Concrete Calculator

What this calculator does

The Concrete Calculator estimates how much concrete a pour will need. It supports two common shapes: a rectangular slab or wall, and a round footing or column base. For each shape it returns the volume in cubic feet, cubic yards, and cubic meters, along with an estimated weight and the number of 60-pound and 80-pound premixed bags.

Concrete is usually ordered by the cubic yard, but small jobs are often mixed from bags. Knowing both numbers makes it easier to compare a ready-mix delivery against a stack of bags from the hardware store. The calculator also includes an optional waste allowance so the estimate can account for uneven subgrade, form deflection, and small spills.

How to use this calculator

1. Choose Slab or wall, or Round footing.
2. For a slab or wall, enter its length, width, and thickness. Length and width are in feet; thickness is in inches.
3. For a round footing, enter the diameter in feet and the depth in inches.
4. Enter the number of identical pours, the waste percentage you want, and click Calculate.

Use whole numbers for the quantity. A waste allowance between 5 and 10 percent is typical for flatwork; leave it at zero if you want the raw geometric volume shown by the reference example.

Concrete formulas

Slab or wall volume (cu ft)
  = length (ft) × width (ft) × thickness (in) ÷ 12

Round footing volume (cu ft)
  = π × (diameter (ft) ÷ 2)² × depth (in) ÷ 12

Ordered volume
  = volume × quantity × (1 + waste % ÷ 100)

Estimated weight (lb)
  = ordered volume × 133 lb per cu ft

Bags
  = weight ÷ bag size, rounded up to the next whole bag

The default example is a slab measuring 5 feet by 2.5 feet by 5 inches. Its volume is 5.21 cubic feet, or 0.19 cubic yards. At the assumed premixed density of 133 pounds per cubic foot, it weighs about 692.7 pounds. That calls for 12 bags of 60-pound mix or 9 bags of 80-pound mix.

Assumptions and limitations

• All dimensions must be positive. Length and width use feet; thickness and footing depth use inches.
• The weight estimate assumes a premixed concrete density of 133 pounds per cubic foot. Different aggregates, admixtures, and water content can change the density.
• Bag counts are rounded up to whole bags. The volume and weight outputs keep their decimal values.
• The waste percentage is applied to the geometric volume before weight and bag counts are calculated.
• The calculator does not include rebar, mesh, gravel base, form lumber, labor, delivery fees, or ready-mix minimum charges.
• A round footing is treated as a cylinder. Square or rectangular footings should be entered as a slab or wall.
• Always confirm bag yield and coverage on the product label before ordering.

Frequently asked questions

How many 80-pound bags are in a cubic yard?

One cubic yard is 27 cubic feet. At 133 pounds per cubic foot, that is about 3,591 pounds of mixed concrete, or roughly 45 bags of 80-pound mix. Bag yield varies by product, so treat this as an estimate.

Should I add a waste allowance?

For most flatwork, a 5 to 10 percent allowance is sensible. Uneven ground, imperfect forms, and material left in the mixer can consume several percent of the total. Add more for a sloped site or a first-time pour.

Why do the volume and bag count not match exactly?

The volume is geometric and can be fractional. Bags are sold as whole units, so the calculator rounds the bag count up. The weight row shows the fractional number of pounds that the rounded bag count is intended to cover.

Reference table

Example concrete quantities

The values are for planning. Actual yield depends on mix design, waste, and subgrade.

Source

Reference: Calculator.net — Concrete Calculator.

This calculator provides planning estimates only. Verify quantities with your concrete supplier and follow local building requirements for structural work.