How Many Bags of Concrete Do I Need?

How to use the concrete bag calculator

1. Pick what you’re pouring. Choose Slab / pad for patios, shed pads, walkways and landings, or Post holes for fence posts, deck posts, mailbox posts and sign posts.
2. Enter the dimensions. Slabs need length, width and thickness. Post holes need the hole diameter, hole depth, the post size and how many holes you’re digging.
3. Add the gravel base for posts. Most fence builders put 4–6 in of gravel in the bottom of each hole so water drains away from the end grain. That part of the hole doesn’t get concrete, so the calculator subtracts it.
4. Choose a bag size and waste allowance. The table shows the count for every common bag size, so you can compare what your store stocks.

Everything updates as you type, and the Metric switch converts inputs to meters and centimeters and volumes to cubic meters if you prefer.

How many bags of concrete: the formula

Bagged concrete is sold by weight, but what you need is volume. Every bag lists its yield — the volume of finished concrete it makes once mixed with water. The math is three steps:

Post-hole dimensions are in inches, so dividing cubic inches by 1,728 (12 × 12 × 12) converts to cubic feet. The concrete depth is the hole depth minus any gravel in the bottom.

Bag yields used

These are the yields printed on QUIKRETE’s Concrete Mix data sheet and on standard bagged concrete mixes generally (see Sources & assumptions below). Fast-setting post mixes and high-strength mixes can yield slightly less per pound — if your bag lists a different number, trust the bag.

Worked examples

How many bags of concrete for a 10×10 slab?

A 10 × 10 ft patio poured 4 in thick holds 10 × 10 × 4 ÷ 12 = 33.3 cu ft (1.23 cubic yards). Divided by 0.60 cu ft per bag, that’s 55.6, so 56 × 80 lb bags with zero waste. Add 10% for spillage and an uneven base and you should buy 62 × 80 lb bags — or 82 × 60 lb bags, or 123 × 40 lb bags.

At 62 bags you’re mixing nearly 5,000 lb of material by hand. Around one cubic yard is where most people start pricing a ready-mix delivery or a towable mixer instead.

How many bags of concrete per fence post?

Take a 4×4 post (actual size 3.5 × 3.5 in) in a 10 in diameter hole dug 36 in deep, with 6 in of gravel in the bottom. Concrete fills the top 30 in:

• Hole: π × 5² × 30 = 2,356 cubic inches
• Post: 3.5 × 3.5 × 30 = 368 cubic inches
• Concrete: 1,989 cubic inches ÷ 1,728 = 1.15 cu ft per post

That is about 2 × 80 lb bags per post (1.92 bags exactly), or 3.07 × 50 lb bags — so 4 × 50 lb bags for a single post. For a 10-post fence, the total is 11.51 cu ft: 11.51 ÷ 0.60 = 19.2, which rounds up to 20 × 80 lb bags with no waste. With 10% waste it is 12.66 cu ft ÷ 0.60 = 21.1, which rounds up to 22 bags. The calculator adds the waste to the whole job and rounds once at the end, rather than rounding hole by hole.

Waste factor and buying tips

• Add 5–10% for waste. Subgrades are never perfectly flat, forms bow slightly, and some mix always stays in the wheelbarrow. Use 10% for slabs on dug-out soil and 5% for neat, formed pours.
• Measure holes after digging. Post holes dug with a clamshell digger or auger often end up wider at the top. One extra inch of diameter on a 10 in hole around a 4×4 post adds about 25% more concrete.
• Pick bag size by job, not just price. 80 lb bags are cheapest per cubic foot, but 40–60 lb bags are easier to carry to a backyard and to mix in small batches for posts.
• Buy one or two extra bags. Unopened bags can usually be returned; running out mid-pour can leave a cold joint in a slab.
• Keep bags dry. Store them off the ground under cover. A bag that has absorbed moisture and gone lumpy won’t reach full strength.

Need several shapes in one pour — a slab plus footings, round columns or steps? Use the multi-shape concrete calculator. Ordering gravel for under the slab? The gravel calculator gives tons and yards.

Recommended thickness and hole size

These are common starting points, not engineering specs. Frost depth varies from a few inches in the Gulf states to more than four feet in the northern US; your local building department can tell you the required footing depth.

Sources & assumptions

How this calculator gets its numbers. Full method: our methodology.

• Bag yields follow the manufacturer data sheet: 0.30, 0.375, 0.45 and 0.60 cu ft for 40, 50, 60 and 80 lb bags (45 × 80 lb bags per cubic yard).
• The default 10% extra covers spillage, uneven subgrade and form bulge; change it in the tool.
• Bag counts are always rounded up to whole bags.

Sources

1. QUIKRETE, Concrete Mix No. 1101 product data sheet. Bag yields: 40 lb ≈ 0.30 cu ft, 50 lb ≈ 0.375, 60 lb ≈ 0.45, 80 lb ≈ 0.60 cu ft.
2. ASTM International, ASTM C387/C387M, Standard Specification for Packaged, Dry, Combined Materials for Concrete and High Strength Mortar. The product standard bagged concrete mixes are made to.

Last reviewed October 25, 2026. Product figures vary by brand; the label on what you buy wins.