Concrete Bag Calculator
Pick your bag size — 40, 60, or 80 lb — then enter your project's dimensions to get an exact bag count, rounded up so you never run short.
Results
- Cubic yards
- 0.93 yd³
- Cubic feet
- 25.0 ft³
- Weight
- 1.88 tons
- Bags
- 84 × 40 lb bag
Assumed density: 4050 lb/yd³ (Concrete (wet mix))
How it works
Choose your bag size first: a 40 lb bag yields 0.30 cu ft, a 60 lb bag 0.45 cu ft, and an 80 lb bag 0.60 cu ft of mixed concrete.
Enter the pour's length and width in feet and depth in inches (e.g., a 12-inch deep footing). The calculator divides total cubic feet by your bag's yield.
The bag count is always rounded up to the next whole bag. Keep 1–2 extra bags on site for adjustments.
Reference chart
Concrete bag yield by size
| Bag weight | Yield (cu ft) | Yd³ per bag | Bags per yd³ |
|---|---|---|---|
| 40 lb | 0.3 | 0.011 | 90 |
| 60 lb | 0.45 | 0.017 | 60 |
| 80 lb | 0.6 | 0.022 | 45 |
Square feet covered per concrete bag
Use this table for small flat slabs where you know the thickness. Post holes, footings, and irregular pours should use the calculator above because they are volume problems.
Approximate slab coverage per bag
| Bag size | 3 in thick | 4 in thick | 6 in thick |
|---|---|---|---|
| 40 lb | 1.2 sq ft | 0.9 sq ft | 0.6 sq ft |
| 60 lb | 1.8 sq ft | 1.4 sq ft | 0.9 sq ft |
| 80 lb | 2.4 sq ft | 1.8 sq ft | 1.2 sq ft |
Frequently asked questions
How many 80 lb bags of concrete do I need for a 4×4 ft slab, 4 inches thick?
4 × 4 × (4/12) = 5.33 cu ft ÷ 0.60 cu ft per bag ≈ 9 bags. Buy 10 bags.
How many bags of concrete do I need for a fence post?
A standard 10-inch diameter, 30-inch deep hole holds about 1.36 cu ft (π × (5/12 ft)² × 2.5 ft). Subtract ~0.21 cu ft for a 4×4 post and you need ~1.15 cu ft — about 2 to 3 × 80 lb bags or 4 to 5 × 40 lb bags per post (rounded up).
What is the difference between 40, 60, and 80 lb bags of concrete?
All three use the same mix; the weight just determines how much you mix at once. 80 lb bags are most cost-effective; 40 lb bags are easier to carry.
How many yards of concrete are in one bag?
An 80 lb bag yields 0.60 cu ft, which is 0.022 yd³. A 60 lb bag yields 0.45 cu ft, or 0.017 yd³. A 40 lb bag yields 0.30 cu ft, or 0.011 yd³.
How many square feet does one bag of concrete cover?
Coverage depends on slab thickness. One 80 lb bag covers about 1.8 sq ft at 4 inches thick, or 2.4 sq ft at 3 inches thick.
Link to this calculator
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<a href="https://diymaterialcalculators.com/concrete-bags/">Concrete Bag Calculator</a> — free online calculator