Concrete Calculator: How Much Concrete Do I Need?

This concrete calculator works out the volume of concrete you need for a slab, footing, column, or wall. You enter the shape and its dimensions (length, width, depth, or diameter), and it returns the volume in cubic yards, cubic feet, and cubic meters, plus an estimated number of 60-lb or 80-lb bags. The math is simple length × width × depth, but the calculator saves you from unit mix-ups and tells you exactly how many bags to order.

Formula

The core formula for a rectangular slab is:

Volume (ft³) = Length (ft) × Width (ft) × Depth (ft)

Because depth is usually given in inches, you convert it first: Depth (ft) = Depth (in) ÷ 12. The result is in cubic feet. To convert to cubic yards, divide by 27 (since 1 yard = 3 feet, and 3 × 3 × 3 = 27). For a round column, the formula is Volume = π × (Diameter/2)² × Height.

Variables:

• L = length of the slab, in feet (or meters)
• W = width of the slab, in feet (or meters)
• D = depth (thickness) of the slab, in inches (or centimeters)
• Volume = cubic feet (ft³), cubic yards (yd³), or cubic meters (m³)

Worked Example

Using the defaults: a slab 10 ft long, 4 ft wide, and 4 in deep.

1. Convert depth: 4 in ÷ 12 = 0.333 ft
2. Volume in cubic feet: 10 × 4 × 0.333 = 13.33 ft³
3. Volume in cubic yards: 13.33 ÷ 27 = 0.49 yd³
4. 60-lb bags: each covers about 0.45 ft³, so 13.33 ÷ 0.45 = 29.6 → round up to 30 bags
5. 80-lb bags: each covers about 0.6 ft³, so 13.33 ÷ 0.6 = 22.2 → round up to 23 bags

The calculator shows 13.33 ft³, 0.49 yd³, 30 bags of 60-lb mix, and 23 bags of 80-lb mix. Always round up—you cannot order a fraction of a bag, and a little extra is safer than running short.

When to Use This Calculator

Use it whenever you are pouring concrete for a home project: a patio slab, a walkway, a shed base, fence post footings, or a small retaining wall footing. It is also useful for estimating the concrete needed for a new driveway or a garage floor. Contractors and DIYers both use this basic volume calculation before ordering ready-mix concrete (which is sold by the cubic yard) or bagged mix (sold by the pound).

If you are pouring a footing that is not a simple rectangle—for example, an L-shaped foundation—split it into two rectangles, calculate each, and add the volumes together. For a circular pad (like a birdbath base or a small column footing), use the cylinder formula: π × radius² × height.

How to Read the Results

The first number, cubic feet, is the pure volume of the space you are filling. The second, cubic yards, is what you will order if you call a ready-mix plant—they deliver by the yard. The bag counts assume you are mixing by hand or with a small mixer, and they already account for the typical volume each bag yields when mixed with water.

Note that bag coverage is approximate. A 60-lb bag of standard concrete mix yields about 0.45 cubic feet; an 80-lb bag yields about 0.6 cubic feet. These are industry-standard approximations, but actual yield can vary slightly by brand and by how much water you add. Always buy one or two extra bags for small jobs—you can return unopened bags to most home centers.

Common Mistakes and How to Avoid Them

• Mixing units: The most common error is entering depth in inches but length in feet without converting. Always convert everything to the same unit first.
• Forgetting the 27: There are 27 cubic feet in a cubic yard, not 3. A slab 3 ft × 3 ft × 3 ft is 27 ft³, which is exactly 1 yd³.
• Rounding down: Never round bag counts down. Concrete shrinks slightly as it cures, and you will waste time if you run out mid-pour.
• Ignoring waste: For a slab on uneven ground, add 5–10% extra volume to account for spillage, uneven subgrade, or forms that bulge slightly.
• Overestimating bag yield: A 60-lb bag does not make a full cubic foot—it makes less than half. Check the bag’s label if you are unsure.

Ready-Mix vs. Bagged Concrete

For small projects (under about 1 cubic yard), bagged concrete is usually more practical. You can mix it in a wheelbarrow or a small electric mixer, and you control the consistency. For larger pours (a driveway, a large patio, a foundation), ready-mix from a truck is faster and cheaper per cubic yard, but you must order the right amount because you cannot easily return the excess.

Ready-mix is typically ordered in quarter-yard increments. If your calculation says 2.3 cubic yards, you will likely order 2.5 yards. The truck holds a minimum load (often 1 yard), so for very small jobs, bagged mix is the only sensible option. Also, ready-mix needs a crew to spread and finish it quickly—concrete sets in about 30–60 minutes, depending on temperature and additives.

Limits of the Formula

This calculator gives the theoretical volume of a solid rectangular or cylindrical space. It does not account for:

• Reinforcement: Rebar or wire mesh takes up space, but the volume is negligible in most cases—you can ignore it for estimating.
• Slope: If your slab is sloped for drainage, the depth varies. Use the average depth or the deepest point if you want to be safe.
• Subgrade preparation: If you are pouring over gravel or sand, the forms may settle, and you might need more concrete than the math suggests.
• Concrete shrinkage: Fresh concrete loses a small amount of volume as water is absorbed and it cures. Adding 5% waste covers this.

For a structural footing or a load-bearing wall, always have a local engineer or building inspector review your design. The volume calculation is only one part of a safe pour—the mix design, reinforcement, and curing method matter just as much.

Frequently Asked Questions

How many 80-lb bags are in a cubic yard?

One cubic yard equals 27 cubic feet. An 80-lb bag yields about 0.6 cubic feet, so 27 ÷ 0.6 = 45 bags. You would need exactly 45 bags of 80-lb mix to make one cubic yard, though in practice you might buy 46–47 to cover waste.

How much does a cubic yard of concrete weigh?

Standard concrete weighs about 4,000 pounds per cubic yard (roughly 2,400 kg per cubic meter). That is about 2 tons. This is why ready-mix trucks are so heavy and why you should never try to move a full yard of concrete by hand.

Can I use this calculator for a round footing?

Yes, but you need to use the cylinder formula instead of the slab formula. For a round footing, measure the diameter, divide by 2 to get the radius, then use: Volume = π × radius² × height. Make sure radius and height are in the same units (both feet or both inches).

Should I order extra concrete?

Yes, for any pour, add 5–10% to your calculated volume. This covers uneven ground, spillage, and the fact that forms often bulge slightly under pressure. For a small slab, one extra bag is usually enough; for a large driveway, add half a yard to your order.

What is the difference between concrete and cement?

Cement is one ingredient in concrete. Concrete is a mixture of cement, water, sand, and gravel (aggregate). Cement is the binding agent; it is about 10–15% of the concrete mix by volume. When you buy a bag of “concrete mix,� it already contains the cement, sand, and aggregate—you just add water.

How long does concrete take to cure?

Concrete reaches about 70% of its full strength in 7 days and nearly full strength in 28 days. It is safe to walk on after 24–48 hours, but you should not drive on a new driveway for at least 7 days. Keep the surface moist for the first week to prevent cracking from rapid drying.

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in l | Length | ft | 10 | 0
in w | Width | ft | 4 | 0
in d | Depth | in | 4 | 0
out vol_ft | Volume | ft³ | l * w * (d / 12) | 2
out vol_yd | Volume | yd³ | (l * w * (d / 12)) / 27 | 2
out bags60 | 60-lb bags | bags | ceil((l * w * (d / 12)) / 0.45) | 0
out bags80 | 80-lb bags | bags | ceil((l * w * (d / 12)) / 0.6) | 0