What people use it for
- The whole job A slab, a walk and the post holes pasted in one line, and one number of bags at the end.
- Round holes A post hole is a cylinder, not a box. Write p12x36 and it counts the hole, not the corner.
- The honest warning When the pour passes a yard and a half it says so, instead of selling you two hundred bags.
The bag is a volume, not a weight
A 60 lb bag and an 80 lb bag of the same mix do not make the same amount of concrete. What counts is the yield printed on the bag, which is the volume of mixed concrete one bag makes. The usual figures are 0.30 cu ft for a 40 lb bag, 0.375 for 50 lb, 0.45 for 60 lb and 0.60 for 80 lb. Divide the volume you need by that number and round up. Divide by the weight instead and you come up short.
A cubic yard is 27 cu ft, so one yard takes 45 bags of 80 lb, 60 of 60 lb, 72 of 50 lb or 90 of 40 lb. Nothing about the brand changes that by much, and it is the reason bags stop making sense somewhere around a yard.
Most jobs are more than one pour
The slab is one pour. The footings under the posts are another. The holes themselves are a third. Counted one at a time, that is three trips to the yard and a leftover pile of a mix you cannot match in a fortnight. Paste the whole list here and the count comes back as one number.
A rectangular pour is written as length x width x depth in inches. So 10x10x4 is a 10 by 10 ft slab 4 in thick, and a pour with only two numbers takes the depth from the box below. A round hole starts with p and is all inches: p12x36 is a hole 12 in across and 36 in deep.
Where the count goes wrong
Rough ground eats volume. A formed slab on level ground takes ten percent waste and is fine. An excavated footing that is wider at the bottom, or a hole with a bell in it, can take fifteen or twenty. When the count sits on the line, take the higher one. A spare bag costs less than a cold joint.
The post standing in the hole is not subtracted here. A 4 by 4 takes about a tenth of a 12 in hole, and less of a wider one, so the count runs a little high on posts. High is the side you want to be wrong on.
The number that decides how you buy
Bagged mix is a good answer up to about a yard. Past that the mixing becomes the job: sixty or more bags, water, a wheelbarrow, and a pour that has to be placed and finished before it sets. The last line of the result says where your job lands, and it is not there to sell you anything.
| Bag | Yield | Bags per cubic yard |
|---|---|---|
| 80 lb | 0.60 cu ft | 45 |
| 60 lb | 0.45 cu ft | 60 |
| 50 lb | 0.375 cu ft | 72 |
| 40 lb | 0.30 cu ft | 90 |