Concrete Needed Calculator: How Much Concrete?

This concrete calculator works out the volume of concrete you need for a slab, footing, column, or other shape, based on the dimensions you enter. You input length, width, and depth (or diameter and depth for a column), and it returns the volume in cubic yards, cubic feet, and cubic meters, along with the number of 60-lb and 80-lb bags required. The result is an estimate—always order a little extra to account for uneven ground, spillage, or changes in depth.

Formula

The basic formula for a rectangular slab is:

Volume = Length × Width × Depth

Where:

• Length (L) – in feet (or meters)
• Width (W) – in feet (or meters)
• Depth (D) – in inches (or centimeters), then converted to feet or meters

For a round column, the formula is:

Volume = π × (Diameter/2)² × Height

All dimensions must be in the same unit before multiplying. For example, if you measure length and width in feet and depth in inches, convert depth to feet by dividing by 12 (since 1 ft = 12 in). The result is in cubic feet. To convert to cubic yards, divide by 27 (since 1 yd³ = 27 ft³). To convert to cubic meters, multiply cubic feet by 0.0283168.

Worked Example

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

1. Convert depth to feet: 4 in ÷ 12 = 0.3333 ft
2. Calculate volume in cubic feet: 10 × 10 × 0.3333 = 33.33 ft³
3. Convert to cubic yards: 33.33 ÷ 27 = 1.23 yd³
4. Convert to cubic meters: 33.33 × 0.0283168 = 0.94 m³
5. Estimate 60-lb bags: 33.33 ÷ 0.45 ≈ 74 bags
6. Estimate 80-lb bags: 33.33 ÷ 0.6 ≈ 56 bags

The calculator shows the same: 33.33 cu ft, 1.23 cu yd, 0.94 cu m, 74 bags of 60-lb, and 56 bags of 80-lb. Note that bag yields are approximate; always check the bag label.

When to Use This Calculator

Use this calculator for any project that requires concrete, such as:

• Pouring a patio, walkway, or driveway slab
• Setting fence posts or deck footings
• Creating a small shed base or workshop floor
• Filling column forms or piers

For irregular shapes, break the area into rectangles or circles, calculate each separately, and add the volumes. For a triangular slab, use (Base × Height × Depth) / 2. For a trapezoid, use ((Base1 + Base2) / 2) × Height × Depth.

How to Read the Result

The first and most important number is the volume in cubic yards, because ready-mix concrete is ordered by the cubic yard. For example, a 10×10 ft slab at 4 inches deep is about 1.23 yd³—you would order at least 1.25 or 1.5 yd³ to be safe. The cubic feet and cubic meters are useful for comparing with bag sizes or for smaller projects. The bag counts assume a standard yield: a 60-lb bag typically yields about 0.45 cubic feet, and an 80-lb bag yields about 0.6 cubic feet. Actual yields vary by mix and water added, so check the manufacturer’s label.

Common Mistakes and How to Avoid Them

• Mixing units: Always convert all dimensions to the same unit before multiplying. The most common error is using inches for depth and feet for length without converting.
• Forgetting to convert depth: Depth is often given in inches, but length and width in feet. Divide inches by 12 before multiplying.
• Rounding too early: Round only at the end. If you round 4/12 to 0.3 instead of 0.333, you’ll underestimate by about 10%.
• Not accounting for waste: Concrete can spill, settle, or be slightly more than expected. Add 5–10% to your order, especially for uneven ground.
• Assuming bag yields are exact: Bag yields vary by mix and water content. Always read the label.

Ready-Mix vs. Bags: Which to Choose?

For small projects (under about 1 cubic yard), bags are convenient and easy to handle. For larger pours, ready-mix delivered by a truck is more economical and consistent. A typical concrete truck carries about 10 cubic yards, but you can order less (some companies have a minimum, often 1–2 yards). For a 10×10 slab at 4 inches, you’d need about 1.23 yards—that’s right at the edge of what many DIYers do with bags (about 56 80-lb bags) versus ordering a short load. Mixing 56 bags by hand is a lot of work; many people prefer to rent a mixer or order ready-mix for anything over 0.5 yard. The calculator helps you compare: if you see more than 20–30 bags, consider ready-mix.

Safety and Accuracy Tips

• Wear gloves, eye protection, and a dust mask when handling dry concrete mix.
• Wet concrete is caustic; avoid skin contact and wash immediately if it gets on you.
• Check your local building codes for required slab thickness and reinforcement. For example, a driveway often needs 4–6 inches, while a shed base may only need 3–4 inches.
• For load-bearing footings, always consult a structural engineer—this calculator gives volume only, not structural specifications.
• Measure twice, pour once. Double-check all dimensions before ordering.

Frequently Asked Questions

How many 80-lb bags of concrete do I need for a 10×10 slab at 4 inches?

You need about 56 bags. The slab is 33.33 cubic feet, and each 80-lb bag yields about 0.6 cubic feet, so 33.33 ÷ 0.6 = 55.6. Round up to 56. Add a few extra bags if you expect waste.

What is the difference between a cubic yard and a cubic foot?

A cubic yard is 27 cubic feet (3 ft × 3 ft × 3 ft). Concrete is usually sold by the cubic yard for ready-mix, while bagged concrete is measured in cubic feet per bag. To convert, divide cubic feet by 27 to get cubic yards.

Can I use this calculator for a round column?

Yes, but you’ll need to use a separate formula: Volume = Ï€ × (diameter/2)² × height. This calculator is set up for rectangular slabs. For a column, you can modify the inputs by entering the diameter as length and width, but that would give a square column, not round. It’s better to use a dedicated column calculator or do the math manually.

How much concrete do I need for a post hole?

For a post hole, calculate the volume of the hole minus the volume of the post. For example, a 12-inch diameter hole 3 feet deep has a volume of π × (0.5 ft)² × 3 = 2.36 ft³. If the post is 4×4 inches (0.33 ft × 0.33 ft) and buried 3 feet, its volume is 0.33 × 0.33 × 3 = 0.33 ft³. Subtract to get 2.03 ft³ of concrete. This calculator works for the hole if you enter diameter as length and width, but remember to subtract the post volume.

Should I order extra concrete?

Yes, always order 5–10% more than the calculated volume. Ground is rarely perfectly level, forms may bulge, and some concrete may stick to the mixer or truck. For a 1.23 yd³ slab, order at least 1.3 yd³. For bags, buy one or two extra bags.

Can I use this calculator for a footing?

Yes, for a rectangular footing, enter the length, width, and depth. For a continuous footing (like under a wall), treat it as a long slab. For stepped footings, calculate each step separately and add the volumes. Remember that footings must meet local building code depth and width requirements—this calculator only gives volume, not structural specs.

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in l | Length | ft | 10 | 0
in w | Width | ft | 10 | 0
in d | Depth | in | 4 | 0
out vol_ft | Volume | cu ft | l * w * (d / 12) | 2
out vol_yd | Volume | cu yd | (l * w * (d / 12)) / 27 | 2
out vol_m | Volume | cu m | (l * w * (d / 12)) * 0.0283168 | 2
out bags60 | 60-lb bags | bags | (l * w * (d / 12)) / 0.45 | 0
out bags80 | 80-lb bags | bags | (l * w * (d / 12)) / 0.6 | 0