Exactly how many bags of concrete you need per square foot based on thickness. Includes 80lb/60lb/40lb bag calculations, conversion tables, and free calculator.
"How many bags of concrete per square foot?" is one of the most common DIY questions. This guide gives you the exact answer with tables for all standard thicknesses, plus practical examples and tips for ordering the right amount of bagged concrete for slabs, patios, and foundations.
Use our concrete calculator for exact bag counts for your project.
Concrete bag coverage depends entirely on thickness. The standard measurement:
To find "bags per square foot":
Example for 4 inches thick: 4 inches = 0.333 feet. One square foot at 0.333 ft thick = 0.333 cubic feet. 0.333 ÷ 0.6 = 0.555 bags per square foot (rounds to 0.56).
This table gives you exact bag counts for all standard slab thicknesses:
| Thickness | 80-lb Bags per sq ft | 60-lb Bags per sq ft | 40-lb Bags per sq ft | Cubic Feet per sq ft |
|---|---|---|---|---|
| 3 inches | 0.42 bags | 0.56 bags | 0.83 bags | 0.25 cubic feet |
| 4 inches | 0.56 bags | 0.75 bags | 1.11 bags | 0.333 cubic feet |
| 5 inches | 0.69 bags | 0.93 bags | 1.39 bags | 0.417 cubic feet |
| 6 inches | 0.83 bags | 1.11 bags | 1.67 bags | 0.5 cubic feet |
How to use this table: Multiply the "bags per sq ft" value by your total square footage. Then add 10% for waste.
If you prefer to calculate manually or understand the math:
Bags needed = (Area in sq ft × Thickness in ft) ÷ Bag yield in cubic feet
Where:
Cubic yards = (Bags × Bag yield in cubic feet) ÷ 27
Example: 45 bags of 80-lb concrete = (45 × 0.6) ÷ 27 = 27 ÷ 27 = 1 cubic yard
Here's how the "bags per square foot" calculation works for real projects:
At 4 inches thick, you need 0.56 bags of 80-lb concrete per square foot. That's about 5-6 bags for every 10 square feet.
Standard thicknesses: 4 inches for patios/walkways, 5-6 inches for driveways/garage floors, 6+ inches for commercial/heavy equipment. Thicker = more bags per square foot.
For 100 sq ft at 4" thick: 56 bags of 80-lb concrete, 75 bags of 60-lb, or 111 bags of 40-lb. Add 10% waste (6-12 extra bags).
Yes, but you'll need more bags (0.75 per sq ft vs 0.56 for 80-lb). 60-lb bags are easier to handle but cost more per cubic yard.
At 4 inches thick: 1.11 bags of 40-lb concrete per square foot. 40-lb bags are easiest to handle but most expensive per yard.
Use our table above for 3", 5", or 6" thickness. Your bags per square foot will be proportionally higher or lower.
Our concrete calculator handles all these variables instantly. Enter dimensions, select thickness, add waste factor, and get precise bag counts and cost estimates.
For larger projects, you might consider ready-mix concrete instead of bags:
| Project Size | 80-lb Bags | Cubic Yards | Cost Difference | Recommendation |
|---|---|---|---|---|
| Small (under 50 sq ft) | Under 28 bags | Under 0.6 yards | Bags slightly cheaper | Use bags |
| Medium (50-200 sq ft) | 28-112 bags | 0.6-2.5 yards | Ready-mix saves 20-40% | Consider delivery |
| Large (200+ sq ft) | 112+ bags | 2.5+ yards | Ready-mix saves 40-60% | Use ready-mix |
In practice: If your project needs more than 100 bags (about 120 sq ft at 4"), seriously consider ready-mix delivery. The labor savings alone often justify the cost.
Now that you know how many bags of concrete per square foot:
Final tip: Bagged concrete is convenient but expensive for large projects. Use our "bags per square foot" calculation to estimate needs, then compare with ready-mix pricing. For anything over 2 cubic yards (90 bags), delivery is usually the better choice.
Use our free Concrete Calculator for precise bag counts tailored to your specific project. Enter dimensions, thickness, and bag size for instant results.
Go to Concrete Calculator