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How Many Bags of Concrete Do I Need?
Enter your slab, wall, footing, sonotube, or fence-post dimensions below and get an exact bag count for 40, 60, and 80 lb bags — always rounded up, with an adjustable waste allowance and an optional cost estimate. No sign-up, no tracking: the math runs entirely in your browser.
By HowMuchBuild Editorial TeamCalculation reviewedUpdated October 2026
ft
in
ft
in
inches — 4″ is standard for patios & walkways
Bag size
10% is a safe default — uneven subgrade and spillage eat more than you think.
Estimated cost
What will it cost?
Enter your local store prices — prices vary by region.
80 lb concrete bags
62 bags
—
Estimate — prices vary by store and region. Check your local store for current pricing.
You need
6280 lb bags
Includes 10% waste · exact pour needs 56 bags (always rounded up)
33.33
total volume (cu ft)
1.23
total volume (cu yd)
0.94
total volume (cu m)
| Bag size | Yield / bag | Exact | With 10% waste |
|---|---|---|---|
| 40 lb | 0.3 cu ft | 112 | 123 |
| 60 lb | 0.45 cu ft | 75 | 82 |
| 80 lb | 0.6 cu ft | 56 | 62 |
How we calculated it
- Volume = Length × Width × Thickness = 10 ft × 10 ft × 0.33 ft
- = 33.33 cu ft ÷ 27 = 1.23 cu yd
- 80 lb bags: 33.33 cu ft ÷ 0.6 cu ft/bag = 55.56 → round up = 56 bags
- With 10% waste: 36.67 cu ft ÷ 0.6 = 61.11 → round up = 62 bags
Estimates for planning. Actual bag yields vary slightly by mix and conditions — always round up and confirm with your supplier for pours over a few cubic yards.
Worked example: 10×10 ft slab at 4 inches thick
This is the most common DIY pour — a patio or shed slab. Here is the full arithmetic, step by step:
- Volume: 10 ft × 10 ft × (4 ÷ 12) ft = 10 × 10 × 0.3333 = 33.33 cu ft
- In cubic yards: 33.33 ÷ 27 = 1.235 cu yd
- 80 lb bags: 33.33 ÷ 0.60 = 55.55 → round up = 56 bags
- With 10% waste: 33.33 × 1.10 = 36.67 cu ft; 36.67 ÷ 0.60 = 61.11 → round up = 62 bags
The same slab needs 75 sixty-pound bags (82 with waste) or 112 forty-pound bags (123 with waste).
Worked example: fence post holes (with post displacement)
A 4×4 post sitting in the hole displaces concrete — skip this and you will overbuy. For 8 holes, each 12 inches in diameter and 36 inches deep, with 4×4 posts (actual 3.5″ × 3.5″):
- Hole volume: π × 0.5² × 3 = 2.356 cu ft per hole
- Post displacement: (3.5 ÷ 12)² × 3 = 0.255 cu ft per hole
- Concrete per hole: 2.356 − 0.255 = 2.101 cu ft
- Total: 2.101 × 8 = 16.81 cu ft
- 80 lb bags: 16.81 ÷ 0.60 = 28.01 → round up = 29 bags (31 with 10% waste)
Concrete bag yield & coverage chart
Standard US premix bags yield these cured-concrete volumes (printed on the bag):
| Bag size | Yield per bag | Bags per cubic yard | Covers at 4″ thick | Covers at 6″ thick |
|---|---|---|---|---|
| 80 lb | 0.60 cu ft | 45 | ≈ 1.8 sq ft | ≈ 1.2 sq ft |
| 60 lb | 0.45 cu ft | 60 | ≈ 1.35 sq ft | ≈ 0.9 sq ft |
| 40 lb | 0.30 cu ft | 90 | ≈ 0.9 sq ft | ≈ 0.6 sq ft |
Quick answers for common projects
Bag counts below use 80 lb bags and include a 10% waste allowance:
| Project | 80 lb bags (with 10% waste) |
|---|---|
| 10×10 ft patio slab, 4″ thick | 62 |
| 12×12 ft patio slab, 4″ thick | 88 |
| 8×10 ft shed slab, 4″ thick | 49 |
| 4×4 ft slab, 4″ thick | 10 |
| Fence post: 12″ dia × 36″ deep hole, 4×4 post | 4 per hole |
| Sonotube: 12″ dia × 4 ft tall | 6 |
| Deck pier: 12″ dia × 36″ deep (no post) | 5 |
The formulas
Slab, wall, or footing (rectangle)
V = Length × Width × Thickness — all in feet. A 4-inch thickness is 4 ÷ 12 = 0.3333 ft.
Column or sonotube (cylinder)
V = π × r² × h — r is half the diameter, h is the height, both in feet.
Fence or deck post hole
V = hole volume − post volume. For a round hole: π × r² × depth. For a square hole: side × side × depth. Subtract the part of the post below grade, using the post's actual dimensions (a nominal 4×4 is really 3.5″ × 3.5″).
Bags from volume
Bags = ⌈ Volume ÷ bag yield ⌉ — always round up. With waste: ⌈ (Volume × 1.10) ÷ bag yield ⌉. One cubic yard equals exactly 27 cubic feet.
Frequently asked questions
How many bags of concrete do I need for a 10×10 slab?
A 10×10 ft slab at 4 inches thick is 33.33 cubic feet. That takes 56 eighty-pound bags (62 with a 10% waste allowance), 75 sixty-pound bags (82 with waste), or 112 forty-pound bags (123 with waste).
How many bags of concrete do I need for a fence post?
For a typical 12-inch-diameter hole, 36 inches deep, with a 4×4 post (actual 3.5×3.5 in), each hole needs about 2.1 cubic feet of concrete after subtracting the post — that's 4 eighty-pound bags per hole.
How many bags of concrete for a 4×4 slab?
A 4×4 ft slab at 4 inches thick is 5.33 cubic feet, which needs 9 eighty-pound bags (10 with a 10% waste allowance).
How many bags of concrete do I need for a sonotube?
A 12-inch-diameter sonotube, 4 feet tall, holds 3.14 cubic feet of concrete — that's 6 eighty-pound bags, always rounded up.
How much does one 80 lb bag of concrete cover?
One 80 lb bag yields 0.60 cubic feet of cured concrete. At 4 inches thick it covers about 1.8 square feet; at 6 inches thick, about 1.2 square feet.
How many 80 lb bags are in a cubic yard?
Exactly 45. One cubic yard is 27 cubic feet, and an 80 lb bag yields 0.60 cubic feet (27 ÷ 0.60 = 45). A 60 lb bag yields 0.45 cu ft, so 60 bags per yard; a 40 lb bag yields 0.30 cu ft, so 90 bags per yard.
Should I add extra concrete for waste?
Yes — add 5–10% for slabs and footings, and 10% is a safe default for post holes. Uneven subgrade, spillage, and over-digging always consume more than the math says. The calculator above includes an adjustable waste slider.
When to order ready-mix instead
Bagged concrete makes sense up to about 1–2 cubic yards (roughly 45–90 eighty-pound bags). Beyond that, a ready-mix truck is usually cheaper per yard and far less labor. If your calculator result is over ~2 cubic yards, call a local batch plant for a quote before buying pallets of bags.
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