Footing

Concrete for a 20 ft footing, 24×12 in

A 20 ft run of 24 in × 12 in strip footing. The volume is the trench cross-section times the run. That is 40 cubic feet, or 1.48 cubic yards: 67 bags of 80 lb premix or 89 bags of 60 lb, before any waste.

By MixRatio · Published 19 July 2026 · Last updated 13 October 2026

Concrete needed

1.48cubic yards· 1.133 m³· 40 ft³

Bags of premix needed for concrete for a 20 ft footing, 24×12 in, by bag size.
Bag size	Bags needed
40 lb	134
60 lb	89
80 lb	67
25 kg	97
30 kg	81

Planning estimate, with no waste allowance included. Add 5-10% for spillage and over-excavation, and check the yield printed on your bag. Structural work must follow your local building code and an engineer's specification.

Bagged or ready-mix?

Either - check the short-load fee

1.48 yd³ clears the usual one-yard minimum but sits in the short-load band, where most suppliers add a surcharge for anything under about 3-4 yd³. That fee is what decides it: get it quoted, add it to the yard price, and compare against the bag count above at your own bag price.

Same job, other sizes

24 × 12 in footing

Volume for each size. The order columns add 10%, the top of the 5-10% waste allowance, and round bags up.
Size	ft³	yd³	yd³ to order (+10%)	80 lb bags (+10%)	60 lb bags (+10%)
10 ft run	20	0.74	0.81	37	49
15 ft run	30	1.11	1.22	55	74
20 ft run (this page)	40	1.48	1.63	74	98
24 ft run	48	1.78	1.96	88	118
30 ft run	60	2.22	2.44	110	147
40 ft run	80	2.96	3.26	147	196
50 ft run	100	3.7	4.07	184	245
60 ft run	120	4.44	4.89	220	294
80 ft run	160	5.93	6.52	294	392
100 ft run	200	7.41	8.15	367	489

Per unit

Each foot of this 24 × 12 in footing holds 2 ft³, so one cubic yard pours about 13.5 ft of it.

One 80 lb bag fills about 4 in of run.

Change the dimensions in the footing calculator. Why every count rounds up: how many bags of concrete you need.

At this size

What changes on a 20 ft footing, 24×12 in

Mixing or ordering it

67 × 80 lb bags is 1.48 yd³ and roughly 34 mixer batches. This is the crossover point: bagging it is a full day with a mixer and at least two people, and the concrete has to keep arriving faster than the placed edge stiffens. Price 1.48 yd³ of ready-mix, short-load surcharge included, against 67 bags at your own bag price before you commit to a day of mixing.

The dimensions used

Dimensions this estimate was worked out from.
Run	20 ft
Width	24 in
Depth	12 in

Change these numbers in the footing calculator

How it is worked out

A strip footing is a long rectangular prism under a wall; a spread footing is a pad under a column. Either way the volume is length × width × depth. Enter the total run of the footing as the length, and the trench cross-section as width × depth.

That volume - 40 ft³ - is divided by 27 to give 1.48 cubic yards, and by each bag's yield to give the bag count. An 80 lb bag of premix yields about 0.60 cubic feet, so this job takes 67 of them; bag counts always round up, because you cannot buy part of a bag.

Ordering ready-mix instead? Suppliers sell by the cubic yard (or cubic metre) and usually have a minimum load and a short-load surcharge - worth a call before you choose between bags and a truck.

Formwork

What is being measured

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