Concrete guide
How to Calculate Concrete Yards for a Slab
Convert slab dimensions and thickness into concrete cubic yards or bag counts with the yardage formula.
Short answer
Multiply slab length by width by thickness in feet to find cubic feet, add a buffer for real-world variation, then divide by 27 for cubic yards. For bagged concrete, divide buffered cubic feet by the yield on the bag and round up.
Use the Concrete CalculatorThe formula
- Cubic feet = length in feet x width in feet x (thickness in inches / 12)
- Buffered volume = cubic feet x (1 + buffer percentage)
- Cubic yards = buffered cubic feet / 27
- Bags = buffered cubic feet / yield per bag, rounded up
Worked example
A 12 ft by 10 ft pad at 4 inches thick
- Thickness in feet: 4 / 12 = 0.333 ft.
- Volume: 12 x 10 x 0.333 = 40 cubic ft.
- Add a 10% buffer: 40 x 1.10 = 44 cubic ft.
- Ready-mix volume: 44 / 27 = 1.63 cubic yd.
- At an assumed 0.6 cubic ft per bag: 44 / 0.6 = 73.34 bags.
Plan for about 1.6 cubic yards or 74 bags, then verify mix yield and supplier ordering rules.
Use the concrete yardage formula
Concrete is ordered by volume, not surface area. A patio and a walkway with the same square footage can need different material amounts when their required thickness differs.
The concrete yardage formula is length in feet x width in feet x thickness in feet, divided by 27. Convert inches of thickness to feet by dividing by 12 before multiplying by length and width.
Figuring concrete cubic yards from slab area
If you already know the slab area, multiply square feet by thickness in feet to get cubic feet. For example, a 120 square foot slab at 4 inches thick is 120 x 0.333, or about 40 cubic feet before buffer.
Divide cubic feet by 27 to convert to cubic yards. The calculator keeps both cubic feet and cubic yards visible because bagged concrete is usually planned from cubic feet while ready-mix is usually ordered by cubic yard.
Allow for the actual pour
Form dimensions, excavation variation, uneven subgrade, spills, and over-excavation can increase required concrete. QUIKRETE notes that product yields are approximate and its bag calculations do not allow for uneven substrate or waste; its calculator rounds bag quantities up to whole bags.
The buffer is a material planning choice, not engineering guidance. Verify slab thickness, base, reinforcement, joints, drainage, frost requirements, loads, and local permit or code requirements before building.
- Use bag yield from the specific concrete mix product data.
- Ask ready-mix suppliers about minimums, delivery fees, and order increments.
- Do not treat a volume estimate as structural design.
Bags or ready-mix
Bagged mixes can be manageable for small pads or repairs. Larger slabs may involve enough bags, mixing time, and placement risk that ready-mix is more practical. Compare the calculated volume, local delivery options, available labor, and time limits of a continuous pour.
References and verification
Use these references together with the instructions and coverage or yield information on the product you select.