Concrete Volume and Bag Calculator
Estimate a rectangular pour in cubic feet, cubic yards, cubic meters, and whole premix bags.
Concrete Calculator (Volume & Bags)
Table of Contents
Volume from dimensions
Enter length L and width W in feet and thickness T in inches. The calculator uses V = L × W × (T ÷ 12) cubic feet, then divides by 27 for cubic yards and by 35.3147 for cubic meters. In metric mode, enter L and W in meters and T in centimeters; it first calculates L × W × (T ÷ 100) cubic meters, converts to cubic feet using 35.3147, and derives the displayed volumes from there.
For a 10 ft by 6 ft slab at 4 in thick, V = 10 × 6 × (4 ÷ 12) = 20 ft³. The calculator displays 0.74 yd³ and 0.57 m³ after rounding to two decimal places.
Premix bag estimate
The calculator assumes yields of 0.45 ft³ per 60 lb bag and 0.60 ft³ per 80 lb bag. It divides the unrounded cubic-foot volume by each yield and rounds up to a whole bag. For the 20 ft³ example, that is 45 bags of 60 lb mix or 34 bags of 80 lb mix. These yields match one manufacturer’s Concrete Mix data; confirm the yield of the product you will buy.
Shape and placement limits
The three dimensions describe one rectangular slab, trench, pad, or square column. The calculator does not model circles, tapered forms, reinforcement displacement, variable thickness, or multiple pours. Measure irregular areas as separate sections and add their volumes yourself. Bag counts and volume contain no automatic waste allowance.
Before ordering
Check actual form dimensions and depth at several points, then ask your supplier about the appropriate mix, minimum delivery, and any site-specific allowance. Do not treat the bag count as a structural mix specification. For a large pour, compare delivered concrete with the labor and equipment needed to mix individual bags.
Related calculators: cubic-yard volume, square footage, and gravel volume.
Sources
- QUIKRETE Concrete Mix product data sheet — approximate 60 lb and 80 lb bag yields.
- NIST Guide to the SI, Appendix B.9 — cubic-foot and cubic-yard conversions to cubic meters.
Last reviewed: 2026-09-28.
Frequently Asked Questions
Does the bag count include extra concrete for spillage?
No. The calculator divides the measured rectangular volume by an assumed bag yield and rounds up to whole bags. Add any allowance for uneven ground, form movement, or spillage separately after checking the actual site.
Can I enter the diameter of a round post hole?
No. All three fields describe a rectangular prism. A round hole needs a cylinder calculation using its radius and depth before choosing a concrete quantity.
Why might the bag label give a different count?
The calculator assumes 0.45 ft³ per 60 lb bag and 0.60 ft³ per 80 lb bag. Yield depends on the product and mix instructions, so use the yield on the specific bag when placing an order.