Calculators

Concrete Bag Calculator

How many 40, 60 or 80 lb bags of premix concrete for a pad, a set of post holes or a known volume.

How it works

Premix bags list how much concrete they make once mixed: about 0.60 ft³ for an 80 lb bag, 0.45 ft³ for 60 lb and 0.30 ft³ for 40 lb. Work out the volume of the pour in cubic feet, add a waste allowance, divide by the yield and round up. Post holes are cylinders: π × (diameter ÷ 2)² × depth.

bags = volume (ft³) × (1 + waste) ÷ yield per bag, rounded up
80 lb
Yields about 0.60 ft³
60 lb
Yields about 0.45 ft³
40 lb
Yields about 0.30 ft³
waste
10% covers spillage and uneven holes

Worked example

A 10 × 4 ft pad, 4 inches thick, with 10% waste, using 80 lb bags.

  1. 1Volume: 10 × 4 × (4 ÷ 12) = 13.3 ft³
  2. 2With waste: 13.3 × 1.10 = 14.7 ft³
  3. 380 lb bags: 14.7 ÷ 0.60 = 24.4, rounded up to 25

25 bags of 80 lb, or 33 of 60 lb, or 49 of 40 lb.

Frequently asked questions

How many 80 lb bags of concrete make a yard?

About 45. A cubic yard is 27 cubic feet, and each 80 lb bag yields about 0.6 cubic feet. It takes 60 bags of 60 lb or 90 bags of 40 lb. At that quantity, a ready-mix delivery is usually both cheaper and faster.

How many bags of concrete per fence post?

A 10-inch hole 3 feet deep holds about 1.6 cubic feet, so roughly three 80 lb bags per post before subtracting the post. A 4×4 post takes up about 0.3 ft³ of that, which leaves room to crown the top so water runs off. Fast-setting mixes are poured dry into the hole and then watered, but use the same yields.

Which bag size should I buy?

80 lb bags are the cheapest per cubic foot and the fastest to mix in a wheelbarrow or mixer. 60 lb and 40 lb bags cost more for the same concrete but are much easier to lift, carry across a yard and pour into post holes. For a few posts, the smaller bag is often worth it.

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