Concrete bag calculator

A 4 × 4 ft pad 4 in thick takes 10 bags of 80 lb concrete mix, 14 bags of 60 lb or 20 bags of 40 lb, with 10% extra. Enter a shape or a volume to count bags for your job.

Shape

Length

Width

Thickness

How many of them

Extra for waste

10 80 lb bags

14 60 lb bags

20 40 lb bags

Volume	5.33 cu ft (0.20 cu yd, 0.151 m³)
With 10% extra	5.87 cu ft (0.22 cu yd)
Ready-mix to order	0.25 cu yd (rounded up to the next ¼ yard)
40 lb bags	20 (0.3 cu ft each)
50 lb bags	16 (0.375 cu ft each)
60 lb bags	14 (0.45 cu ft each)
80 lb bags	10 (0.6 cu ft each)
90 lb bags	9 (0.675 cu ft each)
Weight of the concrete	about 851 lb (0.43 tons)

How it was worked out

4 ft × 4 ft × 4 in = 5.33 cu ft

5.33 cu ft ÷ 27 = 0.20 cu yd

How bags are counted

Each bag size makes a set amount of mixed concrete. The QUIKRETE Concrete Mix data sheet gives 0.30 cu ft for a 40 lb bag, 0.375 for 50 lb, 0.45 for 60 lb, 0.60 for 80 lb and 0.675 for 90 lb. The calculator divides your volume (with the extra you choose) by the yield and rounds up, because a part bag is still a bag at the store.

4 ft × 4 ft × 4 in = 5.33 cu ft

5.33 cu ft ÷ 27 = 0.20 cu yd

With 10% extra: 5.87 cu ft ÷ 0.60 = 9.8, so 10 bags of 80 lb

Bags for common small jobs

Bags with no extra added; add your allowance on top
Job	Cu ft	80 lb	60 lb	50 lb	40 lb
2 × 2 ft pad, 4 in thick	1.33	3	3	4	5
3 × 3 ft pad, 4 in thick	3	5	7	8	10
4 × 4 ft pad, 4 in thick	5.33	9	12	15	18
4 × 8 ft walk section, 4 in thick	10.67	18	24	29	36
10 × 10 ft slab, 4 in thick	33.33	56	75	89	112
10 in hole, 3 ft deep	1.64	3	4	5	6
12 in hole, 3 ft deep	2.36	4	6	7	8
1 cubic yard	27	45	60	72	90

Which bag size to buy

Every size makes the same concrete per pound, so a job needs the same weight of mix whichever bag you pick: about 133 lb of dry mix per cubic foot of concrete. Lighter bags are easier to carry and to mix in a wheelbarrow. For a machine mixer, QUIKRETE asks for at least 1 cu ft of mixer capacity for each 80 lb bag. Sakrete's High-Strength Concrete Mix gives the same 0.6 cu ft for its 80 lb bag, so the counts work for either brand.

Fast-setting mixes are sold for posts. QUIKRETE's 60 lb Fast Setting Concrete Mix yields the same 0.45 cu ft as its regular 60 lb bag. For fence posts, the fence post calculator also allows for the post itself and the gravel in the bottom of the hole.

Sources

QUIKRETE Concrete Mix (No. 1101) data sheet, The QUIKRETE Companies, revised October 2022. Bag yields: 40 lb 0.30 cu ft, 50 lb 0.375, 60 lb 0.45, 80 lb 0.60, 90 lb 0.675; about 140 lb per cubic foot.
QUIKRETE Fast Setting Concrete Mix (No. 1004) data sheet, The QUIKRETE Companies, current sheet. A 60 lb bag yields about 0.45 cu ft.
Sakrete High-Strength Concrete Mix 80 lb product page, Sakrete (Oldcastle APG), current listing. An 80 lb bag yields 0.6 cu ft, the same as QUIKRETE Concrete Mix.
CIP 8: Discrepancies in Yield (Concrete in Practice series), National Ready Mixed Concrete Association, 2016 edition. Order at least 4 to 10 percent more than the measured volume; 1/8 in extra on a 4 in slab is a 3 percent shortfall.