How to use the concrete calculator
- Choose slab for patios, pads, walkways and shed bases, or round post holes for fence posts, deck footings and tube forms.
- Enter dimensions. Slab length and width are in feet and thickness is in inches (4 in is common for walkways and patios). Holes are in inches.
- Keep a waste allowance. Excavations are rarely perfectly flat, and some concrete is always lost, so 5–10% is a sensible cushion. Set it to 0 to see the exact geometric volume.
- Read the cubic yards if you're ordering ready-mix, or the bag counts if you're mixing premix yourself.
How it's calculated
Hole volume (cu ft) = π × (diameter ÷ 2)² × depth ÷ 1,728 × holes
Cubic yards = cubic feet ÷ 27
Bags = cubic feet ÷ yield per bag, rounded up
The tool assumes a yield of about 0.45 cu ft per 60 lb bag and 0.60 cu ft per 80 lb bag, typical of standard concrete mix. Check the yield printed on the brand you buy, as it can differ slightly. There are 1,728 cubic inches in a cubic foot (12 × 12 × 12), which is why the hole volume, measured in inches, is divided by 1,728.
Past roughly one cubic yard, ready-mix delivery is often easier than mixing 45 or more 80 lb bags by hand.
Worked example
Patio slab: 10 × 10 ft at 4 inches, with no waste:
- Volume: 10 × 10 × 4 ÷ 12 = 33.33 cu ft = 1.23 yd³
- 80 lb bags: 33.33 ÷ 0.60 = 55.6, so 56 bags
- 60 lb bags: 33.33 ÷ 0.45 = 74.1, so 75 bags
With the default 10% waste allowance, that becomes 36.67 cu ft, or 62 bags of 80 lb.
Fence posts: four holes, 10 in wide and 36 in deep. Each hole holds π × 5² × 36 ÷ 1,728 = 1.64 cu ft, so four hold 6.54 cu ft. That's 11 bags of 80 lb (or 15 bags of 60 lb) before waste. Remember that the post itself takes up some of the space.
FAQ
How many 80 lb bags of concrete make a cubic yard?
At about 0.60 cubic feet per bag, a cubic yard (27 cubic feet) takes 45 bags of 80 lb mix. With 60 lb bags at 0.45 cubic feet each, it takes 60 bags.
How thick should a concrete slab be?
Four inches is common for patios, walkways and shed floors. Driveways and slabs that carry heavier loads are often 5 to 6 inches. Follow local code and your project plans.
Should I subtract the post from the hole volume?
For an exact figure, yes. A 4 × 4 post (3.5 in square) takes up about 0.09 cubic feet per foot of depth. Many people skip this and treat it as extra waste allowance.
Is this calculator suitable for structural design?
No. It estimates material quantity only. Footings, reinforcement and structural slabs should follow local building codes and, where needed, an engineer's design.