Concrete quantity toolsCalculate the space concrete will fill
The measured concrete volume includes the shapes you are filling, minus openings or other spaces that will not contain concrete. Count overlapping areas only once. A waste allowance is extra material added separately. Keeping these quantities separate explains why an order may be larger than the measured space.
For a round opening inside a round form, subtract the opening’s cross-sectional area from the outer area, then multiply by the common height. The opening must extend through that entire height. A post or object that extends through only part of the height does not displace the full-height volume.
Avoid counting concrete twice
If a slab section contains 40 cubic feet and a separate section adds 8 cubic feet, the total is 48 cubic feet. For a thickened edge already included at the main slab thickness, add only the extra depth. Adding its full depth again would count part of the edge twice.
List each section, its dimensions, and whether you add or subtract it. If a dimension changes, update that section’s volume. Do not hide a measurement change by adjusting a waste percentage elsewhere.
Add a waste allowance only once
The volume calculators do not add a waste allowance. If you include one, calculate adjusted volume = measured volume × (1 + allowance percentage ÷ 100). For example, 48 ft³ with an illustrative 7% allowance becomes 51.36 ft³. Seven percent is an example, not a recommendation.
If a selected product yields an approximate 0.60 ft³ per bag, 51.36 ÷ 0.60 = 85.6, so the purchase estimate rounds up to 86 bags. At that stated yield, those bags represent 51.6 ft³. Keep measured volume, allowance-adjusted volume, and purchase quantity separate. Apply any allowance before converting to bags or another purchase unit, and do not add it again after rounding.