Concrete Calculator by Square Footage

This concrete calculator works out how much concrete you need in cubic yards or cubic meters when you enter the area in square feet and the slab thickness in inches. You get the volume, plus an estimate of how many 60-lb or 80-lb bags to buy. The formula is simple: volume = area × thickness, with unit conversions built in.

Formula

The basic formula for concrete volume is:

Volume (cubic feet) = Area (square feet) × Thickness (feet)

Since thickness is usually given in inches, convert it first: thickness in feet = thickness in inches ÷ 12.

Then convert cubic feet to cubic yards (the standard unit for ordering concrete) by dividing by 27 (because 1 cubic yard = 27 cubic feet).

For bag estimates, a 60-lb bag of concrete mix yields about 0.45 cubic feet, and an 80-lb bag yields about 0.60 cubic feet. Divide the total cubic feet by that yield and round up to the nearest whole bag.

Worked Example

Let’s use the default values: a 100 sq ft area with a 4-inch thickness.

1. Convert thickness to feet: 4 in ÷ 12 = 0.333 ft
2. Volume in cubic feet: 100 sq ft × 0.333 ft = 33.3 cu ft
3. Volume in cubic yards: 33.3 ÷ 27 = 1.23 cu yd
4. 60-lb bags needed: 33.3 ÷ 0.45 = 74 bags (rounded up)
5. 80-lb bags needed: 33.3 ÷ 0.60 = 56 bags (rounded up)

So for a 10×10 slab, 4 inches thick, you need about 1.23 cubic yards of concrete, or 74 of the 60-lb bags (or 56 of the 80-lb bags).

How to Measure Square Footage for a Slab

Measure the length and width of the area in feet. For a rectangle, multiply them. For an L-shaped slab, split it into rectangles, calculate each, and add them. For a circular slab, measure the radius (half the diameter) and use πr². Always measure in the same unit—if you have inches, convert to feet by dividing by 12.

• Rectangular slab: length × width
• Circular slab: Ï€ × (radius)²
• Triangular slab: (base × height) / 2
• Irregular shapes: break into smaller rectangles or circles and sum

Choosing the Right Thickness

Thickness depends on what the concrete will support. A typical patio or walkway is 4 inches. A driveway for passenger cars is usually 4 to 5 inches. A driveway for heavy trucks or a garage floor might be 6 inches. A shed base can be 4 inches, while a foundation for a house is often 8 to 12 inches, but that’s designed by an engineer. Always check local building codes—they may specify minimums for your area.

Common Mistakes and How to Avoid Them

• Mixing units: If you measure length in feet and width in inches, you’ll get a wrong area. Convert everything to feet first.
• Forgetting the thickness conversion: Thickness is often given in inches, but the formula needs feet. Dividing by 12 is easy to forget.
• Rounding too early: Round only at the end. If you round 1.23 cu yd to 1.2, you might under-order.
• Not accounting for waste: It’s wise to order 10% extra to cover spillage, uneven subgrade, or slight miscalculations.
• Assuming bag yield is exact: Bag yield varies by brand and mix. The 0.45 and 0.60 cu ft are typical, but check the bag label.

Ready-Mix vs. Bagged Concrete

For small projects (under about 1 cubic yard), bagged concrete is convenient—you mix it in a wheelbarrow or mixer. For larger slabs, ready-mix delivered by truck is more practical and often cheaper per cubic yard. A typical concrete truck carries about 10 cubic yards, but you can order less (with a short-load fee). If your project needs more than 1–2 cubic yards, ready-mix is usually the way to go. For a 4-inch slab, one cubic yard covers 81 square feet (since 27 cu ft ÷ 0.333 ft = 81 sq ft).

Cost Considerations (General)

Concrete is sold by the cubic yard. Prices vary by region and time, but you can expect to pay more per yard for small loads. Bagged concrete costs more per cubic foot than ready-mix, but you avoid delivery fees. For a 100 sq ft slab, 1.23 cu yd is a small load—many companies have a minimum charge, so it might be cheaper to use bags. Always get quotes from local suppliers.

Safety and Preparation Tips

• Wear gloves, long sleeves, and eye protection when mixing or pouring concrete—wet concrete can cause chemical burns.
• Prepare a solid, level subgrade (compacted gravel or soil) before pouring.
• Use forms (wood or metal) to keep the concrete in place and level.
• If you add reinforcement (rebar or wire mesh), place it in the middle of the slab, not at the bottom.
• After pouring, let the concrete cure for at least 7 days—keep it moist to prevent cracking.

Frequently Asked Questions

How many square feet does a yard of concrete cover?

At 4 inches thick, one cubic yard covers 81 square feet. At 6 inches thick, it covers 54 square feet. The formula is 27 (cubic feet per yard) divided by thickness in feet.

How many 80-lb bags are in a yard?

An 80-lb bag yields about 0.60 cu ft, so 27 ÷ 0.60 = 45 bags per cubic yard. For 60-lb bags (0.45 cu ft), it’s 60 bags per cubic yard.

Can I use this calculator for a footing or column?

Yes, but you need the volume of the footing or column, not just square footage. For a column, use the formula for a cylinder (πr²h). This calculator is for slabs with a uniform thickness.

Should I order extra concrete?

Yes, order 10% more than the calculated volume to account for waste, uneven ground, or spillage. For a 1.23 cu yd slab, order 1.35 cu yd or buy 10% more bags.

What if my slab isn’t a perfect rectangle?

Break it into rectangles, circles, or triangles, calculate each area, and add them. Then use the total area in the calculator.

How long does concrete take to cure?

Concrete reaches about 70% of its strength in 7 days and full strength in 28 days. Keep it moist for the first week to prevent cracking.

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in area | Area | sq ft | 100 | 0
in depth | Thickness | in | 4 | 0
out cubic_ft | Cubic feet | cu ft | area * (depth / 12) | 2
out cubic_yd | Cubic yards | cu yd | (area * (depth / 12)) / 27 | 2
out bags60 | 60-lb bags | bags | ceil(((area * (depth / 12)) / 0.45)) | 0
out bags80 | 80-lb bags | bags | ceil(((area * (depth / 12)) / 0.60)) | 0