Concrete Bag Calculator

See how many bags of concrete mix a job takes in every common size, what they weigh in total, how much mixing water to have on hand and which size costs least per cubic foot.

How to use the concrete bag calculator

1. Choose a slab or footing, a set of post holes, or enter a volume you already know.
2. For post holes, enter the hole diameter and the depth of concrete, then pick the post. The space the post takes up is subtracted.
3. Keep the extra allowance unless your holes are exact, and add bag prices from your store if you want to know which size is cheapest.
4. Read the bag count for each size, the total weight you’ll be moving and the water to have ready.

How the bag count is worked out

Bags = volume (ft³) × (1 + extra %) ÷ yield per bag, rounded up
Post hole volume (in³) = π × (hole diameter ÷ 2)² × depth − post area × depth
Cubic feet = cubic inches ÷ 1,728

Yields are from the QUIKRETE Concrete Mix (No. 1101) data sheet. Weight and yield go together: every size works out to about 0.0075 cubic feet per pound, so a heavier bag simply means fewer trips to the mixer.

Eight pints make a gallon, so an 80 lb bag starts at ¾ gallon of water. Add water gradually toward the maximum only as needed for workability.

Worked example: setting eight fence posts

Those 17 bags weigh 1,360 lb in total and need roughly 12.8 to 19.1 gallons of water.

Which bag size is cheapest?

Compare the price per cubic foot, not the price per bag. With example shelf prices of $4.25 for 40 lb, $5.50 for 60 lb and $6.75 for 80 lb, the cost of 10 ft³ works out as follows:

These prices are only for illustration; enter the prices at your store in the calculator. Smaller bags can cost more per cubic foot, but they are easier to carry, which can matter more on a hillside or a long wheelbarrow run.

Tips

• Enter actual post sizes. A nominal 4 × 4 measures 3½ × 3½ in and a 6 × 6 measures 5½ × 5½ in under the American Softwood Lumber Standard.
• Don’t forget the gravel. If you put gravel at the bottom of each hole for drainage, measure concrete depth from the top of the gravel.
• Buy one spare bag. Returning an unopened bag is easier than a second trip with a half-filled hole.
• For bigger pours, the concrete calculator gives cubic yards for ordering ready-mix.

Questions

How many bags of concrete do I need per fence post?

A 10 in hole with 30 in of concrete around a 4 × 4 takes about 1.15 ft³: 2 bags of 80 lb mix or 4 bags of 40 lb, before any extra. Wider or deeper holes add up quickly, so run your own sizes above.

How many 60 lb bags make a cubic yard?

60. Each 60 lb bag yields about 0.45 ft³, and 27 ÷ 0.45 = 60.

How much water goes in an 80 lb bag of concrete?

QUIKRETE’s data sheet gives 6 pints (¾ gallon) to start and up to 9 pints (1⅛ gallons) for an 80 lb bag of its Concrete Mix.

Is a 90 lb bag worth it?

It yields 0.675 ft³, so 40 bags make a cubic yard instead of 45 with 80 lb bags. That saves five bags per yard but each one is heavier to lift.

Sources

1. Concrete Mix Product No. 1101 data sheet, QUIKRETE.
   Used for: Bag yields (40 lb ≈ 0.30 ft³ to 90 lb ≈ 0.675 ft³) and mixing water per bag.
2. Voluntary Product Standard PS 20-20: American Softwood Lumber Standard, NIST / American Lumber Standard Committee.
   Used for: Board feet are figured on nominal sizes (section 2.2); nominal vs. dry dressed sizes (a 2 × 4 is 1 1/2 × 3 1/2 in).

Related calculators

• Concrete CalculatorCubic yards and bags for slabs, footings, walls and columns.
• Concrete Block CalculatorBlocks per wall, courses and 80 lb mortar bags.
• Cubic Feet CalculatorCubic feet of boxes and cylinders, with gallons.