How many bags of concrete do I need for a 10 × 10 slab?

At 4 inches thick, a 10 ft × 10 ft slab is 10 × 10 × 0.3333 = 33.33 cu ft (1.23 cu yd). With 10% extra that is 36.67 cu ft, or 62 bags of 80 lb (0.60 cu ft each), 82 bags of 60 lb or 123 bags of 40 lb. Without any extra it would be 56 bags of 80 lb.

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

A cubic yard is 27 cu ft. At the published yield of 0.60 cu ft per 80 lb bag that is 27 ÷ 0.6 = 45 bags. It takes 60 bags of 60 lb (0.45 cu ft each), 72 bags of 50 lb (0.375 cu ft) or 90 bags of 40 lb (0.30 cu ft).

How do I calculate cubic yards of concrete?

Convert every measurement to feet (inches ÷ 12), multiply length × width × thickness to get cubic feet, then divide by 27. A 12 ft × 10 ft slab 4 in thick is 12 × 10 × 0.3333 = 40 cu ft, and 40 ÷ 27 = 1.48 cu yd before any waste allowance.

How much concrete do I need for a post hole or Sonotube?

Use the cylinder formula π × (diameter ÷ 2)² × depth. A 12 in tube 4 ft deep holds π × 0.5² × 4 = 3.14 cu ft, about 5.2 bags of 80 lb, so 6 bags each. Choose “Round column, post hole or Sonotube” and enter the number of holes as the quantity.

How thick should a concrete slab, patio or driveway be?

The International Residential Code sets a minimum of 3½ inches for concrete slab-on-ground floors (section R506.1). For driveways, the Michigan Concrete Association recommends 4 inches for light use by cars and light trucks and 5–6 inches for medium use. Your local building department and your contractor have the final say.

Should I use bags or order ready-mix concrete?

There is no single cut-off. Bags suit small pours such as post holes, steps and small pads. For bigger pours, weigh how many bags you would have to lift and mix (the calculator shows the total weight) against a ready-mix delivery, and ask local suppliers for their price per cubic yard, minimum order and any short-load fee.

Concrete Calculator — Cubic Yards & Bags Needed

Work out how much concrete you need for slabs, footings, post holes, circular pads and steps — in cubic yards, cubic feet and 40, 50, 60 or 80 lb bags — with a waste allowance and a cost estimate.

How to use the concrete calculator

Pick the shape of the pour and type its measurements. Lengths take feet and inches in separate boxes, while thickness, rise, tread depth and tube diameter are in inches, the way they are usually measured. Enter a quantity when you are pouring several identical pieces, such as eight post holes, and press Add another pour to combine different shapes, for example a patio plus its steps. Switch to meters and centimeters at the top if you measured in metric; what you typed is converted.

The result gives the volume to order, including the waste allowance, in cubic yards, cubic feet and cubic meters, and the number of 80, 60, 50 and 40 lb bags of premixed concrete. Add a price per bag or per cubic yard to estimate the cost. The working underneath shows every step with your numbers, and the page address keeps your measurements so you can bookmark or share them. Nothing is sent anywhere: the calculation runs in your browser.

Concrete volume formulas

Shape	Formula	Example
Slab, patio, sidewalk	`L × W × T`	12 ft × 10 ft × 4 in = 12 × 10 × 0.3333 = 40 cu ft
Footing or wall	`L × W × H`	20 ft × 1 ft 4 in × 8 in = 20 × 1.3333 × 0.6667 = 17.78 cu ft
Round column, post hole	`π × (D ÷ 2)² × H`	12 in × 4 ft: π × 0.5² × 4 = 3.14 cu ft
Circular slab	`π × (D ÷ 2)² × T`	10 ft × 4 in: π × 5² × 0.3333 = 26.18 cu ft
Solid steps	`W × D × R × N(N + 1) ÷ 2`	3 steps, 4 ft wide, 7 in rise, 11 in tread: 4 × 0.9167 × 0.5833 × 6 = 12.83 cu ft

Every measurement must be in the same unit before you multiply, so convert inches to feet first by dividing by 12: 4 inches is 0.3333 ft, not 0.4 ft. Then convert the answer:

cubic yards = cubic feet ÷ 27
cubic meters = cubic feet × 0.0283168
bags = cubic feet ÷ yield per bag (round up)

The steps formula treats the steps as solid concrete down to the ground: the bottom step is one rise high, the next two rises high and so on, so the heights add up as 1 + 2 + … + N = N(N + 1) ÷ 2 rises. Add a landing at the top as a separate slab.

Worked example

The calculator opens with a patio slab 10 ft wide, 12 ft long and 4 in thick, with a 10% waste allowance:

Volume: 4 in = 4 ÷ 12 = 0.3333 ft, so 10 × 12 × 0.3333 = 40 cu ft (1.48 cu yd).
With 10% extra: 40 × 1.1 = 44 cu ft, which is 44 ÷ 27 = 1.63 cu yd or 1.246 m³.
80 lb bags: 44 ÷ 0.60 = 73.3, rounded up to 74 bags — 5,920 lb of dry mix.
Other bag sizes: 98 bags of 60 lb (44 ÷ 0.45), 118 bags of 50 lb (44 ÷ 0.375) or 147 bags of 40 lb (44 ÷ 0.30).
Cost: at $6.50 per 80 lb bag (an illustration, not a typical price), 74 × $6.50 = $481.00.

How many bags of concrete per cubic yard?

Premixed concrete is sold by weight, but what matters is the volume a bag makes once mixed, called its yield. QUIKRETE’s data sheet for its standard Concrete Mix gives these yields, and Sakrete’s data sheet lists the same figures for its 40, 60 and 80 lb bags:

Bag	Yield per bag	Bags per cu yd
80 lb	0.60 cu ft	45
60 lb	0.45 cu ft	60
50 lb	0.375 cu ft	72
40 lb	0.30 cu ft	90

Bags per cubic yard is simply 27 ÷ yield. Other products — fast-setting, high-strength, crack-resistant or mortar mixes — may list a different yield, so check the bag you actually buy. Always round up to whole bags; a partly used bag is cheaper than a second trip in the middle of a pour.

Bags or a ready-mix truck?

Bagged concrete is convenient for post holes, steps, small pads and repairs because you mix only what you need. As the volume grows, so does the work: the example patio needs 74 bags of 80 lb, almost three tons of dry mix to carry, empty and mix, and a slab should be placed without long pauses. Ready-mix concrete arrives mixed and can be placed in one go, but suppliers usually set minimum orders and may add a short-load fee for small deliveries. Get quotes for both before you decide, and make sure a truck can reach the site.

How thick should concrete be?

The International Residential Code requires concrete slab-on-ground floors to be at least 3½ inches thick (section R506.1). For exterior flatwork, the Michigan Concrete Association’s driveway guidelines recommend 4 inches for light use by passenger cars and light trucks and 5 to 6 inches for medium use by light-to-medium trucks. Footings and walls depend on the load, the soil and your local code, so follow your plans or your building department rather than a rule of thumb.

Tips and common mistakes

Inches are not decimals of a foot. 4 in is 0.3333 ft, and 1 ft 6 in is 1.5 ft. Type feet and inches in their own boxes and the calculator converts them.
Measure the real depth. Ground is rarely flat. Check the depth of the forms or hole in several places and use the average, or the deepest point for small pours.
Keep some extra. Spillage, a soft spot in the base or a form that bows all use concrete. Running short mid-pour is worse than having a bag left over.
Follow the bag’s water amount. Extra water makes concrete easier to place but weaker. The yield on the bag assumes the manufacturer’s mixing instructions.
Check the units of the price. Bags are priced each; ready-mix is priced per cubic yard, so compare using the same volume.

Planning the area first? The square footage calculator handles irregular floor plans, and the gravel calculator works out the stone base under a slab. For unit conversions see cubic feet to cubic meters, inches to feet and pounds to kilograms.

Last reviewed 25 September 2026 · Built and checked by the Reeliy team · How we test our tools

Concrete Calculator

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