Concrete Calculator — Yards & 40, 60, 80 lb Bags

Bags for this pour, by size
Bag	Bags	Per yd³
40 lb	117	90
50 lb	94	72
60 lb	78	60
80 lb	59	45
90 lb	52	40

Bags for this pour, by size

Bag

Bags

Per yd³

40 lb

117

50 lb

60 lb

80 lb

90 lb

How to calculate concrete

Concrete is ordered by volume — cubic yards for ready-mix, bags for small jobs. The calculation is the same for both:

Find the area of the pour in square feet: length × width for a rectangle, π × (diameter ÷ 2)² for a round column or post hole.
Convert the thickness to feet — divide inches by 12 (4 in = 0.333 ft).
Multiply area × thickness to get cubic feet.
Divide by 27 for cubic yards, or divide by the bag yield for bags.

Worked example. A patio 10 ft × 10 ft, 4 in thick:

Volume: 10 × 10 × 4 ÷ 12 = 33.3 ft³ = 33.3 ÷ 27 = 1.23 yd³
With 5 % extra: 33.3 × 1.05 = 35 ft³ = 1.30 yd³
In 80 lb bags: 35 ÷ 0.60 = 58.3 → 59 bags, weighing 4,720 lb

Add every part of the job as its own pour — a slab, the footings under it, the steps up to it — and the calculator totals them. For an odd-shaped slab, choose L-shape, Triangle or Known area; the square footage calculator can work out the area first if it has cut-outs.

Bags of concrete per cubic yard

These yields come from QUIKRETE’s data sheet for its standard Concrete Mix (No. 1101, 4,000 psi). Other brands of standard mix are close, but check the yield printed on your bag and pick the size you are buying.

Bag	Yield per bag	Bags per cubic yard	Covers at 4 in thick
40 lb	0.30 ft³	90	0.9 sq ft
50 lb	0.375 ft³	72	1.1 sq ft
60 lb	0.45 ft³	60	1.35 sq ft
80 lb	0.60 ft³	45	1.8 sq ft
90 lb	0.675 ft³	40	2.0 sq ft

Bigger bags mean fewer trips to the mixer but heavier lifting: 45 bags of 80 lb for one cubic yard is 3,600 lb. The table under the calculator shows the count and total weight for every size, so you can choose before you buy.

Concrete for common projects

All figures include 5 % extra and use the yields above.

Project	Size	Thickness	Cubic yards	80 lb bags	60 lb bags	40 lb bags
Patio slab	10 × 10 ft	4 in	1.30	59	78	117
Sidewalk	30 × 4 ft	4 in	1.56	70	94	140
Shed base	12 × 8 ft	4 in	1.24	56	75	112
Driveway	24 × 20 ft	5 in	7.78	350	467	700
10 fence posts	10 in × 36 in holes	—	0.64	29	39	58
9 deck footings	12 in × 48 in holes	—	1.10	50	66	99
Front steps	3 risers, 4 ft wide, 36 in landing	—	1.07	48	64	96

The driveway row is why ready-mix exists: 350 bags of 80 lb is 14 tons to lift and mix by hand. For fence posts, the post itself takes up some of the hole, so the bag count is slightly generous — which is the safe side to be on.

Slab thickness

Concrete Network gives 4 inches as the usual thickness for residential patios, walkways and driveways that carry passenger cars, and 5–6 inches where trucks or RVs will park. Going from 4 to 5 inches adds about 20 % to the concrete but nearly 50 % to the load the slab can carry. Many contractors also thicken the edges of a driveway by 1–2 inches, where wheels load the slab hardest.

Pour the slab on a compacted gravel base — the gravel calculator works out the stone for it — and check your local building code, which may set a minimum.

Footings, columns and post holes

Choose Column / hole for anything round: fence and deck post holes, sonotube piers, round footings. Enter the diameter (type 12" for inches), set the depth unit, and put the number of identical holes under How many. A 12 in hole 48 in deep holds π × 0.5² × 4 = 3.14 ft³ — about 5¼ bags of 80 lb mix, so buy 6.

Continuous footings and walls are rectangles: the length of the run, the width of the footing, and its depth as the thickness. A footing 20 ft long, 12 in wide and 12 in deep is 20 ft³, or 0.74 yd³.

Concrete steps

Choose Stairs and enter the number of risers, the height of each riser, the depth of each tread, the width of the flight and the depth of the landing at the top. The calculator treats the stairs as solid concrete on the ground: each step is a block reaching from the ground to its tread, and the landing runs the full height.

volume = width × (tread × riser × N(N−1)/2 + landing × N × riser), where N is the number of risers.

Three risers of 7 in, 11 in treads, 4 ft wide with a 36 in landing: 4 × (0.917 × 0.583 × 3 + 3 × 3 × 0.583) = 27.4 ft³ — about 1 cubic yard, or 48 bags of 80 lb mix with 5 % extra.

How much extra to order

Never order the exact amount. The base is never perfectly level, forms bow slightly under the weight, and some concrete stays in the mixer, the wheelbarrow or the truck chute. Running short in the middle of a pour leaves a cold joint that can crack. A margin of about 5 % is sound on a well-prepared, compacted base; add more over uneven or soft ground. Concrete Network describes a 20-yard order that finishes with one yard to spare as a good order.

Bags or ready-mix

	Bagged mix	Ready-mix truck
Best for	Post holes, footings, steps, slabs up to about 1 yd³	Slabs, driveways, anything above about 1 yd³
You do	Lift, mix and pour every bag	Pour and finish as it arrives
Ordering	Bags at a home center, any quantity	Cubic yards, delivered
Watch for	Weight — 45 bags of 80 lb is 3,600 lb	Delivery and small-load fees; have the crew and forms ready

Enter a bag price or a ready-mix price per cubic yard under Price to compare the two for your job. For the volume of any other loose material, use the cubic yards calculator.

Frequently asked questions

How many bags of concrete are in a cubic yard?

45 bags of 80 lb, 60 bags of 60 lb or 90 bags of 40 lb. QUIKRETE's data sheet gives the yield of each bag — 0.60, 0.45 and 0.30 cubic feet — and a cubic yard is 27 cubic feet, so 27 ÷ 0.60 = 45. A 50 lb bag yields 0.375 ft³ (72 per yard) and a 90 lb bag 0.675 ft³ (40 per yard).

How many 80 lb bags do I need for a 10×10 slab?

At 4 inches thick, a 10 × 10 ft slab is 33.3 cubic feet. Each 80 lb bag makes 0.60 ft³, so that is 56 bags exactly, or 59 bags with a 5 % margin. At 6 inches it is 50 ft³ — 84 bags without a margin, 88 with it.

Should I use bags or order ready-mix concrete?

Bags suit small jobs: post holes, footings, steps and small slabs. Above roughly one cubic yard — 45 bags of 80 lb, about 3,600 lb to lift and mix — ready-mix delivered by truck is usually faster and more consistent. Larger slabs and driveways are easier to finish when the concrete arrives all at once.

How thick should a concrete slab be?

4 inches is the usual thickness for residential patios, walkways and driveways for passenger cars. Use 5–6 inches where trucks or RVs will park; going from 4 to 5 inches adds about 20 % to the concrete but nearly 50 % to the load the slab can carry, according to Concrete Network.

How much extra concrete should I order?

Never order the exact amount. Ground is never perfectly flat, forms bow and some concrete is left in the mixer or the truck. A margin of about 5 % covers this on a well-prepared base — Concrete Network calls a 20-yard order with one yard left over a good order. Add more over uneven or soft ground.

How much does a bag of concrete cover?

An 80 lb bag makes 0.60 cubic feet: 1.8 sq ft at 4 inches thick, 2.4 sq ft at 3 inches, or 1.2 sq ft at 6 inches. A 60 lb bag covers three quarters of that and a 40 lb bag half.