How many bags of concrete do I need for a 4x4 slab?

A 4 ft x 4 ft slab at 4 in thick is 5.33 cubic feet. That takes 9 bags of 80 lb, 12 bags of 60 lb, or 18 bags of 40 lb concrete mix (using yields of 0.60, 0.45 and 0.30 cubic feet per bag).

How much concrete does an 80 lb bag make?

An 80 lb bag of standard mix yields about 0.60 cubic feet of mixed concrete; a 60 lb bag yields 0.45 cubic feet and a 40 lb bag 0.30. These are the manufacturers' published yields — check the bag you actually buy and edit the numbers in the calculator if they differ.

How many bags of concrete per fence post?

For a 4x4 post in an 8-inch hole 24 inches deep you need about 0.53 cubic feet of concrete after subtracting the post itself — one 80 lb bag, or two 50 lb fast-setting bags, per post. Run your real hole size in the calculator to total the whole job.

When should I order a ready-mix truck instead of bags?

Above roughly 1 cubic yard (27 cubic feet, about 45 eighty-pound bags), ready-mix delivered by truck is usually cheaper and far less labor. Bags win for small slabs, post holes and repairs where a truck's minimum charge dominates.

Concrete Bag Calculator

Slab or post holes — get the volume and the exact bag count for 80 lb, 60 lb and 40 lb bags before you load the cart.

The short answer

Formula. Cubic feet = length × width × (thickness in inches ÷ 12); bags = cubic feet ÷ yield per bag — an 80-lb bag yields 0.60, a 60-lb bag 0.45, a 40-lb bag 0.30 cubic feet — rounded up.

Example. A 4 ft × 4 ft slab poured 4 in thick is 5.33 cubic feet: nine 80-lb bags of concrete mix (or twelve 60-lb, or eighteen 40-lb).

Job shape

Slab / pad Post holes I know the volume

Slab length (ft)

Slab width (ft)

Thickness (in)

Patio: 4 in. Driveway: 5–6 in.

Bag yields (editable — defaults are the manufacturers' published figures)

80 lb bag yield (cu ft)

60 lb bag yield (cu ft)

40 lb bag yield (cu ft)

How this calculator works

Everything starts with volume in cubic feet. For a slab it is length × width × thickness, with thickness in feet — a 4 × 4 ft slab 4 inches thick is 4 × 4 × (4 ÷ 12) = 5.33 cubic feet. For post holes it is a cylinder per hole (3.1416 × radius² × depth, with radius and depth in feet) minus the part of the post buried in it, which is why an 8-inch hole 24 inches deep for a 4×4 post needs only about 0.53 cubic feet, not the 0.70 the bare hole would suggest.

Then bags. Premixed concrete is sold by weight but yields by volume: the standard published yields are 0.60 cubic feet per 80 lb bag, 0.45 per 60 lb bag, 0.30 per 40 lb bag (figures printed on Quikrete and Sakrete packaging; reviewed October 2026 and editable above). Divide your volume by the yield and round up — you cannot buy a partial bag, and running short mid-pour is the worst way to learn this. The 4 × 4 slab above is 5.33 ÷ 0.60 = 8.9, so nine 80-lb bags, or twelve 60-lb bags.

The buy-bags-or-truck rule falls out of the same arithmetic. One cubic yard is 27 cubic feet — about 45 eighty-pound bags, more than a ton of mix you must mix and place before it starts setting. Below a yard, bags win; above it, ready-mix delivered in a truck is cheaper per foot and immeasurably easier on your back. For post setting specifically, fast-setting mixes save bracing time but follow the same yield math.

Sources & review date

Bag yields are the manufacturers' printed yield-chart figures (80 lb = 0.60, 60 lb = 0.45, 40 lb = 0.30 cubic feet of mixed concrete), checked October 2026 and editable on the page.

Last reviewed: October 2026. Numbers that drift (fees, prices, densities) are editable inputs on this page.

Concrete bag FAQs

How many 80 lb bags is a yard of concrete?

45 bags (27 cubic feet ÷ 0.60 cubic feet per bag). At 60 lb it is 60 bags; at 40 lb, 90. That is the point where nearly everyone switches to ready-mix.

How many bags of concrete for a fence post hole?

With an 8-inch-diameter hole 24 inches deep and a 4×4 post, about 0.53 cubic feet per hole — one 80 lb bag, or two 50-lb fast-setting bags. Bigger posts, wider holes and deeper frost lines push it up; run your real numbers in post-hole mode above.

Should I add extra bags beyond the calculation?

Yes — one or two spare bags for anything over a few bags' worth. Slightly uneven digging, spillage, and a stiff last corner all eat volume, and a spare bag stored dry keeps forever.

Do I need wire mesh or rebar in a small slab?

For a 4-inch patio or shed pad on compacted ground, wire mesh or rebar at 24-in spacing reduces cracking; it adds no volume to the concrete math, only to the shopping list.

Assumptions this tool makes: slab thickness is even and forms are square-sided; post holes are round and the post is centered; bag yields are the manufacturers' published values (80 lb: 0.60, 60 lb: 0.45, 40 lb: 0.30 cu ft — reviewed October 2026, editable above); waste is not included — buy a spare bag or two. Estimates are for planning only; for structural slabs follow local codes.