How Many Bags of Concrete for a Slab? Chart for 4 and 6 in Slabs

A 10 × 10 ft slab 4 in thick holds 33.3 cu ft of concrete. With 10% extra that's 62 bags of 80 lb mix, or about 1.36 cu yd of ready-mix (order 1.5). In metric, a 3 × 3 m slab 100 mm thick takes 106 bags of 20 kg mix.

Yield is printed on the bag. An 80 lb bag makes about 0.60 cu ft (Quikrete data sheet); a 20 kg bag about 9–10 litres.

Bags of concrete for a 4 in slab

Bags for a 4 in thick slab, including 10% extra
Slab size	40 lb bags	60 lb bags	80 lb bags	Or ready-mix
4 × 4 ft	20	14	10	0.25 cu yd
8 × 8 ft	79	53	40	1 cu yd
10 × 10 ft	123	82	62	1.5 cu yd
12 × 12 ft	176	118	88	2 cu yd
10 × 20 ft	245	163	123	2.75 cu yd
20 × 20 ft	489	326	245	5.5 cu yd

Bag yields are the maker's figures for standard mix: 0.30 cu ft for 40 lb, 0.45 for 60 lb and 0.60 for 80 lb. Ready-mix is rounded up to the next quarter yard.

Bags for a 6 in slab

Driveway aprons, shed bases for heavy loads and hot tub pads are often 6 in thick. That's half as much concrete again.

Bags for a 6 in thick slab, including 10% extra
Slab size	60 lb bags	80 lb bags	Or ready-mix
4 × 4 ft	20	15	0.5 cu yd
8 × 8 ft	79	59	1.5 cu yd
10 × 10 ft	123	92	2.25 cu yd
12 × 12 ft	176	132	3 cu yd
10 × 20 ft	245	184	4.25 cu yd
20 × 20 ft	489	367	8.25 cu yd

Metric: bags for a 100 mm slab

Bags for a 100 mm slab, including 10% extra
Slab size	20 kg bags	25 kg bags	30 kg bags	Or ready-mix
1 × 1 m	12	10	8	0.2 m³
2 × 2 m	47	38	32	0.6 m³
3 × 3 m	106	85	71	1 m³
3 × 4 m	141	113	94	1.4 m³
4 × 5 m	235	188	157	2.2 m³
6 × 6 m	423	339	282	4 m³

Metric bags are worked out at about 0.468 litres of concrete per kg of dry mix, so a 20 kg bag makes about 9.4 litres. Makers' figures vary: see our bag yield research.

Worked example: a 10 × 10 ft patio slab

Volume: 10 × 10 × (4 ÷ 12) = 33.3 cu ft.
Add 10% for spillage and an uneven base: 36.7 cu ft.
Divide by 0.60 cu ft per 80 lb bag and round up: 62 bags.
That's about 2.5 US tons of bags to carry, so ready-mix is often easier at this size.

Bags or ready-mix?

Bags suit small pours: posts, steps, small pads. Once a slab needs more than about a cubic yard, mixing by hand gets slow and the first batches start to set before the last go in. Many ready-mix suppliers have a minimum order or a short-load fee, so check before you book.

Common mistakes

Assuming a 2×4 form makes a 4 in slab. A 2×4 is 3.5 in wide, so a slab poured level with its top is 3.5 in thick. Measure the real depth.
No extra allowance. Bases are never perfectly flat. 5–10% extra is normal.
Mixing up bag sizes. An 80 lb bag makes 0.60 cu ft; two 40 lb bags make the same.