Miscellaneous
Concrete Calculator - Slabs, Footings, Columns & Steps
Calculate how much concrete you need for slabs, footings, round columns, or steps. Results in cubic yards or cubic meters, plus bag count for 40, 60, and 80 lb bags.
Total volume
1.23 yd³
33.33 ft³
Per unit
33.33 ft³
Bags needed (includes 10% waste)
123
40 lb bags
0.3 ft³/bag
82
60 lb bags
0.45 ft³/bag
62
80 lb bags
0.6 ft³/bag
How to calculate concrete volume
For a rectangular slab or footing, multiply length × width × thickness (converted to the same unit). For round columns, use V = π r² h. The result in cubic feet can be converted to cubic yards (divide by 27) for ordering ready-mix.
How many bags do I need?
A standard 80-lb bag of pre-mixed concrete yields approximately 0.60 cubic feet when mixed. A 60-lb bag yields 0.45 ft³ and a 40-lb bag yields 0.30 ft³. Always order 5–10% extra for waste. This calculator includes a 10% buffer in the bag count.
When to use ready-mix vs. bagged concrete
Ready-mix delivered by truck is cost-effective for pours larger than about 1 cubic yard. For smaller projects (footings, fence posts, or repair work), bagged concrete is more practical.
Slab thickness guide
| Application | Recommended thickness |
|---|---|
| Sidewalk / residential walkway | 3–4 inches |
| Residential driveway | 4 inches (light vehicles) |
| Commercial driveway / heavy vehicle | 5–6 inches |
| Structural slab / garage floor | 5–6 inches |
| Fence post footing | Diameter: 3× post width; depth: 1/3 of post height + 6” below frost line |
Reinforcement
Unreinforced concrete is strong in compression but brittle in tension. For most applications:
- Wire mesh (6×6 W1.4×W1.4): standard for residential slabs and walkways; controls cracking from shrinkage.
- Rebar: required for structural slabs, driveways subject to heavy loads, and any slab with significant tensile demands. Typically #3 (3/8”) or #4 (1/2”) rebar on a 12”×18” grid.
- Fiber reinforcement: polypropylene or steel fibers mixed into the concrete control plastic shrinkage cracking during curing. Often used instead of wire mesh for residential work.
Mixing ratios
Standard concrete mixing ratios by volume (cement : sand : coarse aggregate):
- 1:2:4 - general-purpose concrete, moderate strength (~20 MPa / 3,000 psi)
- 1:1.5:3 - stronger mix (~25 MPa / 3,600 psi), used for reinforced slabs and beams
- 1:2:3 - standard residential slab mix (roughly M20 grade)
For most DIY projects, pre-mixed bags specify the ratio on the packaging. Achieving consistency by weight (not volume) produces more reliable results.
Curing time
| Time after pour | Approximate strength achieved | Notes |
|---|---|---|
| 24–48 hours | ~15% | Form removal possible for simple slabs |
| 7 days | ~65–70% | Can support normal foot traffic |
| 28 days | ~99% | "Design strength" - rated load-bearing capacity |
| 90 days | ~110%+ | Concrete continues gaining strength slowly |
Keep concrete moist during the first 7 days. Drying too fast causes surface cracking and reduces strength.
Common failure modes
- Plastic shrinkage cracking: occurs in the first few hours as water evaporates from the surface faster than it bleeds from below. Prevent with evaporation retarder, shade, or wind breaks.
- Crazing: a network of fine surface cracks caused by premature surface drying or over-troweling. Cosmetic but affects appearance.
- Spalling and heaving from freeze-thaw: water that seeps into cured concrete expands when it freezes, breaking off the surface layer. Use air-entrained concrete (with chemical admixture) for freeze-thaw climates and apply a penetrating sealer.