A 60-lb bag of concrete yields 0.45 cubic feet; an 80-lb bag yields 0.60 cubic feet; and a 40-lb bag yields 0.30 cubic feet. These figures come directly from Quikrete’s product specification sheets and Sakrete’s published data. Divide the total cubic feet of your project by the bag yield, add 10%, and round up to the nearest whole bag.
Bag Yield Reference
| Bag Size | Yield (cu ft) | Source |
|---|---|---|
| 40 lb | 0.30 | Quikrete/Sakrete product sheets |
| 50 lb | 0.375 | Quikrete product sheet |
| 60 lb | 0.45 | Quikrete/Sakrete product sheets |
| 80 lb | 0.60 | Quikrete/Sakrete product sheets |
The Formula
- Volume (cu ft) = Length (ft) x Width (ft) x Thickness (ft)
- Bags = Volume / yield per bag x 1.10 (10% waste), rounded up
Bags Needed by Slab Size (4-inch thickness, 80-lb bags)
| Slab Size | Area (sq ft) | Volume (cu ft) | 80-lb Bags (+ 10%) |
|---|---|---|---|
| 3×3 | 9 | 3.0 | 6 |
| 4×4 | 16 | 5.3 | 10 |
| 4×8 | 32 | 10.7 | 20 |
| 6×6 | 36 | 12.0 | 22 |
| 8×8 | 64 | 21.3 | 39 |
| 10×10 | 100 | 33.3 | 62 |
| 10×20 | 200 | 66.7 | 123 |
| 12×12 | 144 | 48.0 | 88 |
| 20×20 | 400 | 133.3 | 245 |
At 245 bags for a 20×20 slab, you are looking at 7.4 cubic yards and significant mixing labor. At that scale, ready-mix concrete is far more practical. See bagged-concrete-vs-ready-mix for the cost comparison.
Worked Example: 10×10 ft Patio Slab, 4 in Thick
Step 1 — Find volume:
- 10 x 10 x (4 / 12) = 10 x 10 x 0.333 = 33.3 cubic feet
Step 2 — Choose bag size. Using 80-lb bags:
- 33.3 / 0.60 = 55.5 bags
Step 3 — Add 10% waste:
- 55.5 x 1.10 = 61.1 → round up to 62 bags
Step 4 — Cost estimate (2025 prices):
- 80-lb Quikrete Concrete Mix at hardware stores: roughly $6.50–$8.00/bag
- 62 bags x $7.00 = $434 in materials alone
Step 5 — Compare to 60-lb bags:
- 33.3 / 0.45 = 74 bags x 1.10 = 82 bags x $5.50 = $451
The 80-lb bags are slightly cheaper per cubic foot in most markets but heavier to handle. The 60-lb option is worth considering if you are mixing by hand or working alone.
How Many Bags for a Circular Slab?
For a round pad (such as a hot tub base or fire pit surround), the formula changes:
Volume (cu ft) = 3.14159 x radius² x thickness (all in feet)
Example — 8-foot diameter circle, 4 inches thick:
- Radius = 4 ft
- Volume = 3.14159 x 16 x 0.333 = 16.8 cu ft
- 80-lb bags: 16.8 / 0.60 x 1.10 = 31 bags
The concrete calculator handles circular slabs, rectangular slabs, and post holes without manual math.
How Many Bags for a Concrete Path?
For a 3 ft wide, 20 ft long path at 4 inches thick:
- Volume = 3 x 20 x 0.333 = 20 cu ft
- 80-lb bags: 20 / 0.60 x 1.10 = 37 bags
For a curved or irregular path, break it into sections, calculate each volume separately, and add them together.
Bags for Post Holes vs. Slabs
Concrete for fence posts uses the cylinder formula — not the slab formula. See concrete-for-fence-posts for hole-by-hole math. Quikrete Fast-Setting Concrete (80-lb bag) sets in 20–40 minutes and is the standard product for post-setting; standard Concrete Mix is the right choice for slabs.
Bags for Different Slab Thicknesses
The tables above assume 4-inch thickness. Here is how bag counts change as thickness increases for a 10×10 ft slab:
| Thickness | Volume (cu ft) | 80-lb Bags (+ 10%) | 60-lb Bags (+ 10%) |
|---|---|---|---|
| 3.5 in | 29.2 | 54 | 72 |
| 4 in | 33.3 | 62 | 82 |
| 5 in | 41.7 | 77 | 102 |
| 6 in | 50.0 | 92 | 123 |
At 6 inches thick, you need 92 bags of 80-lb mix for a 10×10 slab — over 7,300 lbs of material to hand-mix. At that volume, ordering ready-mix is the more practical choice.
Mixing Time Estimates
Factor mixing time into your planning, especially for larger projects:
| Mixing Method | Bags per Hour |
|---|---|
| Hand-mixing in wheelbarrow | 5–8 |
| Drill with paddle mixer (5-gal bucket) | 8–12 |
| Rented electric drum mixer | 12–18 |
| Contractor-grade mixer (9+ cu ft) | 20–30 |
For a 62-bag pour (10×10 slab) using a rented drum mixer, plan 3–5 hours of mixing alone. Add screeding, finishing, and cleanup time on top of that. This is why ready-mix is often the faster and cheaper choice once a project exceeds ~0.5 cubic yards, even accounting for the short-load fee.
Quick Reference Conversions
| Volume | 40-lb bags | 60-lb bags | 80-lb bags |
|---|---|---|---|
| 1.0 cu ft | 4 | 3 | 2 |
| 5.0 cu ft | 19 | 13 | 10 |
| 10.0 cu ft | 37 | 25 | 19 |
| 27.0 cu ft (1 yd³) | 100 | 67 | 50 |
All counts include a 10% waste factor and round up to whole bags.
Safety Reminder
Wet concrete is highly alkaline (pH 12–13) and causes chemical burns on skin with prolonged contact. Wear nitrile gloves and safety glasses whenever you mix or handle fresh concrete. Rinse any skin contact immediately with plenty of water.