The Core Formula
Concrete volume is length × width × thickness, all in the same unit. The result is in cubic feet if you use feet throughout. To convert to cubic yards, divide by 27 — one cubic yard equals 27 cubic feet.
Example: A 10 × 10 ft patio at 4 inches thick. Convert thickness: 4 ÷ 12 = 0.333 ft. Volume = 10 × 10 × 0.333 = 33.3 cubic feet. In yards: 33.3 ÷ 27 = 1.23 cubic yards. That's the number you take to the supplier.
The 10% Overage Rule
Always order 10% more than your calculated volume. Concrete is lost to uneven subgrade (the ground is rarely perfectly flat), spillage during mixing or pouring, and the fact that slightly underfilling a form then topping it off produces a weak joint. For small bagged jobs, round up to the nearest bag. For truck orders, tell the dispatcher your calculated volume and they'll apply the standard overage.
Underestimating is the more expensive mistake: a second truck delivery carries the same minimum charge as the first.
Shape Types
Rectangular slab (patios, driveways, floors): Length × Width × Thickness ÷ 27. Most residential concrete pours use this shape.
Circular slab (round pads, hot tub bases): π × radius² × thickness ÷ 27. Use half the diameter as the radius.
Column or cylinder (fence posts, deck piers, sonotubes): Same formula as circular slab, but thickness becomes the depth of the pour. Small per post, but multiplied by 20 posts it becomes significant.
Footing (trench): Treated as a rectangular slab — length × width × depth. For perimeter footings, calculate each run separately and sum them, or treat total linear footage as the length if width and depth are uniform.
How Many Bags of Concrete
Bag coverage depends on weight: 40 lb bag ≈ 0.30 cubic feet, 60 lb bag ≈ 0.45 cubic feet, 80 lb bag ≈ 0.60 cubic feet.
One cubic yard = 27 cubic feet. A 10 × 10 patio at 1.23 cubic yards = 33.2 cubic feet needs roughly 56 × 80 lb bags, 74 × 60 lb bags, or 111 × 40 lb bags. Eighty-pound bags are the most economical per cubic foot but are physically demanding. Sixty-pound bags are the standard DIY choice.
Bags vs. Ready-Mix: When Each Makes Sense
Bagged concrete wins for projects under approximately 1 cubic yard. There's no delivery minimum, no scheduling window, no truck access requirement. Cost runs roughly $6–8 per 60 lb bag.
Ready-mix (truck delivery) wins above 1–2 cubic yards. Typical pricing is $150–200 per cubic yard delivered with a minimum order of 1 yard. At 2 cubic yards, ready-mix costs $300–400 and mixing labor drops to zero. Above 3 yards, the math strongly favors ready-mix regardless of local pricing.
Standard Slab Thickness by Application
Four inches is the residential standard for most applications. Going thinner saves material but reduces load capacity and increases cracking risk.
Curing Time
Concrete gains strength progressively after the pour. As a general guide: 24–48 hours — safe to walk on carefully. 7 days — approximately 70% strength, safe for vehicle traffic on a standard mix. 28 days — full design strength.
Cold temperatures slow curing significantly. Below 50°F curing can stall; below 40°F fresh concrete must be protected from freezing. Hot, dry conditions accelerate surface drying and cause shrinkage cracking — mist the surface or cover with wet burlap for the first few days.