Cement Calculator
Enter your project dimensions to find the exact volume of concrete you need, how many premix bags to buy, and the estimated material cost. Choose from slab, circular slab, footing, column, wall or steps. Switch freely between metric and imperial units and between 40 lb, 60 lb and 80 lb bag sizes.
Formula
Worked example
A 12 ft x 10 ft x 4 in slab: 12 x 10 x (4/12) = 40 ft3. With 10% waste: 40 x 1.10 = 44 ft3 = 1.63 yd3. At 0.60 ft3 per 80 lb bag: ceiling(44 / 0.60) = 74 bags.
How to calculate concrete volume
All concrete calculations start with volume in cubic feet. For a rectangular slab, multiply length x width x thickness, keeping all three in the same unit (feet). For a circular pour such as a footing pad or round slab, use pi x radius squared x depth. For a column or sonotube, the formula is the same as the circular slab but with height instead of thickness. Strip footings and walls are just long rectangles: length x width x depth. Steps use a stacked-prism formula: stair width x tread run x riser height x n(n+1)/2, where n is the number of steps, which accounts for each step adding one more prism of concrete on top of the last. Convert the result to cubic yards by dividing by 27, or to cubic metres by multiplying by 0.0283.
Premix bags vs ready-mix delivery
For pours under about 1 cubic yard (roughly 45 to 75 bags), premix bags from a hardware store are typically the most practical choice. They can be mixed in a wheelbarrow or rented drum mixer, require no minimum order and let you work at your own pace. Between 1 and 3 cubic yards, a rented concrete mixer or a short-load delivery from a ready-mix supplier becomes worthwhile. Above 3 cubic yards, a full truck of ready-mix concrete is almost always faster and cheaper per yard. Ready-mix is ordered in cubic yards; bag calculations here help you decide which route makes sense before you call a supplier. Always buy a few extra bags or order 10% more ready-mix than the raw calculation suggests, as uneven sub-base, form bowing and minor waste are unavoidable on site.
Concrete mix design and strength
Standard premix bags are generally rated at 3000 to 5000 psi after 28 days of curing. For sidewalks and residential slabs a 3000 to 3500 psi mix is usually sufficient. Driveways and garage floors benefit from 4000 psi, and structural elements such as footings and columns often call for 4000 to 5000 psi or higher, depending on local codes and soil conditions. Ready-mix suppliers offer a range of mix designs. When ordering, ask about the compressive strength, water-to-cement ratio and any required admixtures such as air-entrainment for freeze-thaw climates. This calculator computes volume only - always confirm the correct mix design with your local building department or a structural engineer before a structural pour.
Waste allowance and why it matters
A waste allowance accounts for concrete left in the mixer, spilled during placement, absorbed by a dry sub-base and the small volume taken up by reinforcement. Ten percent is the construction-industry standard for most pours. Use 5% only for small, simple pours where you can be very precise. Use 15% for complex forms, steps with many corners or pours over rough or permeable ground. Running short of concrete mid-pour is a serious problem: the partial slab will have a cold joint that weakens it. When in doubt, add more, leftover premix bags can be returned if unopened, and ready-mix suppliers can usually take back a partial yard (for a fee).
Typical slab thickness by application
| Application | Typical thickness | Notes |
|---|---|---|
| Sidewalk / footpath | 3-4 in (7-10 cm) | Residential use |
| Residential driveway | 4-6 in (10-15 cm) | Thicker for heavy vehicles |
| Patio / house slab | 4 in (10 cm) | Standard; 6 in if frost risk |
| Garage floor | 5-6 in (12-15 cm) | Heavier vehicles |
| Commercial / industrial | 6-8 in (15-20 cm) | Forklifts and heavy loads |
| Strip footing (residential) | 8-12 in (20-30 cm) | Depends on load and soil |
| Fence post base | 6-8 in (15-20 cm) | Diameter ~3x post diameter |
These are general guidelines. Consult local codes and a structural engineer for load-bearing pours.
Frequently asked questions
How many bags of concrete do I need for a 10x10 slab?
A 10 ft x 10 ft slab at 4 inches thick holds 33.3 ft3 of concrete. With a 10% waste allowance that is 36.7 ft3. You need 62 bags of 80 lb mix (0.60 ft3 each), 82 bags of 60 lb mix, or 123 bags of 40 lb mix. At 6 inches thick the numbers rise to 92, 123 and 183 bags respectively.
How do I convert cubic feet to cubic yards?
Divide cubic feet by 27 to get cubic yards, because one cubic yard is a 3 ft x 3 ft x 3 ft cube (3 x 3 x 3 = 27). This calculator does the conversion automatically and also shows cubic metres (1 ft3 = 0.02832 m3).
How much does a cubic yard of concrete weigh?
Freshly mixed standard concrete weighs roughly 3,700 to 4,100 pounds per cubic yard (about 150 lb/ft3 or 2,400 kg/m3), depending on aggregate type and moisture. This matters for structural loading estimates and for knowing the capacity of your mixer or truck.
How thick should a concrete slab be?
Residential sidewalks and patios typically use 3 to 4 inches. Driveways and garage floors are usually 5 to 6 inches to handle vehicle weight. Load-bearing slabs, foundations and commercial floors are often 6 to 8 inches or more. Always check local building codes and consult an engineer for structural applications.
What is the difference between cement and concrete?
Cement is a fine powder (usually Portland cement) that acts as the binding agent. Concrete is the finished material: cement mixed with sand (fine aggregate), gravel or crushed stone (coarse aggregate) and water. Premix bags contain cement, sand and aggregate pre-blended; you just add water. When people say "cement slab" or "cement calculator" they usually mean concrete.
How do I calculate concrete for round fence posts?
Use the column/pier mode: enter the hole diameter (typically 3 times the post diameter, so a 4-inch post gets a 12-inch hole) and the hole depth. Multiply by the number of posts in the quantity field. A 12-inch diameter, 36-inch deep hole needs about 2.4 ft3 of concrete, which is four 80 lb bags per post.