Slabs: convert thickness before multiplying
For a rectangular slab measured in feet with thickness in inches:
Volume in cubic feet = length × width × thickness ÷ 12
Volume in cubic yards = cubic feet ÷ 27
The division by 12 converts inches to feet. The division by 27 converts cubic feet to cubic yards. Omitting either conversion can create a large ordering error.
Measure the planned dimensions and distinguish nominal slab thickness from actual excavation depth. A material-volume estimate does not determine suitable thickness, reinforcement, subbase, or load capacity.
Worked example: a 20-by-12-foot slab
Assume a slab 20 feet long, 12 feet wide, and 4 inches thick. These dimensions are an arithmetic example, not a construction specification.
| Step | Calculation | Result |
|---|---|---|
| Area | 20 × 12 | 240 sq ft |
| Thickness in feet | 4 ÷ 12 | 0.3333 ft |
| Net volume | 240 × 4 ÷ 12 | 80 cu ft |
| Net cubic yards | 80 ÷ 27 | 2.963 cu yd |
| With a chosen 10% allowance | 80 × 1.10 | 88 cu ft |
| Adjusted cubic yards | 88 ÷ 27 | 3.259 cu yd |
The order quantity should reflect supplier increments, site conditions, and the project's requirements. Do not automatically round 3.259 down to 3.0. An allowance is intended to cover plausible variation, not to make an uncertain excavation measurement look exact.
Round footings and columns use cylinder volume
For a cylinder:
Volume = π × radius² × depth
If diameter is entered in inches, divide it by 24 to obtain radius in feet. If depth is entered in inches, divide it by 12 as well. Multiply the volume of one cylinder by the number of identical cylinders.
Four cylinders with a 12-inch diameter and 36-inch depth have a combined net volume of 4 × π × 0.5² × 3, approximately 9.425 cubic feet. A chosen 10% allowance brings this to approximately 10.367 cubic feet, or 0.384 cubic yards.
Irregular footings should be divided into shapes that can be calculated separately. Avoid double-counting the overlap where a slab and a thickened edge meet.
Convert volume to bags using mixed yield
Bag counts depend on the product's published yield after mixing. The same bag weight does not guarantee the same yield for every product. QUIKRETE's calculator and product information provide quantity guidance for its specified mixes. [1]
For an illustrative yield of 0.60 cubic feet per bag, the adjusted 88-cubic-foot slab requires 88 ÷ 0.60 = 146.67, or 147 whole bags. Confirm the actual product label before buying and substitute that product’s mixed yield in the calculation.
Do not add more water to stretch yield. Mix, placement, curing, and working-time requirements come from the specified product and project plan. Material-volume arithmetic does not evaluate those requirements.
Understand what an allowance covers
Uneven subgrade, variations in forms, small spills, and handling can increase required material. A complex layout or uncertain excavation may need a different allowance from a precisely formed small pour.
Keep measured volume separate from the allowance so a supplier or contractor can review both. Adding a flat 10% at several stages can overstate the order, while using no allowance can leave a pour short.
The appropriate ordering strategy also depends on delivery minimums, short-load fees, truck access, scheduling, and whether an interruption in placement is acceptable for the design.
Quantity is only one part of the job
Before ordering, confirm the mix specification, reinforcement, base preparation, drainage, weather plan, access, and qualified labor. Local requirements and structural design may determine details that no volume formula can decide.
Wet concrete can create serious skin and eye hazards. Follow the product's safety instructions and use appropriate protection. Treat a calculator result as a purchasing estimate, not as instructions for an otherwise unplanned pour.
Frequently asked questions
How many cubic feet are in a cubic yard?
There are 27 cubic feet in one cubic yard. Divide cubic feet by 27 to convert to cubic yards.
Can I calculate bags from bag weight alone?
No. Use the selected product’s stated mixed yield. The yield in a worked example is an assumption, not a guarantee for every bag of the same weight.
Does a volume estimate determine how thick my slab should be?
No. Thickness, reinforcement, subbase, and load capacity require a suitable design and applicable project requirements.
Sources & calculation notes
Primary references are linked below. Dates, limits, and product terms can change; confirm the applicable details before acting.
Use this guide thoughtfully. Quantity and budgeting guidance only. These calculations do not approve a structural design, determine code compliance, or establish a safe work method. Confirm plans, product instructions, and applicable requirements with qualified professionals.