Construction Material Quantity Calculator

Rectangular pours (slabs, footings, gravel, and topsoil) use length × width × thickness. Round columns use the cylinder formula, π × radius² × height. The result is converted from cubic feet to cubic yards, then a waste percentage is added to cover spillage and site irregularities.

• Rectangular volume (cu ft) = length (ft) × width (ft) × [thickness (in) ÷ 12]
• Column volume (cu ft) = π × [diameter (in) ÷ 12 ÷ 2]² × height (ft)
• Total volume = unit volume × number of units × (1 + waste % ÷ 100)
• Cubic yards = total cubic feet ÷ 27
• Concrete bags = ceil(total cubic feet ÷ bag yield in cubic feet)
• Aggregate tons = cubic yards × estimated density (gravel 1.4, topsoil 1.1 tons per cubic yard)

The volume figure is your raw geometry plus the waste allowance, converted into the units you order in: cubic yards, bags, or tons. The cost is an estimate from the unit price you entered, before delivery or pump charges.

Common mistakes and assumptions

• Entering thickness in inches where the field expects a different unit — check the suffix on the input.
• Ordering the raw volume with no waste allowance; spillage, over-excavation, and uneven subgrade all consume material.
• Treating tons and cubic yards as interchangeable. Conversion depends on the material density used here, and real loads vary with moisture.

A homeowner is pouring a 20 ft by 10 ft concrete patio slab, 4 inches thick, ordering ready-mix at $150 per cubic yard with a 10% waste allowance.

1. 1Volume = 20 ft × 10 ft × (4 ÷ 12 ft) = 66.67 cu ft.
2. 2With a 10% waste allowance: 66.67 × 1.10 = 73.33 cu ft.
3. 3Cubic yards = 73.33 ÷ 27 = 2.72 cu yd.
4. 4Estimated cost = 2.72 cu yd × $150 = about $407.

How much concrete do I need for a slab?

Multiply the slab's length and width in feet by its thickness converted to feet (thickness in inches ÷ 12), which gives cubic feet. Divide by 27 to get cubic yards, then add 5-10% waste for spillage and subgrade unevenness before ordering.

How many 80 lb bags of concrete equal a cubic yard?

An 80 lb bag yields about 0.60 cubic feet of mixed concrete, and a cubic yard is 27 cubic feet, so it takes roughly 45 bags of 80 lb mix to make one cubic yard — before any waste allowance.

Should I order ready-mix or use bagged concrete?

Ready-mix is usually cheaper and faster for anything over about 1 cubic yard, but most trucks carry a minimum load charge. Small footings, post holes, or repairs under a yard are often more economical and less wasteful with bags.

How do I calculate concrete for a round column or post?

Use the cylinder formula: π × radius² × height, where the radius is half the diameter converted to feet. This calculator does that conversion automatically when you select the round column material and enter diameter and height.

How many tons of gravel do I need?

Find the volume in cubic yards from your length, width, and depth, then multiply by an estimated density of about 1.4 tons per cubic yard for typical crushed gravel. Densities vary by material and moisture, so confirm the supplier's conversion factor for an exact order.

Why does the calculator add a waste allowance?

Real pours rarely use exactly the theoretical volume — subgrade isn't perfectly flat, forms shift slightly, and some material spills or is left in the mixer or wheelbarrow. A 5-10% allowance keeps you from running short mid-pour, which is far more costly than having a little extra.