Worked example: concrete volume, bags, bulk cost, and planning total
The default slab is a 10 ft by 8 ft rectangle, 4 in thick, with quantity 1. Raw volume is 10 x 8 x (4 / 12) x 1 = 26.67 cu ft.
With a 10% waste buffer, planning volume is 26.67 x 1.10 = 29.33 cu ft. In cubic yards, that is 29.33 / 27 = 1.09 yd3.
With a 0.6 cu ft bag yield, bags are ceil(29.33 / 0.6) = 49 bags. Bag material cost is 49 x $6.25 = $306.25, displayed as $306.
Bulk material cost is 1.09 x $170 = $184.69, displayed as $185. Bag-path planning total is $306.25 + $85 + $60 = $451.25, displayed as $451.
How to estimate concrete slab volume with waste and shape
For a rectangle, area is length x width. For a round pad, area is pi x (diameter / 2)^2. Volume is area x thickness x quantity, then multiplied by the waste buffer.
How this concrete calculator compares bags versus bulk material
Bags are rounded up from planning volume divided by bag yield. Bulk material cost uses the same planning volume converted to cubic yards or cubic meters, multiplied by the editable bulk price.
How to use bag yield, bag price, and reserve costs in a planning estimate
Bag-path planning total includes bag cost plus delivery or forms reserve and reinforcement reserve. It does not include labor, taxes, trucking minimums, short-load fees, pumps, permits, site prep, compaction, placement, or finishing.
How to read the concrete volume, bag count, bag cost, and bulk cost
Concrete volume is the planning volume with waste. Bags are the rounded shopping quantity. Bag cost is bag material only. Bulk cost is raw material cost before delivery and site-specific charges.
What to verify before ordering concrete
Confirm excavation depth, base material, compaction, drainage, reinforcement, access, mixing plan, finishing help, weather, cure requirements, and whether delivery minimums or short-load fees apply.