Enter your dimensions to get cubic yards, the number of bags, and whether bags or a ready-mix truck fits the job. Add several parts (a slab plus footings, or a row of post holes) and get one total.

How to use it

Pick the shape: slab, footing, wall, round slab, column/tube, or steps.
Enter the size. Long sides go in feet; thickness, step sizes and tube diameter go in inches.
Set the quantity if you have several identical parts, like 8 post holes.
Use + Add another part to combine different shapes in one order.
Optional: enter local prices to compare a ready-mix delivery against bags.

The link button saves your inputs in the page address, so you can send the result to a supplier or open it later.

The formula

Concrete is ordered by volume. Multiply the three dimensions in feet, then divide by 27 to convert cubic feet to cubic yards.

Cubic yards = length (ft) × width (ft) × thickness (in ÷ 12) ÷ 27

For round shapes, use the area of a circle: π × (diameter ÷ 2)² × height.

Worked example: 10 × 10 ft patio, 4 inches thick

10 × 10 × (4 ÷ 12) = 33.3 cubic feet. 33.3 ÷ 27 = 1.23 cubic yards. Add 10% for waste and you need 1.36 cubic yards, or 62 bags of 80 lb mix.

Bags per cubic yard

Bag size	Yield per bag	Bags per cubic yard
80 lb	0.60 ft³	45
60 lb	0.45 ft³	60
50 lb	0.375 ft³	72
40 lb	0.30 ft³	90

Yields are for standard pre-mixed concrete (sand, gravel and cement). High-yield and fast-setting mixes differ, so check the bag label.

Common projects at a glance

Worked out with this calculator, including 10% extra. Use it as a sense check, then enter your own sizes above.

Project	Cubic yards	80 lb bags	60 lb bags	Bags or truck
10 × 10 ft patio, 4 in	1.36	62	82	Compare quotes
3 × 4 ft AC pad, 4 in	0.16	8	10	Bags
12 × 12 ft shed base, 4 in	1.96	88	118	Compare quotes
4 × 30 ft sidewalk, 4 in	1.63	74	98	Compare quotes
24 × 24 ft garage floor, 5 in	9.78	440	587	Ready-mix
10 fence posts, 10 in hole × 30 in	0.56	25	34	Bags
4 deck piers, 12 in tube × 4 ft	0.51	24	31	Bags
3 steps, 4 ft wide, 7 in rise, 11 in run	0.52	24	32	Bags

Coverage per cubic yard

Thickness	Area per cubic yard	Typical use
3 in	108 ft²	Overlays, light walkways
4 in	81 ft²	Patios, sidewalks, shed bases
5 in	65 ft²	Garage floors
6 in	54 ft²	Driveways with heavier vehicles

Questions

How many 80 lb bags of concrete make a cubic yard?

About 45. Each 80 lb bag of standard concrete mix yields roughly 0.60 cubic feet, and a cubic yard is 27 cubic feet, so 27 ÷ 0.60 = 45 bags.

How much area does a cubic yard of concrete cover?

At 4 inches thick, one cubic yard covers 81 square feet. At 6 inches it covers 54 square feet. Divide 324 by the thickness in inches to get square feet per yard.

How much extra concrete should I order?

Most contractors add 5–10%. Uneven ground, soft subgrade and spillage all eat concrete, and running short mid-pour is worse than a little left over. The calculator adds 10% by default; change it to suit your job.

When should I order ready-mix instead of bags?

Under about 1 cubic yard, bags are usually practical. Above about 2 cubic yards, ready-mix is normally the better option: mixing 90+ bags by hand is slow, and batches that set at different times can leave weak joints. In between, compare a delivered quote (including any short-load fee) against the bag cost.

Does the calculator work in metric?

Yes. Switch to metres / cm and enter dimensions in metres, with thickness in centimetres. Results show cubic metres first.

Can I enter feet and inches?

Yes. Long sizes have a feet box and an inches box, so 10 ft 6 in goes in as 10 and 6. You can also type decimal feet, like 10.5, in the feet box.

How much concrete do I need for a fence post?

A 10 in round hole, 30 in deep holds about 1.36 cubic feet, which is 2.3 bags of 80 lb mix before the post takes up some of the space. Use the Column / tube / hole shape, or Square hole for square holes, and set the quantity to the number of posts.

How much gravel goes under a concrete slab?

A compacted base of about 4 inches is common for patios, walkways and shed pads; driveways and poor soils often get more. Check your local code. Turn on Gravel base under Order options and the ticket adds the cubic yards and tons for every slab.

How far apart should rebar be in a slab?

For patios and driveways, 18–24 inches on center is a common range, with bars kept about 3 inches in from the edges. Pick a spacing under Order options and the ticket counts the 20 ft bars, with 10% added for laps and offcuts. Structural slabs follow the engineer’s drawings.

How do I calculate concrete for a curb and gutter?

Choose Curb & gutter, enter the run length and the four section sizes (curb depth and height, gutter width, flag thickness). The section area is curb depth × (curb height + flag thickness) + gutter width × flag thickness, multiplied by the length.

Guides: How to measure for concrete · How thick should a slab be?

Sources

Bag yields: QUIKRETE Concrete Mix No. 1101 data sheet. Unit conversions: 1 cubic yard = 27 cubic feet; 1 cubic metre = 35.31 cubic feet. See methodology & sources.

Concrete Calculator