Concrete Volume Calculator
Concrete volume for a slab, footing or column in cubic yards and cubic metres, with a realistic waste factor and bag counts from published yields.
Volume, plus the part most calculators leave out: waste. Ordering the theoretical volume is how you end up short at the far end of a pour.
| Theoretical volume | — |
|---|---|
| With waste | — |
| 60 lb bags (0.45 ft³ each) | — |
| 80 lb bags (0.60 ft³ each) | — |
The waste factor is subgrade tolerance, not spillage. NRMCA's Concrete in Practice 8 does the arithmetic: an eighth of an inch of extra depth on a 4 in. slab is a 3% shortage, one cubic yard on a 32-yard order. ACI 117‑10 §4.4.5 lets the fine grade under a slab on ground sit ±3/4 in. off. That is six of those eighths, which is why the default in this box is 10% and not zero.
It only runs one way. §4.5.4 holds slab‑on‑ground thickness to −3/8 in. on the average of samples and −3/4 in. on any one. No plus tolerance is stated, and the standard's optional checklist offers that choice for suspended slabs and not for these. R4.4.5 sets the ±3/4 in. grade tolerance so the slab can meet §4.5.4. The rule is symmetric, the target is one-sided, and nobody checks the plus half. CIP 8's ASTM C94 note puts over‑excavation among things none of which are the responsibility of the producer. Concrete is sold by the volume that leaves the drum, so the extra depth goes on your ticket.
Percent is the wrong unit for it. A grading crew works in inches. This box asks for a percentage. Extra depth over thickness: 3/8 in. of low grade is 9.4% on a 4 in. slab and half that on 8 in. The thin slab is the exposed one, and even 15% here buys 0.6 in. against the 3/4 in. §4.4.5 permits. Footings sit higher for another reason. §3.5.2 lets a foundation cast against soil run 3 in. over plan width on a 2 ft dimension, and 1/2 in. under.
Running short is the expensive half. ACI defines a cold joint as a discontinuity from a delay long enough to preclude bonding. That is a plane through the slab nobody detailed and nobody doweled. CIP 8 says to re‑estimate near the end of a placement and adjust the last two or three loads, rather than wait on a short load after the plant closes. Make that call while the plant is still open.
Bag yields are Quikrete's published figures (Technical Data Sheet #1101): an 80 lb bag yields 0.60 ft³, a 60 lb bag 0.45 ft³. That is 45 and 60 bags per cubic yard. Other brands vary, so check the bag. Conversions: 1 yd³ = 27 ft³, 1 ft³ = 0.0283168 m³.
Worked example
How much concrete for a 20 × 12 ft slab, 4 inches thick?
- Volume = 20 × 12 × (4 ÷ 12) = 80 ft³.
- 80 ÷ 27 = 2.96 yd³.
- With the typical 10% slab allowance: 3.26 yd³.
2.96 cubic yards net, 3.26 with waste — order 3.5 from the plant. In bags it is the number the page prints from Quikrete's published yields, and it is a lot of bags.
4 values on this page were driven in a browser and compared with independently derived figures on 2026-10-29 (2808 values across 228 calculators site-wide).
Questions
How many cubic yards for a 20 × 12 ft slab, 4 inches thick?
2.96 yd³ net, about 3.26 with a 10% allowance. Order 3.5.
How many 80 lb bags in a cubic yard?
45, from Quikrete's published yield of 0.60 ft³ per 80 lb bag (27 ÷ 0.6). A 60 lb bag yields 0.45 ft³, so 60 to the yard.
Why is the default waste 10% and not zero?
Because it pays for the subgrade, not for spillage. ACI 117-10 §4.4.5 lets the fine grade under a slab sit ±3/4 in. off, and NRMCA's CIP 8 works out that an eighth of an inch of extra depth on a 4 in. slab is a 3% shortage. Six of those eighths fit inside the tolerance.
What happens if the concrete order comes up short?
A cold joint: ACI defines it as a discontinuity from a delay long enough to stop the concrete bonding, a plane through the slab nobody detailed. CIP 8's remedy is to re-estimate near the end of the placement and adjust the last two or three trucks while the plant is still batching.