix, or 112 bags of 40lb mix. For a project this size, ordering one yard of ready-mix delivery is typically more cost-effective and much less labor-intensive than mixing bags by hand.

How much does a cubic yard of concrete weigh?

A cubic yard of concrete weighs approximately 4,050 pounds (about 1,837 kg). This is based on the standard concrete density of 150 pounds per cubic foot. Wet concrete is slightly heavier than cured concrete due to water content.

What's the difference between 60lb and 80lb bags?

The difference is simply the amount of concrete mix per bag. An 80lb bag yields about 0.6 cubic feet (0.022 cubic yards), while a 60lb bag yields about 0.45 cubic feet (0.017 cubic yards). The 80lb bags are more cost-effective per cubic foot and require fewer bags for a given project, but they are heavier to carry and mix.

How thick should a concrete slab be?

For standard residential use: 4 inches for patios and walkways, 5-6 inches for driveways and garage floors, and 6-8 inches for heavy equipment areas. Always check local building codes, as requirements vary by jurisdiction and intended use.

How long does concrete take to cure?

Concrete reaches about 70% of its rated strength in 7 days and 90% at 28 days. However, initial set occurs within 24-48 hours, after which you can typically walk on it. Full curing continues for months. Keep concrete moist for the first 7 days for optimal strength development.

Should I order bags or ready-mix concrete?

As a rule of thumb, use bags for projects under 1 cubic yard (about 45 bags of 80lb mix). For anything larger, ready-mix delivery is more practical, produces more consistent results, and is usually more cost-effective once you factor in the labor of hand-mixing dozens of bags.

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