Formula
Volume is length times width times thickness. Waste is added before the bags are counted:
bags = ceil(volume × (1 + waste / 100) / bag yield)
An 80-pound bag is taken as 0.60 cubic feet and a 60-pound bag as 0.45 cubic feet, the yields printed on typical US packaged concrete. Meters use the exact foot, 0.3048 m. The slab page reports cubic yards from an inch thickness. The footing page is a strip. The column page is a cylinder. The bag page starts from yards you already know.
Assumptions
- The pour is a rectangular box.
- Length, width, and thickness are in one unit.
- Bags are rounded up. A partial bag still means buying a whole bag.
Worked example
A 10 by 10 by 0.5 foot pour with 10% waste needs 92 bags of the 80-pound mix.
Input definitions
- Thickness
- Depth in the same unit as the length. Six inches is 0.5 feet.
- Waste
- Percent added for spillage, an uneven subgrade, or a small mismeasure.
- Bag
- Packaged mix size. Check the yield printed on the brand you buy.
Limitations
Ready-mix sold by the cubic yard, fiber, and color can change the yield. This page does not price the pour or design the reinforcement.
Questions
When do I use the slab, footing, or column page?
Use the slab page for cubic yards from an inch thickness, the footing page for a strip, and the column page for round piers. This page counts bags from a rectangular box.
Does waste change the bag count?
Yes. The percent is added to the geometric volume before the bags are divided.