Two calculators in one — slabs use length × width × thickness, post holes use the cylinder formula. Both include 10% waste.
⚡ Quick Answers
Slab: A 10×10 patio at 4″ needs 62 bags of 80 lb concrete (1.36 cu yd) — at that size, get a ready-mix quote too. Post hole: A 10″ hole at 3 feet deep needs 3 bags of 80 lb concrete per post. All numbers include a 10% waste allowance.
Why Post Holes Need a Different Formula
Post holes are cylinders, not rectangles. Using L×W×D overestimates by about 27%.
Volume = π × radius² × depth
10″ hole, 3 ft deep:
r = 0.417 ft → V = 1.64 cu ft
+10% waste = 1.80 cu ft = 3 bags
How Deep Should Post Holes Be?
At least 1/3 of the post height underground.
6 ft post → 2 ft minimum
8 ft post → 2.5–3 ft
In freezing climates, go below the frost line.
Fast-Setting vs Regular Fast-setting — sets in 20–40 min. Good for posts.
Regular — full strength in 28 days. Better for slabs.
Both use the same bag volumes.
Slab Reference — Bags by Project Size
80 lb bags · 10% waste included · Values rounded up
Project
Thickness
80 lb Bags
60 lb Bags
Cu Yards
Ready-Mix?
Stepping stone 2×2
4″
3
4
0.05
No — bags
Small pad 3×3
4″
6
8
0.12
No — bags
Small pad 4×4
4″
10
14
0.22
No — bags
Small pad 4×8
4″
20
27
0.43
No — bags
Garden pad 6×6
4″
23
30
0.49
No — bags
Shed base 8×8
4″
40
53
0.87
Worth a quote
10×10 patio
4″
62
82
1.36
Worth a quote
10×10 patio
6″
92
123
2.04
Recommended
10×20 patio
4″
123
163
2.72
Recommended
20×20 patio
4″
245
326
5.43
Recommended
Bags rounded up. Ready-mix pricing varies by location — use cubic yard figures to compare.
Post Hole Reference — Bags Per Hole
80 lb bags · 10% waste · Cylinder formula · Per individual hole
Hole Diameter
Depth
80 lb Bags
60 lb Bags
Cu Ft (+ waste)
8 inches
2 ft
2
2
0.77
8 inches
3 ft
2
3
1.15
10 inches
2 ft
2
3
1.20
10 inches
3 ft
3
4
1.80
10 inches
4 ft
4
6
2.40
12 inches
2 ft
3
4
1.73
12 inches
3 ft
5
6
2.59
12 inches
4 ft
6
8
3.46
Values are per hole — multiply by number of posts for your total.
Frequently Asked Questions
How many bags of concrete do I need for a 10×10 slab at 4 inches?
A 10×10 slab at 4 inches thick needs 62 bags of 80 lb concrete — or 82 bags of 60 lb — including 10% waste. That equals 1.36 cubic yards. At that volume, get a ready-mix quote before committing to bags.
How many bags of concrete do I need per fence post?
For a 10-inch hole at 3 feet deep, you need 3 bags of 80 lb concrete per post. An 8-inch hole at 3 feet needs 2 bags. A 12-inch hole at 3 feet needs 5 bags.
When should I use ready-mix instead of bags?
Once your project exceeds about 1 cubic yard, ready-mix is usually more economical and much faster. The table above flags where a quote makes sense. Pricing varies by location.
How thick should a concrete slab be?
Four inches is standard for patios and walkways. Six inches for driveways. Eight inches or more for heavy loads. Check local building code for footings and foundations.
What is the difference between 60 lb and 80 lb bags?
An 80 lb bag covers 0.60 cubic feet. A 60 lb bag covers 0.45 cubic feet. Both produce the same strength concrete. 80 lb bags are more economical. 60 lb bags are easier to carry solo.
Why does this calculator use a different formula for post holes?
Post holes are cylinders, not rectangles. Using length × width × depth overestimates by about 27%. Our post hole calculator uses π × radius² × depth so you buy the right amount.