How Many Bags of Concrete Do I Need for a Slab?

The volume calculation

Everything starts with cubic feet, then converts to yards or bags.

volume (ft³) = length × width × (thickness in inches ÷ 12)
cubic yards = ft³ ÷ 27

A 10 × 10 ft slab at 4 inches: 10 × 10 × 0.333 = 33.33 ft³, which is 1.24 cubic yards. Add 10% for waste and you are buying for 36.67 ft³.

Bag yields

Bag size	Yield	Bags per cubic yard
80 lb	0.60 ft³	45
60 lb	0.45 ft³	60
40 lb	0.30 ft³	90

Bag counts for common slabs

All at 4 inches thick, including a 10% waste allowance:

Slab	Volume	Cubic yd	80 lb bags	60 lb bags
4 × 4 ft	5.3 ft³	0.20	10	14
8 × 10 ft	26.7 ft³	0.99	49	66
10 × 10 ft	33.3 ft³	1.24	62	82
12 × 12 ft	48.0 ft³	1.78	88	118
20 × 20 ft	133.3 ft³	4.94	245	326

Slabs, footings and columns, in yd³ and bags.Concrete calculator →

Thickness changes everything

Volume scales directly with depth, so a modest thickness change is not modest at all:

10 × 10 ft slab — 80 lb bags needed

4 inch62

5 inch77

6 inch92

Use 4 inches for patios, walkways and shed bases. Use 5 to 6 inches where vehicles park or heavy equipment sits.

The bags-versus-truck line

The practical threshold is about one cubic yard, or 45 bags of 80 lb mix. Above that, three problems compound:

Cold joints. Hand-mixing 60+ bags takes long enough that early batches begin curing before later ones arrive, creating weak seams.
Weight. 62 bags of 80 lb mix is nearly 5,000 lb to move by hand, twice — off the pallet and into the mixer.
Cost. Bagged mix per cubic yard is typically well above ready-mix once you pass a yard or two, even allowing for short-load fees.

Ordering ready-mix? Order in quarter-yard increments and round up. Running short mid-pour is far more expensive than a wheelbarrow of leftovers — and suppliers charge for a return trip.

Before you pour

Compact the sub-base. Four inches of compacted gravel under the slab prevents settlement cracking.
Plan control joints. Cut them at roughly 24 to 36 times the slab thickness in feet — about every 8 to 10 feet for a 4-inch slab. Concrete will crack; joints decide where.
Check the weather. Pouring below 40°F or above 90°F requires specific precautions or the cure suffers.

These figures cover the concrete only. Forms, rebar or mesh, gravel base and a vapour barrier are separate — and on a small slab they often cost more than the mix.

Working through a whole renovation? The same measure-first approach applies to paint, where the usual error is forgetting to subtract doors and windows.

Run your own numbers

Concrete Paint Percentage

FAQ

How many 80 lb bags of concrete make a cubic yard?

45 bags. An 80 lb bag yields 0.60 cubic feet and a cubic yard is 27 cubic feet, so 27 ÷ 0.6 = 45. For 60 lb bags it is 60 bags, and for 40 lb bags it is 90.

How many bags do I need for a 10×10 slab?

At 4 inches thick, a 10×10 ft slab is 33.3 cubic feet, or 1.24 cubic yards. Adding the standard 10% waste allowance, that is about 62 bags of 80 lb mix, 82 bags of 60 lb, or 123 bags of 40 lb.

How thick should a concrete slab be?

Four inches is standard for patios, walkways and shed bases. Go to five or six inches for driveways, vehicle parking or anything carrying heavy point loads. Thickness scales volume directly — moving a 10×10 slab from 4 to 6 inches adds 50% more concrete.

Why add 10% for waste?

Sub-grades are never perfectly level, forms bow slightly under pressure, and some mix is always lost to spillage and the mixer. Ten percent is the common allowance; on uneven ground or hand-dug footings, use 15%.

When should I order ready-mix instead of bags?

Around one cubic yard — roughly 45 bags of 80 lb mix. Beyond that, hand-mixing becomes slow enough that the pour risks setting unevenly between batches, which creates cold joints. Most ready-mix suppliers have a minimum load of one to three yards plus a short-load fee.

Sources

Primary references used for the figures and rules on this page.

1926 Subpart Q — Concrete and Masonry Construction — OSHA
Materials and Structural Systems Division — NIST