Concrete Calculator: How Much Concrete Do I Need?

A 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 height for cylinders), and it returns the volume in cubic yards, cubic feet, or cubic meters, plus the number of 60-lb or 80-lb bags if you’re mixing your own. This is the fastest way to avoid ordering too little (a project-stopping mistake) or too much (a waste of money and material).

Formula

The basic formula for a rectangular slab is:

Volume = Length × Width × Thickness

Where:

• Length (L) – in feet or meters
• Width (W) – in feet or meters
• Thickness (T) – in feet or meters (convert inches to feet by dividing by 12)

The result is in cubic feet (ft³) or cubic meters (m³). To convert to cubic yards (the standard unit for ready-mix orders in the US), divide by 27 (since 1 yard = 3 feet, and 3³ = 27). For a round column, the formula is π × (diameter/2)² × height.

Worked Example

Using the default values from the calculator: a slab that is 10 ft long, 5 ft wide, and 4 inches deep.

1. Convert thickness to feet: 4 in ÷ 12 = 0.333 ft
2. Volume = 10 ft × 5 ft × 0.333 ft = 16.67 ft³
3. Convert to cubic yards: 16.67 ft³ ÷ 27 = 0.62 yd³
4. For 60-lb bags (each covers about 0.45 ft³): 16.67 ÷ 0.45 = 37 bags (rounded up)
5. For 80-lb bags (each covers about 0.60 ft³): 16.67 ÷ 0.60 = 28 bags (rounded up)

The calculator shows 16.67 ft³, 0.62 yd³, 37 bags of 60-lb, and 28 bags of 80-lb. Always round up when ordering bags – you can’t order a fraction of a bag, and it’s better to have a little extra than to run short mid-pour.

When to Use a Concrete Calculator

Use this calculator for any project where you need to pour concrete, including:

• Patio slabs and walkways
• Footings for decks, fences, or posts
• Driveway aprons or garage floors
• Column footings or pier pads
• Steps or curbs

It’s also useful when you’re comparing ready-mix delivery versus bagged concrete. Ready-mix is typically cheaper per cubic yard for large jobs, but you pay for a minimum delivery (often 1–2 cubic yards). For small jobs, bags are more practical, and the calculator tells you exactly how many to buy.

How to Read the Result

The calculator gives you three key numbers:

• Cubic feet – useful if you’re buying concrete by the bag (bags are labeled by how many cubic feet they make).
• Cubic yards – the unit used by ready-mix suppliers. One cubic yard = 27 cubic feet.
• Number of bags – based on standard 60-lb and 80-lb bags. A 60-lb bag yields about 0.45 ft³; an 80-lb bag yields about 0.60 ft³.

If you’re ordering ready-mix, round up to the nearest half or whole yard, because suppliers charge for the full yard even if you use less. If you’re buying bags, round up to the nearest whole bag, and consider buying one extra bag just in case.

Common Mistakes and How to Avoid Them

The most frequent errors when estimating concrete are:

• Mixing units – e.g., entering depth in inches but length in feet. Always convert everything to the same unit first.
• Forgetting to convert thickness – a 4-inch slab is 4/12 = 0.33 ft, not 4 ft.
• Overlooking the “round upâ€� rule – ordering exactly the calculated volume leaves no margin for spillage, uneven ground, or over-excavation. Add 5–10% extra.
• Ignoring the shape – for a cylinder (like a post hole), use the cylinder formula, not the slab formula.

Another common misconception is that a cubic yard is “one yard square� – it’s actually a cube that is 3 ft × 3 ft × 3 ft, which is 27 cubic feet. That’s why a 10×10 slab, 4 inches thick (about 1.23 yd³) looks deceptively small in the hole but produces a surprisingly large pile of concrete.

Limits of the Formula

The calculator assumes a perfectly rectangular or cylindrical shape and does not account for:

• Sloped or uneven ground – you may need more concrete to fill low spots.
• Reinforcement (rebar or wire mesh) – these displace a small amount of concrete, usually negligible.
• Waste due to spillage, over-excavation, or forms that bulge – add 5–10% to the calculated volume.
• Concrete shrinkage during curing – this is minor and typically ignored.

For complex shapes, break the project into rectangles or cylinders, calculate each separately, and add them together. For a curved edge, approximate it with a rectangle that is slightly larger than the curve – you’ll have a bit extra, which is safer than running short.

Bagged Concrete vs. Ready-Mix

Choosing between bags and ready-mix depends on project size and access:

• Bagged concrete – best for small jobs (under about 1 cubic yard). You can mix it in a wheelbarrow or a small mixer. It’s more expensive per cubic yard but you only buy what you need, and there’s no delivery fee.
• Ready-mix – best for large jobs (over 1–2 cubic yards). The cost per cubic yard is lower, but you’ll pay a delivery charge and usually a minimum order fee. You also need a place for the truck to park and a way to move the concrete (wheelbarrow, pump, or chute).

For a typical homeowner, the calculator’s bag count is the most useful number. For a contractor, the cubic yards figure is what goes on the order form.

Frequently Asked Questions

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

Using the formula: 10×10×(4/12) = 33.33 ft³. Each 80-lb bag makes about 0.60 ft³, so you need 33.33 ÷ 0.60 = 55.6 bags, rounded up to 56 bags. Add one or two extra for waste, so buy 57–58 bags.

What is a cubic yard of concrete in square feet?

A cubic yard is 27 cubic feet. If you pour it 4 inches thick (1/3 ft), it covers 27 ÷ (1/3) = 81 square feet. If you pour it 6 inches thick (0.5 ft), it covers 27 ÷ 0.5 = 54 square feet. The thickness determines the coverage.

Should I add extra concrete to the calculator result?

Yes, add 5–10% for waste, spillage, uneven subgrade, and over-excavation. For a small job, buy one extra bag. For a large job, order an extra half yard. It’s cheaper to have a little leftover than to pay for a second delivery or a mid-pour trip to the store.

How do I calculate concrete for a round post hole?

Use the cylinder formula: Volume = π × (diameter/2)² × depth. For example, a 12-inch diameter hole that is 3 feet deep: radius = 6 inches = 0.5 ft, so volume = 3.14 × 0.5² × 3 = 2.36 ft³. That’s about 0.09 yd³, or roughly 4 bags of 60-lb mix.

What is the difference between concrete and cement?

Cement is a powder that acts as a binder; concrete is the finished material made by mixing cement, water, sand, and gravel. You don’t buy “cement� for a slab – you buy concrete mix, which already contains the cement and aggregates. The calculator estimates the volume of concrete, not the amount of cement powder.

Can I use the calculator for a footing that is not perfectly rectangular?

Yes, but you’ll need to break the footing into rectangular or cylindrical sections, calculate each with the same formula, and add them together. For a stepped footing, calculate each step separately. For a trapezoidal shape, use the average width. The result will be slightly approximate, so add extra for safety.

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in l | Length | ft | 10 | 0
in w | Width | ft | 5 | 0
in d | Depth | in | 4 | 0
out v_ft | Volume | cubic ft | l * w * (d / 12) | 2
out v_yd | Volume | cubic yd | v_ft / 27 | 2
out bags60 | 60-lb bags | bags | ceil(v_ft / 0.45) | 0
out bags80 | 80-lb bags | bags | ceil(v_ft / 0.60) | 0