CY Calculator

Concrete Calculator

Estimate concrete in cubic yards and premix bags for slabs, footings and columns. Pick a shape, then enter the size and thickness.

Calculator

Optional: bag size, quantity, waste & cost

Yields are typical; check the bag label.

Quantity is the number of identical areas. Waste is your own allowance; blank means 0%.

Cost uses the total including waste. Per-bag cost uses whole bags.

Enter your dimensions. The result appears here instantly.

How to Calculate Concrete

  1. Measure the length and width of the slab or footing, or the diameter of a round shape.
  2. Measure the thickness (or the height of a column or pier).
  3. Choose the matching shape and enter the values. Units can be mixed: feet, inches, yards, mm, cm or m.
  4. Read the cubic yards for a ready-mix order, or the bags for the premix bag size you will use.

Concrete Formula

Slab: Cubic feet = Length × Width × Thickness (ft)

Round: Cubic feet = π × (Diameter ÷ 2)² × Height (ft)

Cubic yards = Cubic feet ÷ 27

Bags = Cubic feet ÷ Bag yield (ft³), rounded up

All dimensions must be in feet before multiplying. Thickness in inches is divided by 12. The calculator does this for you and keeps full precision until the final display.

Premix Bags per Cubic Yard

Premixed concrete is sold in bags that each make a stated volume. These are typical yields; the exact figure is printed on the bag.

Typical premix bag yields and bags needed per cubic yard
Bag sizeTypical yieldBags per cubic yard
40 lb0.3 ft³90
60 lb0.45 ft³60
80 lb0.6 ft³45

Concrete Examples

Patio slab: 10 ft × 10 ft, 4 in thick

A 4 inch thickness is 0.3333 ft, so the volume is 100 ft² × 0.3333 ft.

100 ft² × 0.3333 ft = 33.33 ft³ ÷ 27 = 1.23 yd³

About 56 bags of 80 lb premix at 0.60 ft³ each.

Footing: 30 ft long, 16 in wide, 8 in deep

Mixed units are fine: 16 in = 1.3333 ft and 8 in = 0.6667 ft.

40 ft² × 0.6667 ft = 26.67 ft³ ÷ 27 = 0.99 yd³

About 45 bags of 80 lb premix at 0.60 ft³ each.

Round pier: 12 in diameter, 4 ft tall

Choose Circle and enter the height as the thickness. Area is π × 0.5² ≈ 0.785 ft².

0.7854 ft² × 4 ft = 3.14 ft³ ÷ 27 = 0.12 yd³

About 6 bags of 80 lb premix at 0.60 ft³ each.

Garage pad with 10% allowance: 12 ft × 12 ft, 4 in thick

The allowance is your own number. Here 10% is added on top of the exact volume.

144 ft² × 0.3333 ft = 48 ft³ ÷ 27 = 1.78 yd³

With 10% allowance: 1.96 cubic yards. About 88 bags of 80 lb premix at 0.60 ft³ each.

Ordering Extra Concrete

The waste field is optional and starts at 0%, so the result is the exact calculated volume until you add an allowance. Bags are counted from the total including your allowance and rounded up.

These are planning estimates. For structural work, reinforcement or large pours, confirm quantities and specifications with your supplier or a qualified professional.

Concrete Calculator FAQ

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

A cubic yard is 27 ft³. At a typical yield of 0.60 ft³ per 80 lb bag that is 45 bags. Yields vary by brand, so check the bag label.

How thick should a concrete slab be?

Thickness depends on the use and local building rules. Patios and walkways are commonly around 4 inches, while driveways and garage floors are often thicker. Follow your local code and ask a qualified professional for structural work.

How do I calculate concrete for a round column?

Choose Circle, enter the diameter, and enter the height in the thickness field. The volume is π × (diameter ÷ 2)² × height, then divided by 27 for cubic yards.

Should I order extra concrete?

Subgrade is rarely perfectly level and some concrete is lost in handling, so people often order a little more than the exact volume. Enter your own percentage in the optional waste field; it is 0% unless you set it.

Should I use bags or ready-mix?

Premix bags suit small jobs. For larger volumes ready-mix delivered by the cubic yard is common, though suppliers may have minimum loads or delivery charges. The optional cost fields support both per-bag and per-cubic-yard prices.

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