I poured a patio slab last summer. Measured the area: 12 feet by 10 feet, 4 inches thick. Calculated the volume: 12 * 10 * (4/12) = 40 cubic feet = 1.48 cubic yards. Ordered 1.5 cubic yards from the concrete plant. The truck arrived, the pour started, and I ran out with about 15% of the slab still exposed. The remaining concrete was not enough to cover the area because my calculation had not accounted for the irregular subgrade, slight over-excavation in one corner, and the concrete that adhered to the chute and wheelbarrow.
I called for a short load (0.5 yards) and paid a premium for the second delivery. The lesson cost me about $200 and three hours of panic. The math is simple. The margins are what matter.
Basic volume calculation
Concrete is sold by the cubic yard (in the US) or cubic meter. The fundamental calculation is length times width times depth, converted to the right unit.
Volume (cubic feet) = Length (ft) x Width (ft) x Depth (ft)
Volume (cubic yards) = Volume (cubic feet) / 27
For a 4-inch slab:
10 ft x 12 ft x (4/12) ft = 40 cubic feet
40 / 27 = 1.48 cubic yards
For metric:
Volume (cubic meters) = Length (m) x Width (m) x Depth (m)
A 3m x 4m slab at 10cm depth:
3 x 4 x 0.10 = 1.2 cubic meters
Common shapes
Not everything is a rectangle. Here are the formulas for common concrete pours:
Circular slab (round patio, fire pit pad):
Volume = pi x radius^2 x depth
A 10-foot diameter circle at 4 inches deep:
pi x 5^2 x (4/12) = 26.18 cubic feet = 0.97 cubic yards
Cylindrical column or pier (footings, posts):
Same formula as a circular slab, but the "depth" is the height of the column.
A 12-inch diameter pier, 42 inches deep:
pi x 0.5^2 x 3.5 = 2.75 cubic feet
For a deck with 12 piers: 12 x 2.75 = 33 cubic feet = 1.22 cubic yards.
Trapezoidal cross-section (driveways with a slope, retaining wall footings):
Volume = ((top_width + bottom_width) / 2) x height x length
A footing that is 24 inches wide at the bottom, 12 inches wide at the top, 12 inches tall, and 40 feet long:
((2 + 1) / 2) x 1 x 40 = 60 cubic feet = 2.22 cubic yards
Steps and stairs:
Each step is a rectangular box. Calculate each step individually and sum the volumes. Remember that the bottom step includes the full height of all steps below it if it is a monolithic pour.
Step 1: 36" wide x 7" rise x 11" run = 2772 cubic inches
Step 2: 36" wide x 7" rise x 11" run = 2772 cubic inches
Step 3: 36" wide x 7" rise x 11" run = 2772 cubic inches
Total: 8316 cubic inches = 4.81 cubic feet = 0.18 cubic yards
But this does not include the solid concrete underneath the steps. The actual volume depends on whether you are pouring steps on grade (with a fill underneath) or cantilevered steps (where each step is a box).
Mix ratios and what they mean
If you are mixing concrete yourself (from bags or raw materials), the mix ratio determines the strength and workability.
The standard mix ratio for general-purpose concrete is:
1 : 2 : 3 (cement : sand : gravel) by volume
With approximately 0.5 parts water (water-to-cement ratio of 0.5 by weight).
A lower water-to-cement ratio produces stronger concrete but is harder to work. A higher ratio is easier to pour but weaker and more prone to cracking. The temptation to add extra water on a hot day is almost irresistible, and it ruins the concrete every time.
Common mix strengths:
- 2,500 PSI: general use, patios, sidewalks
- 3,000 PSI: driveways, garage floors
- 3,500 PSI: foundations, structural applications
- 4,000+ PSI: commercial, high-load applications
When ordering ready-mix, you specify the strength (e.g., "3,000 PSI mix") and the plant adjusts the ratio. When mixing from bags, the bag label specifies the strength. An 80-pound bag of standard concrete mix yields approximately 0.6 cubic feet.
Bags needed = Volume (cubic feet) / 0.6
For a 40 cubic foot slab: 40 / 0.6 = 67 bags. At 80 pounds each, that is 5,360 pounds of concrete. For anything over about 20 cubic feet, ordering ready-mix from a plant is faster, cheaper, and produces a more consistent result.
Why you always order extra
Subgrade irregularity. Even carefully graded soil has variations. A half-inch dip across a 10-foot span does not look like much, but it adds volume.
Formwork displacement. Forms flex under the weight of wet concrete. If your 4-inch form bows outward by a quarter inch on each side, the slab is now 4.5 inches thick in the middle.
Waste. Concrete sticks to tools, wheelbarrows, chutes, and forms. Some is spilled. Some is used for small tasks while you have it (filling a post hole, leveling a step).
Measurement error. A tape measure at an angle reads long. A depth measurement taken at the high point of the subgrade underestimates the average depth.
The industry standard is to order 5-10% extra for simple, regular shapes and 10-15% extra for complex shapes, slopes, or rough subgrades. I now order 10% extra for everything after the patio incident. Returning unused ready-mix is wasteful, but running short mid-pour is worse because a cold joint (where fresh concrete meets partially set concrete) is a structural weak point.
Common mistakes
Confusing cubic feet with cubic yards. This is the most expensive mistake. A 10x10x0.33 foot slab is 33 cubic feet, which is 1.22 cubic yards. If you accidentally order 33 cubic yards, the truck shows up with 891 cubic feet of concrete and nowhere to put it.
Not accounting for rebar or mesh. Steel reinforcement displaces a small amount of concrete, but in a standard slab this is negligible (less than 1% of volume). Do not subtract for it.
Ignoring temperature. Concrete sets faster in heat and slower in cold. Below 50F (10C), the chemical reaction slows dramatically and the concrete may not reach design strength. Above 90F (32C), it sets so fast you may not finish the pour before it begins hardening. Plan your pour for the right conditions.
Mixing different batches without overlap. If you are mixing bag concrete for a slab, pour each batch while the previous one is still wet, and work them together at the seam. Do not let one section set before pouring the adjacent section.
When I need to estimate materials for a project -- whether it is a simple rectangular slab, a set of footings, or an irregular shape -- I use the concrete calculator at zovo.one/free-tools/concrete-calculator to get the volume and bag count before placing orders.
Concrete is unforgiving. Once it is mixed, the clock is ticking. Do the math right before the truck arrives, because you cannot unmix concrete and you cannot unpour a slab.
I'm Michael Lip. I build free developer tools at zovo.one. 350+ tools, all private, all free.
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