Concrete Calculator
Find the cubic yards for a slab, footing, column, or flight of stairs — and how many bags of mix that actually means.
A flat pour — a patio, driveway, shed base, or floor.
5–10% is standard.
Order
1.63 cubic yards
For 44 cu ft, waste included.
Bags of concrete mix
Or, if you're filling this with aggregate
Bulk densities are dry and loose, and swing by 15% or more with moisture and stone size. Use them to size a delivery, not to settle an invoice.
Every concrete job is the same question in different clothing: what volume are you filling? Work that out in cubic feet, convert to cubic yards — the unit concrete is sold in — and add a margin, because running out mid-pour is a genuinely bad afternoon. This calculator does that for the four shapes that cover most residential work, and then tells you what the volume means in bags.
The one conversion that matters
There are 27 cubic feet in a cubic yard — a yard being three feet, and 3 × 3 × 3 = 27. That single number is the source of most confusion on a concrete job, because you measure everything in feet and inches and then order in yards. A 12 × 10 ft patio, 4 inches thick, is 40 cubic feet, which sounds like a lot and is actually 1.48 cubic yards — barely over the minimum a truck will bring.
Note that the thickness is in inches while everything else is in feet, which is exactly how you would measure it and exactly where the arithmetic goes wrong. Four inches is a third of a foot, so the slab above is 12 × 10 × 0.333, not 12 × 10 × 4. The calculator keeps each field in its natural unit so you never do that conversion by hand.
The four shapes
Slabs — patios, driveways, shed bases, floors — are length × width × thickness. Four inches is the standard for foot traffic and patios; go to five or six for a driveway, or anywhere a vehicle will sit.
Footings are the trench pours under a wall, fence, or foundation. Same arithmetic as a slab, but the cross-section is small and the run is long, so both cross-section dimensions are given in inches. A 20 ft run at 16 × 8 inches is 17.78 cubic feet, about two-thirds of a yard.
Columns are round: piers, deck posts, sonotubes. The volume is π × radius² × height, and the field asks for the diameter because that is what is printed on the tube. Watch how fast this one grows — doubling the diameter quadruples the concrete, so four 12-inch piers at 8 ft come to 25.13 cubic feet while four 24-inch piers come to 100.53.
Stairs are the one people get wrong, and the reason is worth understanding. A solid flight is not a wedge, and it is not steps × one-step-volume. Picture it side on: the bottom step is one rise tall, the second is two rises tall, the third is three. So the total is proportional to 1 + 2 + 3 + … + n — the triangular number n(n+1)/2 — not to n. Five steps take fifteen step-blocks of concrete, not five, which is three times what the naive estimate gives. At 7-inch rises, 11-inch treads and 4 feet wide, that is 32.08 cubic feet, or 1.19 cubic yards.
Why the waste allowance is not optional
The default is 10% and it is not padding. Sub-grade is never perfectly flat, so a "4-inch" slab is 4 inches in the high spots and 5 in the low ones. Forms bulge. Some concrete stays in the barrow, the chute, and the mixer. And a slab poured a bit thin is a slab that cracks. Five percent is the floor for a small, well-formed pour on compacted gravel; ten is right for most jobs; go to fifteen on rough or sloping ground, or where you are pouring against earth rather than into forms.
The asymmetry is what settles it. Ordering 10% extra costs you a fraction of a yard. Running out gives you a cold joint — a visible, structurally weak seam where the first pour set before the second arrived — plus a second short-load delivery fee that will dwarf whatever the extra would have cost.
Bags or ready-mix
Bags of pre-mixed concrete yield far less than their weight suggests, because most of that weight is aggregate that was always going to be in there. The published figures are 0.30 cubic feet for a 40 lb bag, 0.45 for a 60, and 0.60 for an 80. So one cubic yard — 27 cubic feet — is 45 of the 80 lb bags: 3,600 pounds of material to buy, move, and mix one bag at a time.
That number is the whole decision. Under about a third of a yard, bags are obviously right — a couple of post footings, a small pad. Above a yard, ready-mix is obviously right. In between, the deciding factors are whether you can get a truck to the pour and whether the pour has to be monolithic: anything structural, or any slab you want free of cold joints, wants to arrive all at once. Renting a towable mixer and buying bulk sand, gravel and cement is the third option, and it mostly makes sense when access rules out a truck.
If you land under a cubic yard, expect a short-load fee. Most suppliers set a minimum around a yard and charge for the difference, so 0.6 of a yard often costs nearly what a full yard costs. The calculator flags this when your total falls below the line.
Gravel, sand, and the base course
Most slabs want a compacted sub-base of gravel or crushed stone underneath — commonly 4 inches — for drainage and to stop the slab moving with the soil. That is the same volume calculation with a different thickness, which is why this tool also converts the volume into tons of aggregate: bulk material is sold by weight, not by the yard.
At roughly 100 pounds per cubic foot dry, a cubic yard of gravel or sand is about 1.35 tons. Topsoil is lighter, near 80 pounds per cubic foot, or about 1.08 tons to the yard. Both figures move by 15% or more with moisture, stone size, and how settled the pile is, so they will size a delivery correctly and should not be used to check a bill. Order by volume where you can and treat the tonnage as the sanity check.
What this doesn't cover
It is a volume calculator, so it assumes the shape you described is solid and the ground under it is flat. Real jobs deviate. Slabs often have a thickened edge or a perimeter beam that adds volume. Stairs are frequently poured over compacted fill or rubble rather than solid, which removes a large fraction of what the stairs figure gives you — if you are filling, work out the fill volume separately and subtract it. And sloping ground means the "4 inch" slab is deeper at one end than the other; measure the thickness at both ends and use the average.
It also says nothing about what goes in the concrete — rebar, mesh, fibre — or about mix strength, which is chosen for the job rather than calculated from it. And it is not a structural design tool. If the pour holds a building up, the sizes should come from someone qualified to specify them; this calculator's job is only to tell you how much to order once they have.
How many cubic yards of concrete do I need for a 12x10 slab?
A 12 × 10 ft slab at the standard 4 inches thick is 40 cubic feet, which is 1.48 cubic yards. With a 10% waste allowance you would order about 1.63 cubic yards. At 6 inches thick the same slab is 60 cubic feet, or 2.22 cubic yards before waste.
How many cubic feet are in a cubic yard?
Twenty-seven. A yard is three feet, so a cubic yard is 3 × 3 × 3 = 27 cubic feet. This is the conversion that trips up most concrete estimates, because you measure the job in feet and inches but buy the concrete in cubic yards.
How many 80 lb bags of concrete make a cubic yard?
Forty-five. An 80 lb bag yields 0.60 cubic feet and a cubic yard is 27 cubic feet. A 60 lb bag yields 0.45 cubic feet, so a yard is 60 of those, and a 40 lb bag yields 0.30, so a yard is 90. Forty-five 80 lb bags is 3,600 pounds of material to carry and mix, which is why ready-mix wins above about a yard.
How much concrete do I need for stairs?
More than you would guess, because the volume grows with the square of the step count rather than linearly. A solid flight is a stack of blocks — the bottom step is one rise tall, the next two rises, and so on — so n steps take n(n+1)/2 step-blocks of concrete. Five steps take fifteen, not five. A 5-step flight at 7-inch rises, 11-inch treads and 4 feet wide is 32.08 cubic feet, or 1.19 cubic yards.
How much extra concrete should I order?
Ten percent is the usual allowance and the default here. Five percent is enough for a small, well-formed pour onto compacted gravel; go to fifteen on rough or sloping ground, or when pouring against earth instead of forms. The asymmetry decides it: extra concrete costs a fraction of a yard, while running out gives you a structurally weak cold joint plus a second delivery fee.
How thick should a concrete slab be?
Four inches is the standard for patios, walkways, shed bases and anything carrying foot traffic. Go to five or six inches for a driveway or anywhere a vehicle parks, and thicker still for heavy equipment. Most slabs also want about four inches of compacted gravel underneath for drainage and stability, which this calculator will size for you if you enter it as its own pour.
How many tons of gravel is a cubic yard?
About 1.35 tons for gravel, crushed stone, or sand at roughly 100 pounds per cubic foot dry. Topsoil is lighter at around 80 pounds per cubic foot, so about 1.08 tons per cubic yard. All of these move by 15% or more with moisture content and stone size, so use the figure to size a delivery rather than to check an invoice.
What is a short-load fee?
It's a surcharge ready-mix suppliers apply below their minimum order, which is typically around one cubic yard. Below that line you are largely paying for the truck and the trip rather than the concrete, so 0.6 of a yard can cost close to what a full yard costs. That is why bags or a rented mixer usually win for small pours, and why this calculator flags totals that fall under a yard.