Concrete Calculator
Calculate cubic yards of concrete and equivalent bag counts for slabs, patios, footings, and other rectangular concrete pours.
How to use this concrete calculator
- Measure the length and width of the slab in feet.
- Enter thickness in inches — 4 inches for walkways and patios, 6 inches for driveways, 8+ inches for structural slabs.
- For pours under 1 cubic yard, bagged concrete is practical. Above 1 cubic yard, consider ready-mix delivery.
- Add 5–10% to your calculated volume to account for spillage, uneven subgrade, and slight measurement errors.
- Order ready-mix by the cubic yard; purchase bags by the 60 lb or 80 lb unit.
Formula
Volume = L × W × (T ÷ 12). Cubic yards = volume ÷ 27. A 60 lb bag yields ~0.45 cu ft; an 80 lb bag yields ~0.60 cu ft.
About the Concrete Calculator
Concrete is one of the most durable construction materials available to homeowners — a well-poured concrete slab can last 50 years or more with minimal maintenance. However, concrete work is also one of the most unforgiving materials for errors: once poured and set, mistakes are expensive to correct. Accurate quantity calculation prevents the twin disasters of running short mid-pour (which creates cold joints — weak planes in the slab) or ordering so much excess that disposal becomes a problem.
The conversion from cubic feet to cubic yards (divide by 27) is the fundamental concrete calculation. A 10×10-foot slab at 4 inches thick is 10 × 10 × (4/12) = 33.33 cubic feet = 1.23 cubic yards. This is close to the ready-mix minimum delivery of most suppliers, making it a borderline case between bag and truck delivery.
Concrete strength is specified in PSI (pounds per square inch) of compressive strength at 28 days. Standard residential concrete is typically 3,000–4,000 PSI. Most bagged concrete mixes provide 4,000 PSI when properly mixed and cured. Higher-strength mixes (5,000 PSI) are available for driveways or structural applications. The water-to-cement ratio is the primary determinant of concrete strength: too much water makes concrete easy to work but significantly weaker. Always follow the manufacturer's mixing instructions precisely and resist the temptation to add extra water for workability.
Control joints are planned cracks — grooves cut or tooled into fresh concrete that direct inevitable shrinkage cracking along intended lines rather than random fractures. For residential slabs, control joints should be placed every 10 feet in each direction (or roughly equal to 1.5 times the slab thickness in feet). A 4-inch slab needs joints every 6 feet; a 6-inch slab every 9 feet. Properly placed control joints produce predictable, tight cracks that are nearly invisible — missed control joints produce random, unsightly cracks that split the slab arbitrarily.
Frequently asked questions
+When should I use ready-mix concrete versus bags?
The practical break-even between bagged concrete and ready-mix is approximately 1 cubic yard. Below 1 yard: bagged concrete is more economical and practical — you control the schedule and mix on site. At 1–2 cubic yards: either approach works; consider whether you can mix and pour fast enough before the first batches start setting. Above 2 cubic yards: ready-mix is almost always the better choice — the physical labor of mixing 90+ bags of concrete is extreme, and maintaining a consistent mix quality across many bags is difficult. Ready-mix trucks typically deliver a minimum of 3–4 yards, with extra charges for smaller loads.
+What thickness of concrete do I need for different applications?
Standard concrete thickness guidelines: sidewalks and garden paths (4 inches), residential patios (4 inches), garage floors (4–6 inches for standard vehicles), driveways (6 inches for passenger cars, 8 inches for heavy trucks or RVs), footings for fences and small structures (check local codes, typically frost depth plus 12 inches), foundation walls (8 inches minimum for most residential), steps (4 inches minimum at each tread). Always check local building codes, which may specify minimum thickness and reinforcement requirements. Thicker concrete is significantly stronger — a 6-inch slab is nearly three times stronger than a 4-inch slab.
+Does concrete need reinforcement?
Reinforcement requirements depend on application and load: unreinforced concrete handles compression well but is brittle under tension and cracking. Rebar (steel reinforcing bars) dramatically increases tensile strength and prevents cracks from spreading — recommended for driveways, structural slabs, and anything supporting significant weight. Wire mesh provides crack control for flatwork like patios and sidewalks and is economical. Fiber additives (polypropylene or steel fibers) mixed into concrete reduce plastic shrinkage cracking during curing. For residential DIY projects, wire mesh in patios and rebar (at least #3 bars at 18-inch grid) in driveways represents good practice even when not strictly code-required.
+How do I prepare the subgrade before pouring concrete?
Proper subgrade preparation is critical for concrete durability. Remove all organic material (topsoil, roots, vegetation) to a depth that reaches stable mineral soil. Compact the subgrade with a plate compactor or hand tamper — loose soil settles and causes concrete to crack. Add a 4-inch compacted gravel base (crusher run or compactible gravel) over the subgrade, particularly for driveways and slabs. The gravel provides drainage and prevents frost heaving in cold climates. Level and screed the gravel layer to ensure uniform concrete thickness. In areas with expansive clay soil, a 4-inch gravel base is even more important as it provides a buffer against soil movement.
+How long does concrete take to cure and when can I use it?
Concrete reaches approximately 70% of its design strength in 7 days and is essentially fully cured at 28 days — concrete continues to strengthen for years but the practical waiting period is 28 days for full design load. Foot traffic is typically safe after 24–48 hours. Light vehicle traffic can resume after 7 days for residential applications. Full vehicle loads should wait the full 28 days. Proper curing (keeping concrete moist or covered) during the first 7 days dramatically improves final strength — concrete that dries too quickly can lose up to 30% of its design strength. Avoid pouring concrete in temperatures below 40°F or above 90°F without special precautions.