Concrete is ordered by volume, not surface area. A slab that looks simple in square feet still needs a third measurement: thickness. Converting all three dimensions into cubic yards is the core of the estimate.
Convert thickness to feet
Length and width are normally measured in feet, while slab depth is often specified in inches. Divide inches by 12 before multiplying. A 4-inch slab is one-third of a foot thick.
Multiply length, width, and depth in feet to get cubic feet. Divide by 27 because one cubic yard contains 27 cubic feet.
- Cubic feet = length × width × (depth in inches ÷ 12)
- Cubic yards = cubic feet ÷ 27
Measure the excavation, not the drawing alone
Forms, subgrade variation, slopes, thickened edges, footings, and trenches can make the real volume different from a simple rectangle. Measure separate sections when the depth changes and add their volumes.
Do not reduce structural thickness to save material. Slab design, reinforcement, base preparation, drainage, and local code requirements belong with a qualified contractor or engineer.
Plan for approximate yield and uneven conditions
QUIKRETE describes its bag yields as approximate and notes that its calculator does not allow for uneven substrate or waste. Ready-mix orders have similar practical uncertainty because excavation is rarely perfect.
A modest extra allowance can prevent a short pour, but the right percentage depends on the job. Confirm the final quantity and minimum-load rules with the supplier.
Bagged mix versus ready-mix
Small repairs and post holes may be practical with bags. Larger slabs quickly require many bags, repeated mixing, and consistent placement. Use product-specific yield data when converting cubic volume into bag count.
The Abelost calculator reports cubic yards and an editable material price. It does not choose the concrete mix, strength, reinforcement, or delivery method.
Keep all dimensions in compatible units, split irregular pours into sections, and verify approximate yield with the selected supplier.
Frequently asked questions
How do you convert slab dimensions to cubic yards?
Multiply length and width in feet by depth in feet to get cubic feet, then divide by 27. Convert an inch depth to feet by dividing it by 12 before multiplying.
How much extra concrete should be ordered?
There is no universal allowance. Uneven excavation, forms, thickened edges, and supplier rules affect the amount, so use a modest project-specific buffer and confirm the order with the supplier.
Sources and further reading
Sources support the methods and practical guidance above. Product specifications and local requirements should take priority for a specific project.
QUIKRETE Concrete Calculator Bag calculations, common slab depths, and approximate-yield caveats. ↗ QUIKRETE Concrete Mix Available package sizes and common general-concrete applications. ↗