Concrete Bag Calculator

1. Choose the shape: a slab or footing (length by width by thickness) or a round hole or column (diameter by depth).
2. Enter the dimensions. Keep length, width and depth in feet, and thickness or diameter in inches, as the fields are labelled.
3. Pick the bag size you are buying and add a waste allowance of 5 to 10 percent for spillage and an uneven base.
4. Read the result: the bag count is rounded up to whole bags. Add a price per bag for the total cost.

Here is what each result line means:

A concrete bag calculator converts the size of a pour into the number of pre-mixed concrete bags you need to buy. It answers the question behind every small concrete job: how many bags do I pick up at the store?

Pre-mixed concrete is sold in 40, 60 and 80 pound bags that each cover a fixed volume. The QUIKRETE Concrete Mix data sheet gives yields of about 0.60 cubic feet for an 80 pound bag, 0.45 for a 60 pound bag and 0.30 for a 40 pound bag, so the calculator finds the volume of your pour and divides by the yield of the bag you choose. It is made for small pours; for a large one, price a ready-mix delivery with the concrete cost calculator instead. To batch your own concrete from separate cement, sand and gravel, use the concrete mix calculator, or size just the binder with the cement calculator.

First the tool finds the volume of the pour in cubic feet, because that is the unit bag yields are quoted in. For a slab that is length times width times thickness, with thickness converted from inches to feet.

1. Convert thickness from inches to feet by dividing by 12 (a 4 inch slab is 0.333 feet).
2. Multiply length by width by that thickness, then by the quantity of identical pours, to get cubic feet.
3. Add the waste allowance for spillage and an uneven base.
4. Divide by the bag yield (0.60 cubic feet for an 80 pound bag) and round up to whole bags.

For a round hole or column the footprint is a circle, so the volume is pi times the radius squared times the depth, with the radius taken as half the diameter.

Suppose you are pouring a patio slab 10 feet long, 10 feet wide and 4 inches thick, using 80 pound bags with a 10 percent waste allowance.

Calculation: volume = 10 x 10 x (4 / 12) = 33.33 cu ft. Adding 10 percent waste gives 36.67 cu ft. At 0.60 cubic feet per 80 pound bag that is 36.67 / 0.60 = 61.1, rounded up to 62 bags. Those bags weigh 62 x 80 = 4,960 pounds and fill about 1.36 cubic yards.

The lesson: 62 bags is roughly two and a half tons to haul, lift and mix. At this size most people price a short-load ready-mix delivery before deciding to bag it.

Bag Size

The bag size sets the yield: an 80 pound bag covers twice the volume of a 40 pound bag, so switching sizes changes the count even though the volume is the same. Bigger bags mean fewer to carry but heavier lifts.

Thickness and Depth

Thickness multiplies the whole area, so it drives the bag count more than people expect. Moving a slab from 4 to 6 inches adds half as many bags again.

Waste and Spillage

Some mix is lost to the mixer, the barrow and trimming. A 5 to 10 percent allowance covers it, and rounding up to whole bags adds a little more headroom.

Posts and Displaced Volume

A fence post or pier in the hole displaces concrete. If you set a post, the hole needs fewer bags than its full volume suggests, so subtract the post or treat the extra as waste.

Bags suit small pours; ready-mix by the cubic yard suits large ones. The crossover is usually somewhere between one and two cubic yards, which is about 45 to 90 eighty-pound bags.

One cubic yard is 27 cubic feet, so it takes about 45 eighty-pound bags to equal a single yard of ready-mix.

Setting Fence Posts

Use the round shape for post holes, enter the diameter and depth, and set the quantity to the number of posts to get the total bags in one step.

Small Slabs and Pads

Patios, shed bases, AC pads and steppingstones are classic bag jobs. Size them as a slab and the tool gives the bag count and the weight to carry.

Repairs and Footings

Small footings, curb repairs and fillets are easy to bag. Break an odd shape into rectangles and round holes, add the bags for each, and round up.

1. Rounding the Bag Count Down

Always round up. A part bag still has to be bought, and being one bag short stops the pour. The tool rounds up for you.

2. Mixing Up Feet and Inches

Entering thickness in feet instead of inches shrinks the result twelvefold. Keep length and width in feet and thickness in inches, as the fields are labelled.

3. Forgetting the Post Displaces Concrete

Filling a post hole, the post itself takes up space. Using the full hole volume over-buys, though a little extra is safe insurance.

4. Ignoring Waste on a Rough Base

An uneven or over-dug base swallows mix. Without a waste allowance you run short, so keep 5 to 10 percent.

5. Bagging a Pour That Should Be Ready-mix

Past roughly a cubic yard, dozens of bags cost more and take far longer than a delivery. Check the volume line before you buy a pallet of bags.

The arithmetic is exact and the bag yields are the manufacturer’s published figures; the usefulness of the answer depends on the dimensions and the waste allowance you enter.

A 10 by 10 foot slab at 4 inches thick needs about 62 eighty-pound bags with a 10 percent waste allowance, 82 sixty-pound bags, or 123 forty-pound bags. Enter your slab size above to get the count for the bag size you are buying.

About 45. An 80 pound bag yields roughly 0.60 cubic feet, and a cubic yard is 27 cubic feet, so 27 divided by 0.60 is 45 bags. A 60 pound bag yields about 0.45 cubic feet, which works out to about 60 bags per cubic yard.

About 0.60 cubic feet of finished concrete, per the QUIKRETE data sheet. That is a patch roughly 1.8 square feet at 4 inches thick. A 60 pound bag covers about 0.45 cubic feet and a 40 pound bag about 0.30 cubic feet.

It depends on the hole and the post. For a 10 inch diameter hole 2 feet deep, the hole holds about 1.1 cubic feet, so you need about 2 eighty-pound bags before allowing for the post, which displaces some concrete. Set the quantity to the number of posts.

For small pours under about a cubic yard, bags are convenient and often cheaper once delivery and short-load fees are counted. Past one to two cubic yards, ready-mix usually wins on both cost and effort. The volume line here helps you judge the crossover.

The weight is the bag size: 40, 60 or 80 pounds dry. The tool multiplies the bag count by that weight so you can plan the load. Sixty-two 80 pound bags, for example, weigh 4,960 pounds, close to two and a half short tons.

Because you cannot buy part of a bag, and running one bag short stops the pour partway through, risking a cold joint. The tool always rounds up to whole bags, and the waste allowance adds a little extra for spillage.

No. The 0.60, 0.45 and 0.30 cubic foot figures are for standard concrete mix. Fast-setting, high-early-strength and sand or mortar mixes have different yields, so check the data sheet for the exact product you are buying.

Follow the bag: the mixing water is printed on it and varies by product and bag size. This calculator estimates bag count and volume, not mixing water, so read the data sheet for the amount per bag.

No. The calculation runs entirely in your browser and nothing you enter is stored or sent anywhere.

• QUIKRETE Concrete Mix 1101 data sheet (QUIKRETE; bag yields 0.60, 0.45 and 0.30 cu ft and unit weight).
• QUIKRETE Concrete Mix product line (QUIKRETE; available bag sizes and product data).
• National Ready Mixed Concrete Association (NRMCA; ready-mixed concrete and the bag versus delivery comparison).
• American Cement Association (American Cement Association; what concrete and cement are).