Cubic feet, cubic yards, and ordering concrete

Concrete is measured with a tape in feet and inches, sold by the truck in cubic yards, and specified on metric plans in cubic meters. This guide walks the conversion chain the site's concrete calculators run: footprint area times thickness to cubic feet, cubic feet to cubic yards and cubic meters, a waste allowance, and the round-up that turns a volume into a ready-mix order or a bag count. Every figure below is computed at build time by the same engines behind the concrete slab calculator, concrete footing calculator, and concrete column calculator, so this page and those tools cannot disagree.

Three units, one volume

A cubic foot is a cube one foot on each side. A cubic yard is a cube three feet on each side, so it holds exactly 27 cubic feet. Metric volume rests on the international foot, defined as exactly 0.3048 m, which makes one cubic foot exactly 0.028316846592 m³. From those two definitions the geometry engine derives the rest: one cubic yard is 0.7646 m³, and one cubic meter is 35.31 cu ft.

Cubic yards are the unit US ready-mix is sold in, so every concrete tool here computes in cubic feet and reports cubic yards and cubic meters beside it. Nothing is rounded between the units — only the digits on screen are trimmed — and package counts always round up.

From area and thickness to cubic feet

A slab is a footprint area with a thickness. The geometry engine takes the area in square feet and the thickness in inches, converts the thickness to feet by dividing by 12, and multiplies: cubic feet = area sq ft × (thickness in ÷ 12). The methodology page's example slab makes the chain concrete. A 10 × 10 ft footprint is 100 sq ft. At 4 in thick, the thickness is 4 ÷ 12 = 0.3333 ft (one third of a foot), so the volume is 33.33 cu ft — 1.23 cu yd once divided by 27, and 0.94 m³.

Length and width set the footprint. Thickness is the input to watch: it is entered in a different unit from the other two, and it comes from the plans, not the calculator. If the footprint is not one rectangle, settle the area first with the square footage calculator and bring the total to the thickness step.

The inches-versus-feet mistake

Because thickness is entered in inches while length and width are in feet, the classic estimating error is a unit slip on that one field. Treat the 4 in slab as 4 feet thick — the engine sees 48 in — and the same 100 sq ft footprint becomes 400 cu ft, or 14.81 cu yd: exactly 12 times the correct figure. The slip runs the other way too. Type the feet value 0.3333 into the inches box and the volume collapses to 0.1029 cu yd, 12 times too small. If the cubic-yard figure looks wildly high or oddly small, check the thickness field before anything else.

Waste, then the quarter-yard order

The slab engine then runs the rest of the pipeline in a fixed order. First the waste allowance: adjusted = net × (1 + waste% ÷ 100). The default for concrete pours is 10% — a trade convention covering spillage and an uneven subgrade, adjustable in every tool. For the example slab that lifts 33.33 cu ft to 36.67 cu ft, or 1.36 cu yd.

Second, the round-up. Ready-mix suppliers commonly take orders in 0.25 cu yd steps, so the engine rounds the adjusted volume up to the next quarter yard: order = ceil(adjusted ÷ 0.25) × 0.25. The 1.36 cu yd becomes an order of 1.5 cu yd, with 0.14 cu yd as spare. Waste is applied before this round-up, never after — padding an already-rounded figure would count the margin twice. Quarter-yard ordering is a widespread convention, not a rule: confirm the increment and any short-load policy when you call the plant. Third, a cost line appears only if you enter a price per cubic yard; the site never supplies one.

Worked slab examples

Five slabs through the same chain, all at the 10% default allowance except the last, at 5%. The metric row was entered in meters and centimeters and converted exactly (0.3048 m per foot, 2.54 cm per inch) before the engine ran.

The 24 × 24 ft pad at 5 in shows how area dominates: 576 sq ft at 5 ÷ 12 ft is 240 cu ft, 8.89 cu yd net, and an order of 10 cu yd with only 0.22 cu yd of rounding.

The 6 × 4 ft pad at 4 in is the opposite case: 8 cu ft, just 0.3 cu yd net and 0.33 cu yd with waste. Quarter-yard ordering rounds that to 0.5 cu yd, so 0.17 cu yd of the order is spare — the size of job where bags make sense. The same 8 cu ft with the same allowance is 8.8 cu ft to mix, or 30 bags of 40 lb, 24 bags of 50 lb, 20 bags of 60 lb, 15 bags of 80 lb mix at the manufacturer's yields.

The 5 × 4 m slab at 10 cm comes out the way it went in: 2 m³ net, held by the engine as 70.63 cu ft and 2.62 cu yd. With 10% waste the adjusted volume is 2.2 m³, and the US quarter-yard convention (still reported in metric mode) rounds it to 3 cu yd, 2.29 m³. A supplier with a different increment changes only that last step.

The 30 × 12 ft slab at 6 in with 5% waste shows the allowance control at work: 180 cu ft, 6.67 cu yd net, and 7 cu yd once adjusted — an exact quarter-yard fit, so the order is 7 cu yd with 0 cu yd spare. The percentage is yours to set; the engine applies whatever allowance you enter before the round-up.

Footings and columns: the same chain, ending in bags

Footings and columns are solids rather than slabs, so only the geometry step changes shape. A rectangular footing is length ft × (width in ÷ 12) × (depth in ÷ 12), times the footing count. A round column is a cylinder, π × r² × height, with the radius converted from inches to feet; a square pier is width × depth × height. The bag engine then computes, for each bag size, bags = ceil(volume × (1 + waste% ÷ 100) ÷ yield per bag).

The yields are the one sourced input: the QUIKRETE Concrete Mix (No. 1101) product data sheet lists 0.3, 0.375, 0.45, and 0.6 cu ft per 40, 50, 60, and 80 lb bag — yields the manufacturer states as approximate — and Target Products, the licensed QUIKRETE manufacturer in western Canada, publishes 0.43 and 0.52 cu ft for its 25 and 30 kg bags. The bag-yields guide covers each size.

Two 20 ft × 16 in × 8 in footings are each 20 × (16 ÷ 12) × (8 ÷ 12) = 17.78 cu ft, 35.56 cu ft together: 1.32 cu yd or 1.01 m³. With 10% waste there is 39.11 cu ft to mix — 66 bags of 80 lb mix at 0.6 cu ft each, with 0.49 cu ft left in the last bag.

One continuous 40 ft × 12 in × 12 in footing is 40 cu ft — a 12 in square section is one square foot, so cubic feet equal length. That is 1.48 cu yd, well past the third-of-a-yard mark below which bags are the practical route; as bags it is 74 × 80 lb or 98 × 60 lb after the allowance.

Six 3 ft × 24 in × 10 in pads with metric bags use the footing engine's metric table: 5 cu ft each, 30 cu ft and 0.85 m³ in all, and with waste 33 cu ft — 77 bags of 25 kg or 64 bags of 30 kg.

Four round 10 in × 8 ft columns are cylinders with a 5 in (0.4167 ft) radius: 4.36 cu ft each, 17.45 cu ft and 0.65 cu yd for the four, and 32 bags of 80 lb after waste. Six square 12 × 12 in piers, 3 ft tall, are one square foot of section by 3 ft: 3 cu ft each, 18 cu ft total, 0.67 cu yd, and 33 bags of 80 lb — or 66 of 40 lb.

Bags or a truck?

Volume decides. Below roughly a third of a yard — 9 cu ft, or 17 bags of 80 lb mix with the default allowance — bags are practical; a full slab usually justifies a truck. One cubic yard with no waste allowance is 90 × 40 lb, 72 × 50 lb, 60 × 60 lb, 45 × 80 lb, or in metric bags 63 × 25 kg, 52 × 30 kg. To compare the two routes for your own numbers, bring the cubic-feet line to the concrete bag calculator.

Reading the output, and where the model stops

Each concrete tool shows the same lines in order: the net pour, the volume with your allowance applied, the rounded order or bag count, the spare that rounding adds, and a cost line only when you typed a price: the net line checks your geometry, the adjusted line your allowance, and the order line is what you give the plant.

These calculators estimate material quantities only. They do not decide slab thickness, footing or column dimensions, reinforcement, load-bearing requirements, code compliance, permits, or soil conditions — those belong to a licensed engineer, architect, code official, or qualified contractor. The models also assume the shapes you enter are what gets built: they do not cover subgrade preparation, formwork, reinforcement layout, delivery minimums, or moisture and temperature effects, and the waste allowance is one flat percentage. Treat the order figure as a well-organized starting point for the call to the plant, not a guarantee.

Frequently asked questions

Run your own dimensions through the concrete slab calculator, concrete footing calculator, or concrete column calculator, compare bag sizes in the concrete bag calculator, and see the guide to concrete bag yields by size. More concept guides are on the guides page.

Every volume in this guide is computed at build time by the engines that run the concrete calculators, and dimensions you enter in those tools never leave your browser. The full estimation pipeline is documented on the methodology page.