How to measure for concrete
Concrete is sold by volume, so every job comes down to length × width × depth. The catch is mixing units: slab sizes are usually measured in feet but thickness in inches. Divide inches by 12 before multiplying, then divide cubic feet by 27 to get cubic yards, the unit ready-mix suppliers use.
For a slab or patio, measure the inside of the forms. For a footing, add up the length of every run and use the trench width and depth in inches. For a round column, pier or post hole, the volume is a cylinder: π × radius² × height, with the radius being half the diameter.
Measure thickness in several places. A slab planned at 4 inches that averages 4.5 inches because the gravel base dips needs 12.5% more concrete, which is why the extra-for-waste figure matters.
Bags or ready-mix?
Premix bags make sense for post holes, small pads, steps and repairs. QUIKRETE publishes approximate yields for its standard Concrete Mix: 0.30 cubic feet for a 40 lb bag, 0.45 for a 60 lb bag and 0.60 for an 80 lb bag. That means one cubic yard (27 cubic feet) takes 45 bags of 80 lb, or 3,600 lb of dry mix.
Mixing and placing dozens of bags by hand is heavy, slow work, and a big slab poured in several batches can show joints between them. For larger pours, compare the bag count with a delivered ready-mix quote; the table under the result shows how many bags of each size you would need and how much they weigh in total.
How much extra to add
Running short in the middle of a pour is far worse than having a few bags left over. The default here adds 10%, which covers spillage, a slightly uneven base and forms that bow outward. Raise it if the ground is rough or you are digging holes by hand, since holes rarely come out as neat cylinders.
Leftover dry mix keeps if it stays dry and sealed, and it is handy for setting a post or patching later, so rounding up costs little.
The formulas
Slab: V = L × W × (T ÷ 12)Footing: V = L × (W ÷ 12) × (D ÷ 12)Column: V = π × (d ÷ 24)² × HCubic yards = V × (1 + waste ÷ 100) × count ÷ 27Bags = round up (V × (1 + waste ÷ 100) × count ÷ bag yield)- V = volume of one pour in cubic feet
- L, W, H = length, width and height in feet (footing width in inches)
- T, D = thickness or depth in inches; d = column diameter in inches
- bag yield = cubic feet per bag (0.30 for 40 lb, 0.45 for 60 lb, 0.60 for 80 lb)
Example: a 10 × 10 ft patio, 4 inches thick
- Volume: 10 × 10 × (4 ÷ 12) = 33.33 cubic feet, or 33.33 ÷ 27 = 1.23 cubic yards.
- Add 10% for waste: 33.33 × 1.10 = 36.67 cubic feet, or 1.36 cubic yards (1.04 cubic meters).
- 80 lb bags at 0.60 cu ft each: 36.67 ÷ 0.60 = 61.1, so buy 62 bags.
- With 60 lb bags (0.45 cu ft) you would need 82 bags; with 40 lb bags (0.30 cu ft), 123 bags.
Frequently asked questions
How many 80 lb bags of concrete make a cubic yard?
At QUIKRETE's published yield of 0.60 cubic feet per 80 lb bag, a cubic yard (27 cubic feet) takes 27 ÷ 0.60 = 45 bags, before any extra for waste.
How many bags do I need for a post hole?
A hole 12 inches wide and 3 feet deep holds π × 0.5² × 3 = 2.36 cubic feet. With 10% extra that is 2.59 cubic feet, or 5 bags of 80 lb (4.32 rounded up). The post itself takes up some room, so you may have a little left over.
How do I convert cubic feet to cubic yards?
Divide by 27, because a cubic yard is 3 ft × 3 ft × 3 ft. So 36.67 cubic feet is 36.67 ÷ 27 = 1.36 cubic yards.
Is a 60 lb bag a better deal than an 80 lb bag?
Yield is proportional to weight, so compare the price per cubic foot: divide the bag price by 0.45 for a 60 lb bag or 0.60 for an 80 lb bag. Smaller bags cost more per cubic foot but are easier to lift.
Does this work for steps?
Solid steps can be split into boxes: each step is a box as wide as the stairs, one tread deep and as tall as the top of that step. Run each box through slab mode (with its height in inches as the thickness), add the cubic feet, then divide by the bag yield.
Sources
Last reviewed for 2026. How we calculate.