Add slabs, footings and round holes or columns, include a waste allowance, and see the volume and number of premix bags.
Slab length and width in feet, thickness in inches. Round holes: diameter in inches, depth in feet.
Bag yields are typical values: about 0.60 cu ft per 80 lb bag, 0.45 per 60 lb, 0.30 per 40 lb and roughly 0.012 m³ per 25 kg bag. Check the yield printed on the bag you buy.
A slab or footing is a box, so its volume is length × width × thickness. A round hole or column is a cylinder: π × (diameter ÷ 2)² × depth. The calculator converts everything to cubic feet (or cubic meters in metric mode), multiplies by the quantity of each item, adds them up and then adds your waste percentage. There are 27 cubic feet in a cubic yard and about 35.31 cubic feet in a cubic meter.
Bag counts divide the total by a typical yield for each premix size and round up to whole bags. Common U.S. premix yields are about 0.60 cu ft for an 80 lb bag, 0.45 cu ft for 60 lb and 0.30 cu ft for 40 lb. A 25 kg bag of a similar mix makes roughly 0.012 m³ (25 kg is about 69% of an 80 lb bag, and 0.69 × 0.017 m³ ≈ 0.012 m³). Products differ, so always confirm with the bag label.
Example: a 10 ft × 10 ft patio, 4 in thick, is 10 × 10 × (4 ÷ 12) = 33.33 cu ft, or 1.23 cu yd. With 10% extra that becomes 36.67 cu ft (1.36 cu yd), or 62 bags of 80 lb premix.
For more than about a cubic yard, many people compare the cost and effort of bags against a ready-mix delivery, which is usually ordered in quarter-yard steps. Enter the quote you received to compare.
How thick should a concrete slab be?
Thickness depends on the use, soil and local code. Patios and walkways are often around 4 in (10 cm), while driveways and slabs that carry vehicles are frequently thicker and may need reinforcement. Check local building requirements or ask a professional for anything structural.
How much extra concrete should I order?
Many builders add 5–10% for spills, uneven subgrade and forms that bow out. Use more for rough excavations such as post holes dug by hand.
How many 80 lb bags are in a cubic yard?
At a typical yield of 0.60 cu ft per bag, one cubic yard (27 cu ft) takes 45 bags of 80 lb premix. Using 60 lb bags it takes 60, and with 40 lb bags it takes 90.
Should I subtract the post from a fence-post hole?
For an accurate count, yes. Add the hole as one round row and the post as a second row set to the same depth, then subtract the post volume by hand, or simply use the waste allowance as a buffer. A 4×4 post takes up only a small part of a 10–12 in hole.
Does the calculator include gravel base or rebar?
No. It only covers the concrete itself. A compacted gravel base, forms, reinforcement and finishing supplies are separate.