Concrete calculator: how many bags or yards to buy
An 80 lb bag of concrete mix makes about 0.60 cubic feet. So a 10 by 10 ft patio poured 4 inches thick takes 56 bags before any waste, or 1.25 cubic yards from a ready-mix truck.
Where the bag count comes from
Concrete is bought by volume, so the calculator works out the cubic feet inside your forms and divides by what one bag makes. For a slab that is length times width times thickness, with the thickness turned into feet first (4 inches is 0.333 ft). A round column or form tube is pi times the radius squared times the height. A post hole is the same cylinder with the post's own volume taken back out, because the wood sits where concrete would otherwise go. Steps are stacked blocks: each step is as deep as every tread above it plus the landing.
Cubic feet = length (ft) × width (ft) × thickness (ft). Cubic yards = cubic feet ÷ 27. Bags = cubic feet ÷ yield per bag (0.60 cu ft for 80 lb, 0.45 for 60 lb, 0.30 for 40 lb).
Quikrete prints these yields on its data sheet. Sakrete's high-strength sheet matches them for the 40, 60 and 80 lb bags, with 0.66 cubic feet for the 90 lb size. So the two brands differ by at most a bag per yard. The waste allowance is added before dividing into bags, so the count you see already includes it.
Bags per cubic yard and per common pour
| Bag size | Yield per bag | Bags in a cubic yard | Bags for a 10 x 10 ft slab, 4 in |
|---|---|---|---|
| 40 lb | 0.30 cu ft | 90 | 112 |
| 50 lb | 0.375 cu ft | 72 | 89 |
| 60 lb | 0.45 cu ft | 60 | 75 |
| 80 lb | 0.60 cu ft | 45 | 56 |
| 90 lb | 0.675 cu ft | 40 | 50 |
The slab column is before waste. Multiply by 1.1 for the usual 10% extra.
A 12 by 16 ft shed slab, bags against a truck
A 12 by 16 ft slab is 192 square feet. Poured 4 inches thick that is 192 × 0.333 = 64 cubic feet, or 2.37 cubic yards. With 10% extra the order is 70.4 cubic feet. In 80 lb bags that is 70.4 ÷ 0.60 = 117.3, so 118 bags. They weigh 9,440 pounds dry and, at the data sheet's 6 to 9 pints a bag, need about 90 to 130 gallons of water. The same pour from a ready-mix plant is 2.75 cubic yards once it is rounded up to the quarter yard.
At the $148 to $182 per yard in the two public bids, that is $407 to $500 for the concrete. That is before any short-load fee for an order under the plant's minimum. Compare the delivered price plus any short-load fee with the bag total; 2.75 yards is below the 3 to 6 yard minimums in those bids. A thickened edge or footing under the shed walls adds to the count. Put the footing in as a second pour in the calculator.
Measuring the pour so the count is right
Measure the inside of the forms, not the outside of the lumber, because a 2x4 form takes an inch and a half off each side. Check the thickness at several points. A 4 inch slab over a base that dips to 5 inches in the middle needs a quarter more concrete across that area. For a post hole, measure the hole you will actually dig, not the auger size. Quikrete's rule makes it three times the post width across and a third of the post's overall length deep. That gives 32 inches for an 8 ft post. For steps, count the risers from the ground to the top landing, and measure the landing from its front edge to the wall. Concrete per fence post works the hole and depth rules through by post size.
Permits and the limits of a buying estimate
The figures here are for buying material. They do not tell you whether a slab, footing or column is strong enough or how much steel it needs. They also do not tell you how thick your local code says it must be. Footings for a building, retaining walls over a few feet tall, and any slab that carries a structure usually need a permit and an inspection. The inspector's required dimensions are the ones to put into the calculator.
Questions about buying concrete
How many bags of concrete are in a cubic yard?
A cubic yard is 27 cubic feet. Using the Quikrete yields, that is 45 bags of 80 lb, 60 bags of 60 lb and 72 bags of 50 lb. It is also 90 bags of 40 lb, or 40 bags of 90 lb. Other brands print their own yield on the bag and land within a bag or two of these.
How many 80 lb bags do I need for a 10x10 slab?
At 4 inches thick, a 10 by 10 ft slab is 33.3 cubic feet. Divide by the 0.60 cubic feet an 80 lb bag yields and you get 55.6, so 56 bags with no waste. With the usual 10% extra it is 62 bags. At 6 inches thick it is 84 bags before waste.
How much concrete do I need for a fence post?
Quikrete's rule is a hole three times the post's width and one third of the post's overall length deep. So a 4x4 that is 8 ft long gets a 10 to 12 inch hole 32 inches deep. Its data sheet says two 50 lb bags of fast-setting mix set a 3 inch post in a 9 inch hole 30 inches deep. For a 10 inch hole 32 inches deep around a 4x4, the hole volume minus the post is about 1.2 cubic feet. That is 3.3 bags of 50 lb, so buy 4 per hole. Fill to 3 or 4 inches below grade, as the sheet says.
Should I buy bags or order a ready-mix truck?
Bags make sense up to roughly a cubic yard, which is 45 bags of 80 lb and many batches in a mixer. Above that, a ready-mix truck delivers the whole pour at once. One continuous pour also has no cold joints between batches. Two public bids priced delivered 4,000 psi ready-mix at $148 to $182 per cubic yard, with minimum orders of 3 to 6 yards. They were Kingsport, Tennessee in April 2025 and Ottawa County, Oklahoma in December 2025. Ask the plant for its minimum and any short-load fee when you call.
How much extra concrete should I order?
The calculator's default allowance is 10%. The option goes to 15% or 20% when the subgrade is rough or the forms are not perfectly level. A slab that ends up half an inch thicker than planned across 100 square feet swallows 4 cubic feet. That is seven 80 lb bags, so an uneven subgrade is the usual reason a pour runs short.
How thick should a concrete slab be?
Patios and walks are usually 3 1/2 to 4 inches, driveways 4 inches, and the apron at the street 5 to 6 inches in city standards. The residential code sets 3 1/2 inches as the minimum for a floor on the ground. The slab thickness guide lists the documents, and your building department's figure wins.
How much does a bag of concrete weigh once it is mixed?
The Quikrete data sheets put the unit weight of mixed concrete at about 140 pounds per cubic foot. An 80 lb bag that yields 0.60 cubic feet ends up as roughly 84 pounds of concrete in the form. A cubic yard of bagged mix weighs about 3,800 pounds in place.
6 source documents
- The Quikrete Companies, Concrete Mix product data sheet, No. 1101 (yield per bag size and unit weight), revised October 2022. Read 2 November 2026.
- The Quikrete Companies, Fast-Setting Concrete Mix data sheet, No. 1004 (post hole rule and bags per hole), revised November 2025. Read 2 November 2026.
- Sakrete (Oldcastle APG), High-Strength Concrete Mix technical data sheet (yields: 40 lb 0.30, 60 lb 0.45, 80 lb 0.60, 90 lb 0.66 cu ft). Read 2 November 2026.
- The Quikrete Companies, Concrete calculator page (slab calculations at 4 in and 6 in). Read 2 November 2026.
- City of Kingsport, Tennessee, Bid opening minutes, ready-mix concrete, delivered price per cubic yard by strength, 16 April 2025. Read 2 November 2026.
- Ottawa County, Oklahoma, Highway Districts, Bid tally 2025-2026.15, six-month ready-mix concrete, priced per cubic yard delivered, opened 15 December 2025. Read 2 November 2026.