Multiply length x width x thickness (in feet) to get cubic feet, then divide by the yield of the bag and add 10 percent. A 60 lb. bag yields about 0.45 cu. ft. and an 80 lb. bag about 0.6 cu. ft., so a 10 x 10 ft. slab at 4 in. thick needs about 82 bags of 60 lb. or 62 bags of 80 lb. This guide gives the formula, a bag-count table and fence post numbers for Los Angeles slab, footing and fence work.
The bag-count formula
Concrete is ordered by volume. Convert the thickness to feet (4 in. = 0.333 ft., 6 in. = 0.5 ft.), multiply the three dimensions, then divide by the yield printed on the bag. Always add 5 to 10 percent, because forms are never perfectly level and subgrade settles. Yields vary slightly by brand, so check the label before a big order.
| Bag size |
Typical yield |
Bags per cu yd |
| 50 lb. |
0.375 cu ft |
72 |
| 60 lb. |
0.45 cu ft |
60 |
| 80 lb. |
0.6 cu ft |
45 |
| 90 lb. |
0.67 cu ft |
40 |
Bag counts for common pours
The table below includes a 10 percent allowance. Once a pour passes about 1 cubic yard, which is roughly 45 bags of 80 lb., compare the labor and cost of bags against ready-mix delivery.
| Pour |
Volume |
60 lb. bags |
80 lb. bags |
| 4x4 ft. pad, 4 in. |
5.3 cu ft |
13 |
10 |
| 10x10 ft. slab, 4 in. |
33 cu ft |
82 |
62 |
| 10x10 ft. slab, 6 in. |
50 cu ft |
123 |
92 |
| Footing, 12x12 in., 10 ft. |
10 cu ft |
25 |
19 |
Fence posts and small holes
A 4x4 post in a 10 in. wide, 30 in. deep hole takes about 1.2 cu. ft. of concrete around the post, which is three 50 lb. bags or three 60 lb. bags. Dig the hole one-third the post's length deep, set the post on 4 in. of gravel, and brace it plumb while the mix sets. Fast-setting mix lets you hang the fence rails the same day.
Which mix for which job?
Standard 4000 psi mix suits slabs, footings and posts. Fast-setting mix saves time on posts and small repairs, and crack-resistant mix adds fibers for thin slabs and walkways. Our bagged concrete mix comparison sorts them by use. Examples from the concrete mix collection include the PSI-4000 60 lb. mix, the fast-setting 50 lb. mix and the crack-resistant 50 lb. mix.
Prepare the base before you pour
A compacted 4 in. gravel base keeps a slab from cracking unevenly. The sand, gravel and base rock guide explains which aggregate to order, and the concrete aggregates collection carries it. Reinforcement matters too: see rebar vs wire mesh vs fiber before you set the forms.
Los Angeles pour tips
Southern California heat shortens working time in summer, so mix only what you can place in 20 to 30 minutes and keep the surface damp while it cures. A slab that will carry vehicles or a footing for a structure needs a permit and an inspection in most cities, so confirm requirements with your local building department first. Once the slab has cured, protect it with a sealer; see concrete sealers compared. For deck footings, the deck materials checklist covers the rest of the build.
Concrete bag-count checklist
- Measure length, width and thickness, and convert inches to feet
- Multiply to get cubic feet
- Divide by the bag yield: 0.45 for 60 lb., 0.6 for 80 lb.
- Add 10 percent and round up
- Compare against ready-mix above about 1 cubic yard
- Order gravel base and reinforcement with the mix
- Check permit rules for structural pours
Conclusions
Volume divided by bag yield, plus 10 percent, is the whole calculation. Use 80 lb. bags to cut the number you have to lift, keep fast-setting mix for posts, and switch to ready-mix when the pour grows past a cubic yard. The yields here are typical values; always confirm the figure on the bag you buy.
