Reference formulas and worked examples below keep their stated units. Monetary examples use USD; dimensional examples use US customary units unless labeled otherwise. Metric calculator inputs are converted to these reference units before calculation. Choosing another currency does not convert example amounts or exchange money.
How to use this concrete calculator
Estimate concrete volume and whole bags for a rectangular slab. Set thickness, waste allowance, and the yield printed on your bag.
How the calculation works
Cubic feet = length × width × thickness in inches ÷ 12. Multiply by 1 + allowance ÷ 100.
Divide by 27 for cubic yards. Whole bags = adjusted cubic feet ÷ yield per bag, rounded upward.
Worked example
A 10 × 10 ft slab at 4 in thick is 33.33 cu ft before allowance. At 10% extra, order 36.67 cu ft (1.36 cu yd), or 62 bags at 0.6 cu ft each.
Limits and assumptions
One rectangular slab only. Does not design reinforcement, thickness, footings, curing, or structural capacity. Bag yields vary by product.
Frequently asked questions
Does the allowance affect the bag count?
Yes. The bag count uses the volume after the allowance.
What bag size should I choose?
Enter its labeled finished yield in cubic feet; do not enter the dry bag weight.
Can I calculate several slabs?
Calculate each size and add volumes before rounding the final bag order.