Concrete Calculator – Cubic Yards, m³ & Bags Needed

Concrete Calculator

Concrete volume is length × width × thickness, all in feet, divided by 27 to get cubic yards. A 10 × 10 ft slab 4 inches thick needs 1.23 cubic yards, or about 1.36 yd³ with 10% extra, which is 62 bags of 80 lb premix.

By the Calculators With Steps team · Updated November 10, 2026 · How we check our math

How to use the concrete calculator

Choose what you are pouring: a slab (patio, driveway, shed base), a footing, a wall, a round column (post hole or tube form) or steps.
Enter each measurement and pick its unit. You can mix units, such as a slab measured in feet with its thickness in inches.
Set how many identical pours you need, for example six post holes.
Keep the waste allowance at 10% unless you have a reason to change it.

The calculator gives the volume in cubic yards, cubic feet and cubic metres, and the number of premixed bags in 40, 60 and 80 lb and 20 and 25 kg sizes.

Concrete volume formulas

Project	Volume formula
Slab, footing or wall	length × width × thickness (or height)
Round column or post hole	π × (diameter ÷ 2)² × height
Solid steps	width × run × rise × (1 + 2 + … + n)

Convert every measurement to feet first (inches ÷ 12), multiply, and you have cubic feet. Then:

Cubic yards = cubic feet ÷ 27

Cubic metres = cubic feet × 0.0283168

Bags = cubic feet ÷ bag yield, rounded up

Worked example: a 10 × 10 ft patio

A patio slab is 10 ft long, 10 ft wide and 4 in thick.

Step	Working	Result
Thickness in feet	4 ÷ 12	0.333 ft
Volume	10 × 10 × 0.333	33.33 ft³
Add 10%	33.33 × 1.1	36.67 ft³
Cubic yards	36.67 ÷ 27	1.36 yd³
Cubic metres	36.67 × 0.0283168	1.038 m³
80 lb bags	36.67 ÷ 0.60	62 bags
60 lb bags	36.67 ÷ 0.45	82 bags

How many bags of concrete do I need?

Bag yields below are from the QUIKRETE Concrete Mix data sheet. Other brands are similar, but check the yield printed on your bag.

Bag size	Yield per bag	Bags per cubic yard	Bags per cubic metre
40 lb	0.30 ft³	90	118
60 lb	0.45 ft³	60	79
80 lb	0.60 ft³	45	59
20 kg	about 9.4 L	82	107
25 kg	about 11.7 L	66	86

The metric bag yields are worked out from the same yield per kilogram as the U.S. data sheet (about 0.47 litres per kg). Manufacturer sheets for 20 kg bags typically list 9 to 10 litres per bag.

Slab thickness and coverage

How much concrete a 100 sq ft slab (for example 10 × 10 ft) needs at common thicknesses, before any waste allowance:

Thickness	Cubic feet	Cubic yards
4 in	33.33	1.23
5 in	41.67	1.54
6 in	50	1.85

A 4-inch slab is standard for patios, sidewalks and shed bases. Driveways are commonly 4 to 6 inches thick, and local codes may set a minimum. Thicker slabs, footings and anything structural should follow your building plans or an engineer’s specification.

Other projects

Footing. A trench footing 20 ft long, 16 in wide and 8 in deep holds 17.78 ft³. With 10% extra that is 0.72 yd³.

Wall. A wall 20 ft long, 4 ft high and 8 in thick holds 53.33 ft³, or 2.17 yd³ with 10% extra, which is a job for ready-mix.

Post holes and tube forms. Six holes 12 in across and 4 ft deep need 20.73 ft³ with 10% extra, or 35 bags of 80 lb mix. For deck footings poured in cardboard tube forms, enter the tube’s diameter.

Steps. Four steps 4 ft wide with a 7 in rise and an 11 in run need 23.53 ft³ with 10% extra (0.87 yd³), or 40 bags of 80 lb mix. The formula assumes the steps are solid down to the ground. For steps built against an existing slab or with a hollow fill, the real volume is less.

Metric slab. A 3 m × 3 m slab 10 cm thick is 0.9 m³. With 10% extra it is 0.99 m³, which is 85 bags of 25 kg mix or 106 bags of 20 kg mix.

Tips for ordering concrete

Add 5% to 10% for spillage, uneven subgrade and forms that bow slightly. Running short partway through a pour leaves a weak joint.
Measure the depth in several places. An excavation that averages 4.5 in instead of 4 in needs 12.5% more concrete.
Ready-mix suppliers sell by the cubic yard (or cubic metre) and many charge a short-load fee for small orders, so compare that with the cost of bags.
To size the area first, use the square footage calculator. For tanks, cylinders and other solid shapes, the volume calculator has more formulas.

Concrete Calculator: frequently asked questions

How do I calculate concrete for a slab?

Multiply length × width × thickness in feet to get cubic feet, then divide by 27 for cubic yards. A 10 × 10 ft slab 4 in (0.333 ft) thick is 33.33 ft³, or 1.23 yd³. Add about 10% for spillage and uneven ground.

How many bags of concrete are in a cubic yard?

A cubic yard is 27 cubic feet. An 80 lb bag yields about 0.60 ft³, so you need 45 bags per cubic yard. With 60 lb bags (0.45 ft³) you need 60, and with 40 lb bags (0.30 ft³) you need 90.

How much concrete do I need for a post hole?

Use π × radius² × depth. A 12 in (1 ft) diameter hole 4 ft deep holds 3.14 ft³. Six of them with 10% extra need 20.73 ft³, or 35 bags of 80 lb mix. Subtract the post's own volume if you want a closer figure.

How do I calculate concrete for steps?

Treat solid steps as stacked blocks: volume = width × tread depth × riser height × (1 + 2 + … + number of steps). Four steps 4 ft wide with 7 in risers and 11 in treads need 21.39 ft³, or 0.87 yd³ with 10% extra.

How do I calculate concrete in cubic metres?

Multiply length × width × depth in metres. A 3 m × 3 m slab 10 cm (0.1 m) thick is 0.9 m³, or 0.99 m³ with 10% extra. That is about 85 bags of 25 kg premix or 106 bags of 20 kg premix.

How thick should a concrete slab be?

Four inches (about 10 cm) is the usual thickness for patios, walkways and shed bases. Driveways are commonly 4 to 6 inches, thicker where heavy vehicles park. Each extra inch on a 100 sq ft slab adds about 0.31 cubic yards.

Should I use bags or order ready-mix concrete?

Bags suit small jobs such as posts, steps and pads under about a cubic yard. Above that, mixing 45 or more 80 lb bags by hand is slow and hard work, and a ready-mix delivery is usually more practical.