See the calculation with sample numbers
A 10 × 12 ft rectangular slab at 4 inches thick is 40 cu ft. Adding 5% gives 42 cu ft, or about 1.56 cu yd.
What can change the result?
This is only a volume estimate. It does not determine mix, strength, reinforcement, grade, or structural design.
How this was calculated: Cubic feet and yards of concrete; clearly labeled planning estimate.