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Concrete: the Formula, the Bags and What a Yard Weighs

How much concrete a slab needs

Concrete is sold by volume, so «how much» is a volume question, and a rectangular slab answers it with two multiplications and one division:

cubic yards = width × length × thickness in inches ÷ 324

The 324 is not a rule of thumb. A cubic yard is 3 × 3 × 3 feet = 27 cubic feet; spread one inch deep it becomes a sheet 27 × 12 = 324 square feet across. Divide 324 by the thickness you want and you have the coverage of one yard; multiply the area by the thickness and divide by 324 and you have the yards you need. Nothing in that chain depends on what the concrete is made of, which is why the volume is exact and the weight is not.

Step by step

  1. Measure the two sides in feet. Width × length for a rectangle; split an L-shape into rectangles and add the results.
  2. Choose the thickness in inches. 4 inches for a footpath, a patio or a shed floor; 5 to 6 inches for a slab a car stands on.
  3. Multiply and divide by 324. That is the answer in cubic yards. For cubic feet, divide by 12 instead of 324 — or take the cubic yards you just got and multiply them by 27.
  4. Add the ordering allowance. NRMCA advises 4% to 10% over the volume calculated from plan dimensions, for waste, over-excavation, form deflection and settlement.

How thick to pour

Thickness is an input on this page, and every value gets its own answer. Here is what 10 × 10 feet takes at each of the usual thicknesses:

Thickness One cubic yard covers Concrete for 10 × 10 feet Typical use
3 inches108 square feet0.93 cubic yards thin pad on firm, well drained ground
4 inches81 square feet1.23 cubic yards footpath, patio, shed floor
5 inches64.8 square feet1.54 cubic yards driveway for a car, garage floor
6 inches54 square feet1.85 cubic yards driveway with vehicles, machinery pad
8 inches40.5 square feet2.47 cubic yards heavy pad, thickened edge, footing

A strip foundation on this form

A strip footing is not a rectangle, but it is a rectangle unrolled: a long, narrow band. Feed the form the width of the strip as one side and the length of its run as the other, and put the height of the strip in the thickness field.

Measure the run along the centreline of the strip, not around the outside of the house. The outer perimeter counts each of the four corners twice. For a 24 × 36 ft house on a 1 ft strip the centreline is 2 × ((24 − 1) + (36 − 1)) = 116 ft.

So the form takes 1 × 116 feet at 8 inches, which is 116 square feet and 2.86 cubic yards of concrete — 129 bags of 80 lbs, or 3.16 cubic yards ordered ready mixed. The same trick works for a path, a kerb or a beam: anything long and narrow is a band.

Bags or a mixer

Bagged dry mix and ready mixed concrete are two ways to buy the same volume, and the honest comparison is made at the same volume. A bag of QUIKRETE Concrete Mix yields this much of mixed concrete:

Bag Yield Bags per cubic yard
40 lbs 0.3 cubic feet 90
50 lbs 0.375 cubic feet 72
60 lbs 0.45 cubic feet 60
80 lbs 0.6 cubic feet 45
90 lbs 0.675 cubic feet 40

The numbers come from the QUIKRETE Concrete Mix data sheet (Product No. 1101, revised October 2022), section YIELD; this page calculates with the 80 lbs bag, the common US size.

Where bags stop making sense. One cubic yard is 45 bags — 3,600 lbs of dry mix to lift, and every batch has to be mixed and placed before the last one sets. A truck mixer carries 8–12 cubic yards (NRMCA, Concrete in Practice, CIP 31), and it arrives mixed.

What a cubic yard of concrete weighs

One number, and the source gives exactly one: 140 lbs per cubic foot, measured to ASTM C138 — QUIKRETE Concrete Mix data sheet, Table 1, «Typical Physical Properties». A cubic yard is 27 cubic feet, so that is 3,780 lbs per cubic yard. A 10 × 10 feet slab 4 inches thick therefore weighs 4,649 lbs.

Three honest limits. First, the word on the data sheet is «approximately»: this is a nominal, not a measurement of your mix. Second, it is ordinary normal-weight concrete on dense aggregate — structural lightweight runs roughly 15 to 40 per cent lighter (NRMCA CIP 36) rather than at any fixed fraction of it, and reinforcement adds the weight of the steel on top. Third, it is the density of the fresh placed concrete, not of the dry bag: the bag weighs less than what it becomes, because you add the water.

Strength: the grade does not change the volume

Nothing on this page depends on the strength you order — a stronger mix fills the same forms. It does decide what the slab may carry, so here is the usual reading of the numbers:

Strength Where it goes
2,500 psi (17 MPa)non-structural fill, blinding, mud slabs
3,000 psi (21 MPa)footings and foundation walls in mild exposure
3,500 psi (24 MPa)interior slabs, basement floors, shed bases
4,000 psi (28 MPa)driveways, garage slabs, exterior flatwork in freeze–thaw
4,500–5,000 psi (31–35 MPa)heavy-duty pavement and structural work
The bagged mix on this page reaches 4,000 psi (28 MPa) at 28 days by ASTM C39, per its own data sheet, and NRMCA puts the typical specified strength for most concrete applications at 3,500 to 5,000 psi (25 to 35 MPa) (Concrete in Practice, CIP 31). One caution: American psi figures are measured on a cylinder. A European class is a pair — in C25/30 the first number is the cylinder strength and the second the cube — while Russian class B is a cube value. The families are not converted into one another by arithmetic.

Concrete for common slab sizes

At 4 inches thick, with the bags counted for the 80 lbs size:

Slab Area Concrete Bags
2 × 2 feet4 square feet0.0494 cubic yards3 bags
2 × 3 feet6 square feet0.0741 cubic yards4 bags
2 × 4 feet8 square feet0.0988 cubic yards5 bags
3 × 3 feet9 square feet0.11 cubic yards5 bags
4 × 4 feet16 square feet0.2 cubic yards9 bags
4 × 6 feet24 square feet0.3 cubic yards14 bags
4 × 8 feet32 square feet0.4 cubic yards18 bags
6 × 6 feet36 square feet0.44 cubic yards20 bags
6 × 8 feet48 square feet0.59 cubic yards27 bags
8 × 8 feet64 square feet0.79 cubic yards36 bags
8 × 10 feet80 square feet0.99 cubic yards45 bags
8 × 12 feet96 square feet1.19 cubic yards54 bags
10 × 10 feet100 square feet1.23 cubic yards56 bags
10 × 12 feet120 square feet1.48 cubic yards67 bags
10 × 14 feet140 square feet1.73 cubic yards78 bags
10 × 15 feet150 square feet1.85 cubic yards84 bags
10 × 16 feet160 square feet1.98 cubic yards89 bags
10 × 20 feet200 square feet2.47 cubic yards112 bags
12 × 12 feet144 square feet1.78 cubic yards80 bags
12 × 16 feet192 square feet2.37 cubic yards107 bags
12 × 20 feet240 square feet2.96 cubic yards134 bags
12 × 24 feet288 square feet3.56 cubic yards160 bags
14 × 14 feet196 square feet2.42 cubic yards109 bags
15 × 15 feet225 square feet2.78 cubic yards125 bags
15 × 20 feet300 square feet3.7 cubic yards167 bags
16 × 16 feet256 square feet3.16 cubic yards143 bags
16 × 20 feet320 square feet3.95 cubic yards178 bags
16 × 24 feet384 square feet4.74 cubic yards214 bags
20 × 20 feet400 square feet4.94 cubic yards223 bags
20 × 30 feet600 square feet7.41 cubic yards334 bags
20 × 40 feet800 square feet9.88 cubic yards445 bags
24 × 24 feet576 square feet7.11 cubic yards320 bags
24 × 30 feet720 square feet8.89 cubic yards400 bags
25 × 25 feet625 square feet7.72 cubic yards348 bags
30 × 30 feet900 square feet11.11 cubic yards500 bags
30 × 40 feet1,200 square feet14.81 cubic yards667 bags
30 × 50 feet1,500 square feet18.52 cubic yards834 bags
30 × 60 feet1,800 square feet22.22 cubic yards1,000 bags
36 × 36 feet1,296 square feet16 cubic yards720 bags
40 × 40 feet1,600 square feet19.75 cubic yards889 bags
40 × 50 feet2,000 square feet24.69 cubic yards1,112 bags
40 × 60 feet2,400 square feet29.63 cubic yards1,334 bags

Where this stops being the right calculator

  • The slab must be flat and rectangular. Split an L-shape into rectangles and add them; for a circle use π × radius² and enter the result as one side against a width of 1.
  • This calculator does not design the slab. Thickness, reinforcement, sub-base and control joints are decided by the load and the ground, and no volume calculator answers that.
  • Footings, columns and steps are separate pours. Each has its own dimensions; add the volumes, do not average the thicknesses.
  • Measuring the area first? That is a different question and it has its own page — square footage of 10 × 10 feet answers «how many square feet», this page answers «how many cubic yards, bags and pounds».
  • The weight is a nominal. It comes from one packaged product; a different mix design lands near it, not on it.

Doing it in your head

Remember one number: 324. One cubic yard, one inch deep, covers 324 square feet — so at 4 inches a yard does 81 square feet, at 6 inches 54. And one case comes out exact and is worth keeping: 9 × 9 feet at 4 inches is 81 × 4 ÷ 324 = 1 cubic yard, on the nose.

No prices here, on purpose. What a yard of concrete costs depends on the plant, the haul distance and the week; we have no data on any of the three, so this page answers volume, bags and weight and leaves the price to your supplier.

See Also

Concrete Calculator

This calculator works out how much concrete a slab needs: enter the two sides and the thickness and it returns the volume in cubic yards and cubic feet, the number of 80 lbs bags of dry mix, what the finished slab weighs and how much ground one cubic yard covers. For example, it can help you find out how much concrete do I need for a 10 × 10 feet slab? (The answer is: 1.23 cubic yards). Enter the sides (e.g. 10 × 10 feet), choose the unit and set the thickness — 4 inches by default, anything from 1–24 inches is accepted and gets its own page.

FAQ

How much concrete do I need for a slab?

Multiply the two sides in feet to get the area, multiply that by the thickness in inches and divide by 324 — that gives cubic yards. A 10 × 10 ft slab 4 inches thick is 100 square feet, and 100 × 4 ÷ 324 ≈ 1.23 cubic yards, or 33.33 cubic feet. One case comes out exact and is worth remembering: 9 × 9 ft at 4 inches is 81 × 4 ÷ 324 = 1 cubic yard on the nose.

How many bags of concrete are in a cubic yard?

A cubic yard is 27 cubic feet and an 80 lb bag of QUIKRETE Concrete Mix yields approximately 0.60 cubic feet, so it takes 45 bags — 3,600 lb of dry mix. The figures are from the QUIKRETE Concrete Mix (No. 1101) data sheet, revised October 2022.

How much does a yard of concrete weigh?

The same data sheet gives a unit weight of approximately 140 lb per cubic foot measured to ASTM C138, so a cubic yard of that mix is about 3,780 lb — near enough 1.9 tons. It is the nominal figure for one bagged product, not a norm for concrete as a category: ordinary normal-weight mixes sit in a narrow band around it, structural lightweight runs roughly 15 to 40 per cent lighter (NRMCA CIP 36) rather than at any fixed fraction, and reinforcement adds the weight of the steel.

How much extra concrete should I order?

NRMCA advises ordering 4% to 10% more than the volume calculated from plan dimensions, to cover waste, over-excavation, form deflection and settlement (Concrete in Practice, CIP 31 and CIP 8). This page uses the top of that band, 10%, taken from the exact volume and rounded up, because running short mid-pour costs a cold joint, not just a second trip.

How thick should a concrete slab be?

4 inches is the usual thickness for a footpath, a patio or a shed floor, and it is what this page uses by default. QUIKRETE puts 4 inches at the low end for a driveway carrying passenger cars and 5 to 6 inches where trucks or heavy vehicles stand; the sub-base, the soil and the local code decide as much as the slab does. Anything thicker is designed by load, not by area.

Should I buy bags or order ready mixed concrete?

They are two ways to buy the same volume. Bags win on small jobs because you mix only what you need; they lose quickly on size, since one cubic yard is 45 bags and 3,600 lb of dry mix to carry and mix by hand. A truck mixer holds 8 to 12 cubic yards (NRMCA CIP 31), so ready mixed concrete takes over well before a job fills one. Where exactly is a local question rather than a rule: suppliers charge a minimum load and a short-load fee, and the delivery distance often decides it.