Concrete footing calculator

A 40 ft wall footing 16 in wide and 8 in deep holds 1.32 cubic yards of concrete; order 1.45 yards with 10% extra. Use it for continuous wall footings or for a set of identical pad footings.

Footing length

For a wall footing, the total run

Width

Depth (thickness)

Number of footings

Extra for waste

1.45 cubic yards

66 80 lb bags

87 60 lb bags

Volume	35.56 cu ft (1.32 cu yd, 1.007 m³)
With 10% extra	39.11 cu ft (1.45 cu yd)
Ready-mix to order	1.5 cu yd (rounded up to the next ¼ yard)
40 lb bags	131 (0.3 cu ft each)
50 lb bags	105 (0.375 cu ft each)
60 lb bags	87 (0.45 cu ft each)
80 lb bags	66 (0.6 cu ft each)
90 lb bags	58 (0.675 cu ft each)
Weight of the concrete	about 5,671 lb (2.84 tons)

How it was worked out

40 ft × 16 in × 8 in = 35.56 cu ft

35.56 cu ft ÷ 27 = 1.32 cu yd

Wall footings and pad footings

A continuous footing runs under a wall. Enter the total length of the run (for a rectangle, the perimeter measured along the footing's centerline), its width and its depth, and leave the number of footings at 1. A pad or spot footing sits under a post or column. Enter one pad's length and width, its depth, and how many there are.

Trenches are rarely as neat as the plan. If the trench is dug wider than the footing and the concrete is poured against the soil, measure the trench itself, because the concrete fills whatever is there.

Worked example

40 ft × 16 in × 8 in = 35.56 cu ft

35.56 cu ft ÷ 27 = 1.32 cu yd

With 10% extra: 1.45 cu yd, or 66 bags of 80 lb mix

Minimum footing size in the residential code

The International Residential Code (section R403.1.1) sets the footing width from the load and the soil's bearing value, using Table R403.1. Its smallest width for any case in those tables is 12 in. Spread footings must be at least 6 in thick, and the footing must stick out at least 2 in past each face of the wall, but no more than the footing's thickness. Footings must also go below the frost depth set for your area. Local codes can amend all of this, so your building department has the final word.

Concrete per length of wall footing

Continuous footings
Width × depth	Cu ft per 10 ft	Cu yd per 10 ft	Cu yd per 100 ft	80 lb bags per 10 ft (with 10%)
12 × 6 in	5	0.19	1.85	10
12 × 8 in	6.67	0.25	2.47	13
16 × 8 in	8.89	0.33	3.29	17
16 × 10 in	11.11	0.41	4.12	21
20 × 8 in	11.11	0.41	4.12	21
20 × 10 in	13.89	0.51	5.14	26
24 × 12 in	20	0.74	7.41	37

Concrete per pad footing

Square pad footings
Pad	Cu ft each	80 lb bags each (with 10%)	60 lb bags each (with 10%)
12 × 12 × 8 in	0.67	2	2
16 × 16 × 8 in	1.19	3	3
18 × 18 × 10 in	1.88	4	5
24 × 24 × 12 in	4	8	10
30 × 30 × 12 in	6.25	12	16
36 × 36 × 12 in	9	17	22

Bag counts use 0.60 cu ft for an 80 lb bag and 0.45 cu ft for a 60 lb bag, the yields on the QUIKRETE Concrete Mix data sheet.

Sources

International Residential Code, sections R403.1.1 and R506.1, International Code Council, 2021 edition. Spread footings at least 6 in thick with projections of 2 in or more; Table R403.1 widths start at 12 in; concrete floor slabs at least 3.5 in thick.
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.
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.