Concrete Bags Calculator

How many bags of concrete do I need for a slab?

Convert the pour to cubic feet first, then divide by the yield printed on the bag. Volume in cubic feet equals length × width × thickness, all in feet, so a 4 in thickness enters the formula as 0.333 ft. Quikrete Concrete Mix yields approximately 0.30 ft³ for a 40 lb bag, 0.375 ft³ for 50 lb, 0.45 ft³ for 60 lb, 0.60 ft³ for 80 lb, and 0.675 ft³ for a 90 lb bag [1]. Sakrete High Strength Concrete Mix lists the same 0.30, 0.45, and 0.60 ft³ figures with 0.66 ft³ for its 90 lb bag [2]. Because bags are sold whole, the calculator always rounds the final count up.

A worked slab makes the sequence concrete. A 4 ft × 6 ft equipment pad at 4 in thick is 24 ft² × 0.333 ft = 8.0 ft³. With the 10 percent allowance that becomes 8.8 ft³. Dividing by 0.60 ft³ gives 14.7, so buy 15 bags of 80 lb mix, which is 1,200 lb of material. The same pad in 60 lb bags is 8.8 divided by 0.45, or 19.6, so 20 bags. Pick one bag weight per order and stay with it, because mixing sizes in a single calculation produces a count that matches nothing on the pallet.

How much does an 80 lb bag of concrete cover?

Coverage depends entirely on thickness, because the bag holds a fixed volume. An 80 lb bag yields about 0.60 ft³, so at 4 in thick it covers 0.60 divided by 0.333, or 1.8 ft². At 2 in the same bag covers 3.6 ft², at 3 in it covers 2.4 ft², at 5 in it covers 1.44 ft², and at 6 in only 1.2 ft². Those numbers explain why bag counts climb so quickly on flatwork: a modest 10 ft × 10 ft patio at 4 in is 33.3 ft³, which is 56 bags of 80 lb mix before any waste allowance. Sakrete gives the same arithmetic in its own coverage instructions [2].

Do not confuse bag weight with volume or with strength. The 80 lb figure is the shipping weight of dry material; the useful number for estimating is the cubic-foot yield. Quikrete lists a unit weight of roughly 140 lb per ft³ for cured concrete mix, which is why 0.60 ft³ of finished concrete comes from an 80 lb bag plus about 6 pints of water [1]. Both Quikrete Concrete Mix and Sakrete High Strength Concrete Mix are rated at 4,000 psi at 28 days when tested to ASTM C39, and both meet ASTM C387 for packaged dry combined materials [1][2].

How many bags of concrete for a fence post hole?

Post holes are where bag counts are most often wrong, because a hole is deeper than people picture. Take a square-sided hole 12 in × 12 in and 36 in deep: that is 1 ft × 1 ft × 3 ft = 3.0 ft³. With the 10 percent allowance the volume is 3.3 ft³, and 3.3 divided by 0.60 gives 5.5, so the named result is 6 bags of 80 lb mix per hole. Ten posts is 60 bags, or 4,800 lb. Quikrete recommends digging the hole about three times the post diameter with a depth of one third of the overall post height, which is what pushes these volumes up [1].

Round holes take the cylinder formula, and the calculator accepts either equivalent square dimensions or a direct volume entry. A 10 in diameter sonotube 42 in deep is pi × (5 divided by 12)² × 3.5 ft = 1.91 ft³, so 2.1 ft³ with waste, which is 4 bags of 80 lb mix. A 12 in tube at the same depth is 2.75 ft³, or 3.02 ft³ with waste, needing 6 bags. Subtract the volume the post itself occupies only if the post is large relative to the hole; for a 4 × 4 in wood post in a 12 in hole the difference is small enough to leave in the allowance.

Is it cheaper to buy bags of concrete or ready-mix?

Bags win on small, scattered, or awkward pours. Anything under roughly one cubic yard, meaning fewer than about 45 bags of 80 lb mix, is usually cheaper and simpler in bags because ready-mix carries minimum-load and short-load charges. Bags also let you pour one post today and another next weekend, store leftovers dry, and reach places a truck chute cannot. The cost per cubic foot is higher, but you pay only for what you open, and Quikrete notes that you should allow at least 1 ft³ of mixer capacity for each 80 lb bag mixed at one time [1].

Larger flatwork flips the comparison. Two cubic yards is 90 bags of 80 lb mix, about 7,200 lb, and both manufacturers instruct you to place concrete to the full depth of the form and work continuously to avoid cold joints [1][2]. Hand mixing that quantity in batches makes continuity hard. Water control is another factor: Sakrete warns that excess water reduces strength and durability and can cause cracking, dusting, or scaling [2]. This calculator converts volume to a bag count. It does not price bags, compare delivery quotes, or judge whether your crew can keep up with the pour.

How much water do you add to a bag of concrete?

Follow the label, because water content is a strength decision. Quikrete gives a starting water content of 6 pints for an 80 lb bag with a maximum expected content of 9 pints, and lists 3 pints starting for 40 lb, 3.5 pints for 50 lb, 4 pints for 60 lb, and 7 pints for 90 lb [1]. Sakrete lists 1.75 quarts for 40 lb, 2.5 quarts for 60 lb, 3.5 quarts for 80 lb, and 4.5 quarts for 90 lb [2]. If a slump cone is available, Quikrete says to adjust water to reach a 2 in to 3 in slump per ASTM C143 [1].

Aim for a stiff, moldable consistency with no dry pockets and no standing puddles of water on the surface. Quikrete directs you to work the mix with a shovel, rake, or hoe and to add water only as needed [1]; Sakrete tells you to avoid a soupy mix outright [2]. Temperature matters too: both manufacturers want material, air, and substrate between roughly 40 and 90 degrees Fahrenheit, and both call for protecting fresh concrete from freezing for 48 hours [1][2]. None of that changes the bag count, but it decides whether the finished pour reaches the strength on the bag.

How much does each concrete bag size yield?

Yields from the Quikrete Concrete Mix data sheet. Sakrete High Strength lists 0.66 ft³ for its 90 lb bag, which is 41 bags per yd³.

How much mixing water does each bag size need?

Manufacturer figures are approximate. Excess water reduces strength; target a 2 in to 3 in slump.

How many 80 lb bags does a small pour need?

Counts round up to whole bags. At 56 bags a 10 × 10 patio is already past the point where ready-mix usually wins.

Work these out first

• slab volume in cubic yardsConcrete Slab Calculator
• area of the pourSquare Footage Calculator

What to do with this result

• cost per bag across the whole orderMaterial Cost Calculator

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• waste allowance on bagged mixProject Waste Factor Calculator