Concrete guide
How to Calculate Concrete Yards for a Slab
Convert slab dimensions and thickness into cubic yards or bag counts, then plan a practical concrete buffer.
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.
Thickness belongs in the volume calculation
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.
Convert inches of thickness to feet by dividing by 12 before multiplying by length and width. One cubic yard equals 27 cubic feet.
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.