How to use the concrete calculator
- Pick the shape you are pouring: slab, footing, wall, round column or steps.
- Enter the dimensions in feet and inches (thickness and diameter are in inches). If you have several identical pieces, such as six piers, set the quantity.
- Keep the 10% waste allowance or change it. Open More options to edit the yield of each bag size if your brand lists a different number.
- Read the cubic yards to order and the number of bags for each bag size. Add a price per yard or per bag for a cost estimate.
Formula
Slab, footing or wall: cubic feet = length x width (or height) x thickness in inches / 12
Round column: cubic feet = 3.14159 x (diameter / 2 / 12)^2 x height
Steps: cubic feet = width x run x rise x n(n+1)/2, poured solid to the ground
Cubic yards = cubic feet / 27
Bags = cubic feet with waste / yield per bag, rounded up to a whole bag
There are 27 cubic feet in a cubic yard because a yard is 3 feet on each side. The waste allowance multiplies the volume by (1 + waste percent).
Worked example: a 10 ft by 10 ft patio slab, 4 inches thick
Volume is 10 x 10 x (4 / 12) = 33.33 cubic feet, which is 33.33 / 27 = 1.235 cubic yards.
With a 10% waste allowance the volume is 33.33 x 1.10 = 36.67 cubic feet, or 1.358 cubic yards to order.
In 80 lb bags with a typical yield of 0.60 cubic feet, you need 36.67 / 0.60 = 61.1, so 62 bags. In 60 lb bags (0.45 cubic feet) it is 81.5, so 82 bags.
At $150 per cubic yard the ready-mix cost would be 1.358 x $150 = $203.70, before delivery fees.
Ready-mix or bags
Bagged mix is practical for small jobs such as a fence post, a small pad or a set of steps. A single 80 lb bag makes roughly 0.6 cubic feet, so a full cubic yard takes about 45 bags, which is a lot of mixing and lifting. For larger pours, ready-mix delivered by truck is usually easier and more consistent.
Suppliers set their own minimums, delivery charges and ordering increments, and many sell ready-mix in fractions of a yard. Ask your supplier how they round and whether a short-load fee applies, then compare the cost line here with their quote.
Why a waste allowance matters
Subgrades are rarely perfectly flat, forms bow a little under load and some concrete is lost to spillage or stays in the wheelbarrow. A few percent of extra material is cheap insurance, because running short in the middle of a pour can leave a cold joint that weakens the slab. This tool starts at 10% and lets you change it to match your site.
Steps and columns
The steps option assumes each step is poured solid down to the ground, so the first step is one rise tall, the second two rises tall and so on. If your steps sit on a landing, over a hollow base or on fill, subtract that volume or calculate the landing separately as a slab. Round columns use the cylinder formula, so enter the diameter of the form tube in inches.
Assumptions and limits
- Bag yields are typical published figures: 80 lb about 0.60, 60 lb about 0.45, 50 lb about 0.375 and 40 lb about 0.30 cubic feet. Yields differ by brand and product, so edit them to match the label on your bag.
- The shape is treated as a solid with square corners and a uniform thickness. Reinforcement, sloped surfaces, rounded edges and thickened edges are not modeled.
- The waste allowance is an input you control. Nothing here tells you what allowance your site needs.
- Cost uses the price you enter and ignores delivery, pump, short-load, tax and disposal charges.
- This is a planning estimate. Confirm quantities with your supplier and follow local building codes and your engineer or plan for thickness, reinforcement and mix specifications.
Frequently asked questions
How many cubic feet are in a cubic yard of concrete?
A cubic yard is 27 cubic feet, because it measures 3 feet by 3 feet by 3 feet. Divide cubic feet by 27 to convert.
How many 80 lb bags make a cubic yard?
Using the typical 0.60 cubic foot yield, one cubic yard needs 27 / 0.60 = 45 bags of 80 lb mix. Your brand may list a slightly different yield, which you can edit under More options.
Should I order extra concrete?
Most people add a margin for uneven ground and spillage. This calculator starts with 10 percent and shows the exact figure too, so you can decide how much of that margin to keep.
Does this work for a driveway or a garage slab?
Yes, use the slab shape and enter the length, width and thickness. For a very large pour, ask your supplier about delivery increments and whether the base needs a different thickness in different areas.
How do I measure a slab that is not a rectangle?
Split it into rectangles and run the calculator once for each, or measure its area with the square footage calculator and multiply by thickness in feet.