Concrete Calculator (Slab and Yardage)
Work out how much concrete a slab, wall, footing, round pad or post needs, in cubic yards, cubic meters and pre-mixed bags, with waste and cost.
How it works
Pick a shape: a slab or wall (rectangle), a footing, a round slab or a column (cylinder). Enter the dimensions in feet and inches, or switch to metric for meters and centimeters, plus the number of identical pieces.
The calculator turns every dimension into feet (or meters), multiplies them into a volume, then divides cubic feet by 27 to get cubic yards. A waste allowance of 5, 10 or 15% covers spillage, uneven ground and form flex.
Results show cubic yards, cubic feet and cubic meters, a ready-mix order rounded up to the nearest quarter yard, and how many 40, 60 or 80 lb bags you need, always rounded up. Add a price per yard and a delivery fee to see the estimated cost.
Formula
Slab: ft³
Where:
- L, W
- = Length and width in feet (meters in metric)
- T
- = Thickness or depth in inches (centimeters in metric)
- D
- = Round slab diameter in feet
- d, H
- = Column diameter in inches and height in feet
- qty
- = Number of identical pieces
Worked example
A 10 ft by 10 ft patio slab poured 4 inches thick: 10 × 10 × 4 ÷ 12 = 33.33 cubic feet, and 33.33 ÷ 27 = 1.2346 cubic yards.
Adding 10% waste gives 1.3580 cubic yards, so you would order 1.5 yards of ready-mix. With 80 lb bags at 0.60 cubic feet each, that is 56 bags with no waste, or 62 bags with the 10% allowance.
Planning a Concrete Pour
Measure the finished size of the pour, not the size of the forms from outside edge to outside edge. For slabs, check the depth in several spots, because a subgrade that dips by even half an inch across a large patio can add a surprising amount of concrete. Compact the base gravel well and set the forms level before you calculate the final order.
Call your ready-mix supplier a few days ahead and ask about their minimum load, short-load fee, and how long the truck can wait on site. Have enough helpers, wheelbarrows and screeds ready, since concrete starts to set within an hour or two in warm weather. Bagged mix is easier to manage for small pads, steps and fence posts.
Common Mistakes to Avoid
The most common mistake is forgetting to convert inches to feet. A 4 inch slab is 0.333 feet thick, not 4 feet, and mixing units can make an estimate wrong by a factor of twelve. This calculator handles the conversion for you, but double check that each field uses the unit shown beside it.
Ordering exactly the calculated amount is another trap. Spillage, form bulge and an uneven base all eat into the volume, and a second delivery costs time and money. Order a little extra, plan a spot for any leftover such as a small pad or a few blocks, and always confirm thickness and reinforcement with local code.
Frequently asked questions
One 80 lb bag yields about 0.60 cubic feet, and a cubic yard is 27 cubic feet, so you need 45 bags per cubic yard. With 60 lb bags it takes 60, and with 40 lb bags it takes 90.
Related calculators
Estimate only. Check local building codes and ask your supplier before ordering, and order a little extra.