Concrete bag yield guide (how many bags per cubic foot?)
How to estimate concrete bags: why yield matters, common bag sizes, and when ready-mix is a better option.
Yield is the key number
Bagged concrete is sold by weight, but you need volume. The label yield (cubic feet per bag) varies by product and brand, so it is more reliable than generic charts.
Quick volume per 100 sq ft (common slab thicknesses)
| Thickness | Volume per 100 sq ft | Notes |
|---|---|---|
| 3" (0.25 ft) | 0.93 cu yd | Thin slab; confirm suitability |
| 4" (0.33 ft) | 1.23 cu yd | Very common for pads/driveways |
| 5" (0.42 ft) | 1.54 cu yd | Heavier loads / some driveways |
| 6" (0.50 ft) | 1.85 cu yd | Common for heavier duty slabs |
Rule of thumb (use the label when possible)
- 80 lb bags often yield around 0.6 cu ft (varies by product).
- 60 lb bags often yield around 0.45 cu ft (varies by product).
- 50 lb bags often yield around 0.375 cu ft (varies by product).
Do not forget thickened edges (a common under-order)
Many slabs include thickened edges or footings. If you ignore them, you can come up short even if the slab volume is correct. Treat thickened edges as a separate volume line item.
Example: a 12 ft × 20 ft slab at 4 inches is ~2.96 cu yd. If all four edges are thickened to 12 inches deep and 12 inches wide, the extra thickened portion is ~1.58 cu yd—often a meaningful add-on.
- Slab volume = area × slab thickness.
- Thickened edge volume = edge length × thickened width × (thickened depth − slab thickness).
- If you have multiple thickened zones, calculate each one and add them.
When to consider ready-mix
- Large pours take many bags and a lot of labor time.
- Consistency is easier with ready-mix.
- Ordering short can stop a pour; buffers are usually cheaper than a second delivery.
How to estimate bags (step-by-step)
Step 1: calculate total concrete volume in cubic feet (or cubic yards). Step 2: divide by the bag yield on the label. Step 3: round up and add a buffer.
Example: if you need 40 cu ft and your 80 lb bag yields 0.60 cu ft, you need 40 / 0.60 = 66.7 → 67 bags (plus buffer).
- If the job is tight-tolerance (posts, footings), the buffer is usually worth it.
- If you’re mixing by hand, smaller bag sizes can reduce fatigue but increase batch count.
Consistency tips when mixing many bags
- Measure water consistently; “a splash more” changes strength and finish quality.
- Mix long enough for uniform color and texture, then place quickly to avoid cold joints.
- Stagger mixing and placing so you’re not dumping half-set material into fresh work.
Common mistakes that reduce yield or strength
- Adding extra water to improve workability (can reduce strength and increase cracking).
- Estimating with generic bag yield instead of the label yield for your product.
- Ignoring forms and over-excavation: extra void space can consume many bags.