Free concrete bag calculator: enter slab size, post holes, or a known volume to get cubic yards and 80, 60, and 40 pound bag counts with waste.
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Enter your values and press Calculate.
Choose the project type, then fill in only the fields for that type: slab length, width, and thickness for a slab; hole diameter, depth, and count for post holes; or a known volume in cubic feet. Pick the bag size you want the waste figure for, set a waste allowance, and press Calculate. The calculator works out the volume in cubic feet and cubic yards, gives bag counts for 80, 60, and 40 pound bags side by side, and adds your waste allowance to the selected size. Bag counts are always rounded up, because a part bag still has to be bought. This is a planning estimate, not structural design advice.
Concrete volume is length times width times thickness, with thickness converted from inches to feet by dividing by 12. A 10 by 10 foot slab at 4 inches thick is 10 × 10 × 4 / 12 = 33.33 cubic feet, and one cubic yard holds 27 cubic feet, so that slab is 1.23 cubic yards. Bag counts come from bag yield, the volume one bag makes: an 80 pound bag yields 0.60 cubic feet, a 60 pound bag 0.45, and a 40 pound bag 0.30. Divide volume by yield and round up, because you cannot buy part of a bag. That is why the same slab needs 56 eighty pound bags, 75 sixty pound bags, or 112 forty pound bags before waste.
Pick the project type first. For a slab, patio, walkway, driveway, or shed base, enter length and width in feet and thickness in inches, and ignore the post hole fields. For fence or deck posts, enter hole diameter and depth in inches and the hole count, and ignore the slab fields. If you already know the volume, for example from a ready mix quote, choose known volume and enter cubic feet directly. Choose the bag size for the waste figure, set waste, and press Calculate. Compare the three bag sizes before buying: smaller bags are easier to carry but need more bags and usually cost more per cubic yard.
One cubic yard needs 45 eighty pound bags, 60 sixty pound bags, 72 fifty pound bags, or 90 forty pound bags. Those are the anchor numbers to remember when a supplier quotes in yards and you buy in bags. In metric terms, one cubic metre needs about 55 twenty five kilogram bags at the usual yield, and bag weights of 20, 25, and 40 kilograms are common outside the United States. This calculator works in feet and inches with pound bags; if your supplier sells kilogram bags, convert the volume to cubic feet first, then divide by the yield printed on the bag, since yields vary slightly by brand and mix.
Coverage per bag is the area one bag covers at a given thickness: an 80 lb bag covers 1.8 square feet at 4 inches thick, since 0.60 cubic feet divided by one third of a foot is 1.8, and the same bag covers 1.2 square feet at 6 inches. The cubic yards to bags conversion is yards times 45 for 80 lb bags, and the bags to cubic yards conversion is bags divided by 45 for the same size.
A walkway or patio slab is usually 4 inches thick, which suits foot traffic and light furniture. A driveway or shed base that carries vehicles needs 6 inches, and heavy vehicle areas may need more, plus reinforcement. Thicker slabs use much more concrete: doubling thickness from 4 to 6 inches raises volume by half, so a 20 by 10 foot slab jumps from about 67 cubic feet to 100 cubic feet. Do not guess thickness for anything structural, such as a foundation or a retaining wall footing: those need an engineer's specification, and this calculator only counts bags once the dimensions are decided.
A round hole's volume is pi times radius squared times depth, with radius and depth in feet. A 12 inch diameter hole 36 inches deep holds about 2.36 cubic feet, so four such holes need about 9.42 cubic feet, or 16 eighty pound bags before waste. One detail most calculators skip: the post itself displaces concrete, so the true concrete volume is slightly less than the hole volume. The difference is small for a fence post in a wide hole, so this calculator does not subtract it, which keeps the estimate safely on the generous side. For a large post in a tight hole, subtract the post cross section times depth from the hole volume before ordering.
The same round volume formula covers a sonotube column: a sonotube is a cardboard tube form for a round concrete column, so its volume is pi times radius squared times depth, just like a post hole without the post.
Use 5 percent waste for a clean formed slab with exact dimensions, 10 percent for a normal pour, and 15 percent for rough excavation where the ground is uneven. Water per bag is printed on the bag and is typically about 3 quarts for an 80 pound bag: add water gradually, because too much water weakens concrete even though it makes mixing easier. A pallet usually holds 40 to 56 bags depending on bag size and supplier, so check before ordering a full pallet. When the job passes about one cubic yard, or roughly 45 eighty pound bags, compare ready mix delivery: mixing that many bags by hand is slow, and ready mix often costs less per yard at that scale.
A 12x12 slab at 4 inches thick is 48 cubic feet, which needs 80 eighty pound bags before waste, and 88 with a 10 percent allowance.
A 10 by 10 foot slab at 4 inches thick is 33.33 cubic feet, or 1.23 cubic yards: 56 eighty pound bags before waste, and 62 with a 10 percent allowance. A 20 by 10 foot slab at the same thickness is 66.67 cubic feet: 112 eighty pound bags before waste, and 123 with 10 percent. Four post holes, 12 inches across and 36 inches deep, total 9.42 cubic feet: 16 eighty pound bags before waste, and 18 with 10 percent. In every case the calculator rounds up, because a fraction of a bag still means buying one more bag.
Work out the volume in cubic feet, divide by the bag yield, and round up. A 10 by 10 foot slab at 4 inches thick needs 56 eighty pound bags before waste. Enter your dimensions above for your exact count.
45 bags. An 80 pound bag yields 0.60 cubic feet, and a cubic yard holds 27 cubic feet, so 27 / 0.60 = 45. For 60 pound bags the figure is 60 per yard, and for 40 pound bags it is 90 per yard.
60 bags. A 60 pound bag yields 0.45 cubic feet, and 27 / 0.45 = 60. If you buy 50 pound bags, the figure is 72 per yard.
0.30 cubic feet. A 50 pound bag yields 0.375 cubic feet, a 60 pound bag 0.45, and an 80 pound bag 0.60. Divide your total cubic feet by the yield and round up for the bag count.
At 4 inches thick, 56 eighty pound bags before waste, or 62 with a 10 percent waste allowance. At 6 inches thick the same slab needs 84 bags before waste, because thickness drives volume directly.
A 12 inch diameter hole 36 inches deep needs about 2.36 cubic feet, which is 4 eighty pound bags before waste. Four such holes need 16 bags. Wider or deeper holes rise fast, so measure the actual hole, not the planned hole.
Around one cubic yard, about 45 eighty pound bags, ready mix delivery usually wins on time and often on price. Bags suit small jobs, remote spots a truck cannot reach, and work spread over several days.
Follow the bag instructions, typically about 3 quarts for an 80 pound bag. Add water gradually and mix thoroughly: extra water makes mixing easier but weakens the finished concrete, so never flood the mix.
Use 5 percent for a clean formed slab, 10 percent for a normal pour, and 15 percent for rough excavation. This calculator applies your chosen percentage to the selected bag size and rounds up.
Usually 40 to 56 bags, depending on bag size and supplier. Check with your supplier before ordering a pallet, because a pallet of 80 pound bags weighs far more than a pallet of 40 pound bags.
Concrete sets in hours, can usually take light foot traffic in 24 to 48 hours, and reaches most of its strength in about 28 days. Keep it damp during the first week in hot or dry weather, and avoid heavy loads until it has cured.
Fast-setting concrete suits fence posts and small repairs, because it sets in under an hour and often needs no mixing, just water in the hole. It costs more per bag than standard mix, so it rarely pays for large slabs.