Concrete Bag Calculator

Multiply length by width by thickness in feet to get cubic feet, add your waste allowance, then divide by the yield of one bag. An 80 lb bag yields about 0.60 cubic feet. Round the result up to the next whole bag, because partial bags cannot be bought.

Worked example: 10 ft x 10 ft x 4 in

1. Measure the slab. Measure length and width in feet and thickness in inches. Here it is 10 ft, 10 ft and 4 in.
2. Convert to cubic feet. Multiply length by width by thickness divided by 12. 10 x 10 x (4 / 12) = 33.33 ft³.
3. Add a waste allowance. Multiply the volume by 1.10 for a 10% allowance. 33.33 x 1.10 = 36.67 ft³.
4. Divide by bag yield. Divide by the yield of one bag, 0.60 cubic feet for an 80 lb bag. 36.67 / 0.60 = 61.1 bags.
5. Round up. Round the result up to the next whole bag. Buy 62 bags.

If you already know the volume, the Volume tab skips the first two steps. The Cubic Yards Calculator gives that figure from your dimensions. The default allowance is 10%.

A cubic yard holds 27 cubic feet, so it takes 45 bags at 80 lb, 60 bags at 60 lb, or 90 bags at 40 lb. These counts use the manufacturer yields of 0.60, 0.45 and 0.30 cubic feet per bag and include no waste allowance.

To size a known volume, multiply cubic yards by the bags per yard for your bag weight. Two cubic yards at 80 lb is 2 x 45 = 90 bags before any allowance. Table 1 also lists each bag weight in kilograms and each yield in litres for metric orders.

Use Table 2 to check an area. At 4 in thick one 80 lb bag covers 1.8 sq ft, so 100 sq ft needs 55.6 bags before allowance. Thicker pours cover less per bag, so the count rises quickly with depth. Other brands and mixes differ, so check the label.

Yield and bags per cubic yard

Coverage per bag by thickness

At 4 in thick with a 10% waste allowance, a 4 ft x 4 ft slab needs 10 bags of 80 lb mix, a 10 ft x 10 ft slab needs 62, and a 20 ft x 20 ft slab needs 245. The table also lists the 60 lb and 40 lb counts for each size.

The 10 ft x 10 ft row matches the hero example: 33.33 ft³, 36.67 ft³ with the allowance, and 62 bags of 80 lb mix. The allowance adds 6 bags on that slab. Bag counts scale with area, so a 20 ft x 20 ft slab needs about four times the bags of a 10 ft x 10 ft slab.

For a different thickness, or a slab that is not a rectangle, enter the area on the Area tab, or use the Square Footage Calculator first.

Pick the bag weight you can lift and mix without breaks. The sizes carry the same product and the same strength rating, so the choice changes bag count and lifting, not the mix. An 80 lb order for a 10 ft x 10 ft slab weighs 4,960 lb across 62 bags.

The total load barely changes: the 40 lb and 60 lb orders both weigh 4,920 lb, against 4,960 lb at 80 lb. What changes is how many bags you open and lift.

A bag of concrete mix is not bagged cement. The mix already combines cement, sand and gravel, which is what ASTM C387 covers for packaged, dry, combined materials for concrete.

For QUIKRETE Concrete Mix, an 80 lb bag starts with 6 pt of water and has a maximum expected water of 9 pt. A 60 lb bag runs 4 to 7 pt and a 40 lb bag runs 3 to 4-1/2 pt. The target slump is 2 to 3 in.

Start at the lower figure and add water until the mix is workable, then check slump if you have a cone.

Ready-mix wins when the bag cost per cubic foot, plus mixing labor, exceeds the delivered price per cubic yard divided by 27, plus any short-load fee. Compare both per cubic foot using your own local quotes. The Concrete Calculator explains when delivery makes sense.

• Bags: cost per cubic foot = bag price / yield per bag, such as 0.60 ft³ for an 80 lb bag.
• Ready-mix: cost per cubic foot = delivered price per yd³ / 27, plus any short-load fee.

Enter your two prices in the optional cost box under the calculator. For a full project estimate, use the Concrete Cost Calculator. For large pours, see the Concrete Calculator.

The common errors are rounding down, skipping the waste allowance, entering inches where feet are expected, and applying one yield to every bag weight. Skipping a 10% allowance alone turns a 62-bag order for a 10 ft x 10 ft slab into 56.

1. Rounding down. Truncating 61.1 to 61 bags leaves the pour 0.07 ft³ short.
2. Skipping the allowance. The count drops from 62 bags to 56.
3. Entering inches as feet. A 4 in slab typed as 4 ft gives 734 bags, not 62. Convert first with the Feet and Inches Calculator.
4. Using one yield for every size. Dividing a 60 lb order by 0.60 gives 62 bags. The 60 lb yield is 0.45, which gives 82.

• Round up, then buy one extra bag if the pour cannot pause.
• Check the order weight against your vehicle and your helpers.
• Add the starting water from Table 4, then adjust to a 2 to 3 in slump.
• Set rebar before the pour, and size it with the Rebar Calculator.
• Compact a gravel base first, and size it with the Gravel Calculator.

Open the Rebar Calculator and the Gravel Calculator next, using the same slab size. Then price the job with the Concrete Cost Calculator.

Bags = ceil(volume in ft³ x (1 + waste) / yield per bag). Yields, bag weights and water come from the QUIKRETE Concrete Mix 1101 data sheet, revised October 2022. Counts round up as the QUIKRETE calculator does. The 10% default follows the Sakrete concrete calculators. Yields apply to QUIKRETE Concrete Mix only.

Verified against QUIKRETE Concrete Mix 1101 yields, October 2022. Results are for reference only.

Update log

• October 2026: version 1.0.0, first publication.