Planning sequence
Start with volume, not square footage
Concrete is ordered by volume. Length and width give area, but thickness turns the project into cubic feet or cubic yards. Always convert inches to feet before multiplying. A 4 inch slab is one third of a foot thick.
Separate slab, footing, and post-hole shapes
Flat slabs, thickened edges, continuous footings, piers, and round post holes use different formulas. Estimate each shape separately, then add them together before applying waste.
Pick bagged concrete or ready-mix early
Bagged concrete can work for small patches, posts, and small pads. Ready-mix delivery is usually easier for larger slabs because hand mixing many bags is slow and physically demanding.
Plan the base and forms before ordering
Concrete volume assumes the form depth is accurate and the base is level. Soft soil, low spots, thick edges, and uneven digging can quickly increase volume.
Planning assumptions
| Cubic yard conversion | 1 cubic yard equals 27 cubic feet. |
|---|---|
| 4 inch slab factor | Use 0.333 ft thickness for rough planning. |
| Default waste | Use 5-10% for many small slabs; use more for uneven grade or holes. |
| Bag yield | Check the actual bag label; yield varies by product and bag size. |
Common mistakes
Forgetting thickened edges
A slab with deeper edges needs extra concrete beyond the flat slab volume.
Estimating holes as perfect cylinders
Post holes are often wider or deeper than planned, especially in loose soil.
Ordering exact volume
Short concrete is difficult to fix during a pour. Add practical waste before ordering.
Related measuring guides
Worked examples
Concrete Estimate for a 10x12 Slab at 4 Inches
Worked concrete estimate for a 10x12 slab with 4 inch thickness, cubic yards, waste, and bag count.
View worked exampleConcrete Estimate for a 12x16 Slab at 4 Inches
Worked concrete estimate for a 12x16 four-inch slab with cubic yards, waste, and 80 lb bag count.
View worked exampleConcrete Estimate for an 8x10 Shed Slab
Worked estimate for an 8x10 shed slab at 4 inches thick with cubic yards, waste, and bag count.
View worked exampleFAQ
How do I estimate concrete for a slab?
Multiply length by width by thickness in feet, divide by 27 for cubic yards, then add waste.
When should I use ready-mix?
Ready-mix is often better when the bag count becomes high or the pour needs to be placed quickly.
Does the concrete calculator size reinforcement?
No. Rebar, mesh, base, and structural requirements depend on the project and local conditions.