Concrete Qty Calculator: Yards, Bags & Mix

This concrete quantity calculator works out how much concrete you need for a slab, footing, column, or wall. You enter the dimensions (length, width, and depth or height) and the calculator returns the volume in cubic yards, cubic feet, and cubic meters, plus an estimate of how many 60-lb or 80-lb bags you’ll need. It applies the standard formula volume = length × width × depth, then converts that volume into the units your supplier or store uses.

Formula

The core formula is the same for any rectangular pour:

Volume = Length × Width × Depth

• Length (L) – in feet (ft) or meters (m)
• Width (W) – in feet (ft) or meters (m)
• Depth (D) – in inches (in) or centimeters (cm), usually converted to feet or meters first

For a slab, the depth is the slab thickness. For a footing, it’s the footing height. For a column, you’d use the cross-section area (Ï€ × radius² for a round column) times the height. The calculator above handles rectangular shapes; for round columns, replace length × width with Ï€ × radius².

Conversions used:

• 1 cubic yard = 27 cubic feet
• 1 cubic foot = 0.0283168 cubic meters
• 1 cubic meter = 1.30795 cubic yards

Worked Example

Using the default values: length = 10 ft, width = 4 ft, depth = 4 in.

1. Convert depth to feet: 4 in ÷ 12 = 0.3333 ft
2. Volume in cubic feet: 10 × 4 × 0.3333 = 13.33 cu ft
3. Volume in cubic yards: 13.33 ÷ 27 = 0.49 cu yd
4. Volume in cubic meters: 13.33 × 0.0283168 = 0.38 cu m
5. 60-lb bags: 13.33 ÷ 0.45 = 29.6 → round up to 30 bags
6. 80-lb bags: 13.33 ÷ 0.60 = 22.2 → round up to 23 bags

The calculator shows 13.33 cu ft, 0.49 cu yd, 0.38 cu m, 30 bags (60-lb), and 23 bags (80-lb).

When to Use This Calculator

Use it for any rectangular concrete pour: patios, sidewalks, footings, shed slabs, or small foundations. It’s also useful for estimating the concrete needed for a set of steps (each step is a separate rectangular block) or a wall (length × height × thickness). For irregular shapes, break the area into rectangles, calculate each, and add them together.

The bag estimates assume a standard mix ratio and typical bag yield. A 60-lb bag yields about 0.45 cubic feet of wet concrete; an 80-lb bag yields about 0.60 cubic feet. These are industry-standard approximations used by most ready-mix and bagged concrete manufacturers.

If you’re ordering ready-mix from a plant, you’ll order in cubic yards. Most plants require a minimum order (often 1 cubic yard) and charge for the full yard even if you use less. For small projects, bags are usually more practical.

How to Read the Result

The first output (cubic feet) is the raw volume. The second (cubic yards) is what you’d order from a ready-mix company. The third (cubic meters) is useful if your supplier uses metric. The bag counts tell you how many bags to buy at the store.

Always round up to the next whole bag or the next quarter-yard when ordering. Concrete is sold by the whole bag or by the fraction of a yard, and you can’t return unused mixed concrete. If the calculator says 0.49 cu yd, order 0.5 or 0.75 cu yd to have a small margin.

For a slab, the depth is the thickness of the concrete after it’s placed. If you’re pouring over a gravel base, the concrete depth is measured from the top of the gravel to the top of the slab, not the total excavation depth.

Common Mistakes and How to Avoid Them

• Mixing units: The biggest error. If you enter length in feet and width in inches, the volume will be wrong. Convert everything to the same unit before calculating.
• Forgetting to convert depth: Depth is often given in inches, but length and width are in feet. Divide inches by 12 to get feet.
• Not accounting for waste: Concrete shrinks slightly when it dries, and you’ll spill or overfill forms. Add 5–10% to your final volume. For a 13.33 cu ft pour, add 0.67–1.33 cu ft, so order about 14–15 cu ft.
• Assuming bag yield is exact: Bag yields vary slightly by brand and mix. The 0.45 and 0.60 figures are close but not universal. Check the bag label if you need precision.
• Ignoring the minimum order: Ready-mix plants often have a minimum of 1 cubic yard. If your project needs 0.5 yards, you’ll pay for 1 yard. Bags might be cheaper for small jobs.

Limitations of the Formula

This formula assumes a perfectly rectangular, flat pour with uniform depth. It doesn’t account for:

• Sloped surfaces (like a ramp) – you’d need the average depth.
• Curved or irregular shapes – approximate with rectangles or use a more advanced method.
• Reinforcement (rebar or wire mesh) – these displace a tiny amount of concrete, but the effect is negligible for most projects.
• Subgrade irregularities – if the ground isn’t level, your actual depth will vary.

For anything structural (footings for a house, retaining walls, or a driveway that will bear heavy vehicles), consult a structural engineer. The calculator gives volume, not structural design.

Ready-Mix vs. Bags: Which to Choose?

For pours under about 0.5 cubic yards (roughly 40–50 bags), bags are usually more practical. You can mix them in a wheelbarrow or a small mixer, and you don’t have to worry about a truck minimum or scheduling a delivery.

For larger pours, ready-mix is faster and often cheaper per cubic yard. A typical 10×10×4-inch slab is about 1.23 cubic yards – that’s 82 bags of 60-lb concrete, which is a lot of mixing. Ready-mix delivers it ready to pour.

The calculator’s bag counts help you compare: if the number of bags is more than you can reasonably mix in a day, go with ready-mix.

Frequently Asked Questions

How many 80-lb bags of concrete do I need for a 10×10 slab?

Assuming a 4-inch depth, the volume is 10×10×(4/12) = 33.33 cubic feet. Each 80-lb bag yields about 0.60 cu ft, so 33.33 ÷ 0.60 = 55.6, meaning you’d need 56 bags. Add 5–10% for waste, so plan for 59–62 bags.

What is the difference between a 60-lb and 80-lb bag of concrete?

The weight of the dry mix. A 60-lb bag yields about 0.45 cubic feet of concrete; an 80-lb bag yields about 0.60 cubic feet. The 80-lb bag covers about 33% more volume per bag, so you need fewer bags, but each one is heavier to carry and mix.

How do I calculate concrete for a round column?

Use the formula for the volume of a cylinder: Ï€ × radius² × height. For a column 12 inches in diameter and 4 feet tall, the radius is 6 inches (0.5 ft), so volume = 3.1416 × 0.5² × 4 = 3.14 cubic feet. That’s about 7 bags of 60-lb mix.

Should I order extra concrete?

Yes, order 5–10% more than the calculated volume. This covers spillage, uneven subgrade, and slight over-excavation. For a 1-cubic-yard pour, order 1.05–1.10 cubic yards. For bags, just buy one or two extra bags.

Can I use this calculator for a footing?

Yes, a footing is just a rectangular (or trapezoidal) pour. Enter the footing’s length, width, and depth. If the footing is trapezoidal (wider at the bottom), calculate the average width and use that.

How do I convert cubic feet to cubic yards?

Divide by 27. There are 27 cubic feet in one cubic yard. For example, 54 cubic feet ÷ 27 = 2 cubic yards. This is the number you’ll order from a ready-mix plant.

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in len | Length | ft | 10 | 0
in wid | Width | ft | 4 | 0
in dep | Depth | in | 4 | 0
out vol_ft | Volume | cu ft | len * wid * (dep / 12) | 2
out vol_yd | Volume | cu yd | (len * wid * (dep / 12)) / 27 | 2
out vol_m | Volume | cu m | (len * wid * (dep / 12)) * 0.0283168 | 2
out bags60 | 60-lb bags | bags | ceil((len * wid * (dep / 12)) / 0.45) | 0
out bags80 | 80-lb bags | bags | ceil((len * wid * (dep / 12)) / 0.60) | 0