It's the most-asked question on any concrete job. The answer comes from just two numbers: the volume you're filling and what one bag actually makes.
What one bag makes
| Bag | Yield | Bags per cu yd |
|---|---|---|
| 80 lb concrete mix | 0.60 cu ft | 45 |
| 60 lb concrete mix | 0.45 cu ft | 60 |
| 40 lb concrete mix | 0.30 cu ft | 90 |
| 50 lb fast-setting | 0.375 cu ft | 72 |
Yields vary a little by brand, so check the bag. These are the figures our calculators use by default.
Quick slab table
Bags of 80 lb mix per 10 sq ft of slab: about 2.3 at 4" thick, 2.8 at 5" and 3.4 at 6", before waste.
Post holes
Post holes use less than you'd expect, because the post takes up part of the hole. A 4×4 post in a 10" × 30" hole needs about 1 cu ft of concrete: two 50 lb bags of fast-setting mix, or two 80 lb bags of regular mix.
Add waste
Add 10% for spills, uneven ground and the half-bag that gets wet. On anything over a cubic yard, seriously consider ready-mix: see bags vs ready-mix.
Water
An 80 lb bag takes roughly 3 quarts of water. Too much water is the most common way to weaken bagged concrete. Add it gradually until the mix is workable, not soupy.
This guide is general information for estimating. It is not engineering, legal or tax advice. Check local codes and your inspector for your project.