For a standard 4-inch concrete slab, you need 45 bags of 80lb concrete per cubic yard, 60 bags of 60lb, or 90 bags of 40lb. To find your cubic yards, multiply length × width × depth (in feet) and divide by 27. Always order 10% extra for waste.Use our free Concrete Calculator to get an instant bag count for your exact dimensions — no math needed.
The Formula: How to Calculate Bags of Concrete
Concrete is measured in cubic yards. Here is the three-step formula to find out how many bags you need:
- Calculate cubic feet: Length (ft) × Width (ft) × Depth (ft)
- Convert to cubic yards: Divide cubic feet by 27
- Multiply by bags per yard: 45 for 80lb bags, 60 for 60lb, 90 for 40lb
Worked Example: 10×10 ft Patio at 4 Inches Deep
- 10 × 10 × 0.333 = 33.3 cubic feet
- 33.3 ÷ 27 = 1.23 cubic yards
- 1.23 × 45 = 56 bags of 80lb concrete
Add 10% waste: 56 × 1.10 = 62 bags total to purchase.
Skip the math entirely with our free Concrete Calculator — enter your dimensions and get an instant result.
Bags Per Cubic Yard: 40lb, 60lb, 80lb
One cubic yard = 27 cubic feet. Here is how many bags of each size you need to fill one cubic yard:
| Bag Size | Coverage per Bag (cu ft) | Bags per Cubic Yard | Approx. Cost per Bag |
| 40lb bag | 0.30 cu ft | 90 bags | $4–$5 |
| 60lb bag | 0.45 cu ft | 60 bags | $5–$6 |
| 80lb bag | 0.60 cu ft | 45 bags | $6–$8 |
Prices based on national averages at home improvement stores in 2026. Regional prices may vary.
The 80lb bag gives the best value per cubic foot. The 40lb bag is easier to handle — useful if you are working alone or on uneven ground.
Bags Needed by Project Size
Here is a quick reference for common concrete projects at the standard 4-inch depth:
| Project | Dimensions | Cubic Yards | 80lb Bags | 60lb Bags |
| Small patio | 8 × 8 ft, 4 in | 0.79 | 36 | 48 |
| Standard patio | 10 × 10 ft, 4 in | 1.23 | 56 | 74 |
| Large patio | 12 × 12 ft, 4 in | 1.78 | 80 | 107 |
| Sidewalk (40 ft) | 40 × 3 ft, 4 in | 1.48 | 67 | 89 |
| Garage floor (1-car) | 12 × 20 ft, 4 in | 2.96 | 134 | 178 |
| Garage floor (2-car) | 20 × 20 ft, 4 in | 4.94 | 223 | 297 |
| Fence post (per post) | 10 in dia, 2 ft deep | 0.09 | 4 | 5 |
All quantities include a 10% waste buffer. For projects over 2 cubic yards, consider ready-mix delivery instead.
Need the exact number for your project? Our Concrete Calculator calculates bags for any shape including round footings, rectangular slabs, columns, and steps.
Bags vs Ready-Mix Concrete: Which Should You Use?
This is one of the most common questions homeowners ask before starting a concrete project. The answer depends on project size.
| Factor | Concrete Bags (DIY) | Ready-Mix Truck |
| Best for | Under 1 cubic yard | Over 1 cubic yard |
| Cost per yard | $162–$240 | $125–$165 |
| Labor | High (mixing by hand or machine) | Low (pour directly) |
| Quality consistency | Variable | Consistent, plant-mixed |
| Minimum order | None | Typically 1–2 yards |
| Delivery fee | None | $50–$100+ |
Rule of thumb: If your project needs more than 1 cubic yard of concrete (about 45 bags of 80lb), call a ready-mix supplier. Mixing 50+ bags by hand or with a small mixer takes a full day and produces inconsistent results.
See our concrete slab cost guide for a full breakdown of ready-mix pricing vs bag costs by project type.
Always Add a Waste Factor
Never order the exact amount you calculate. Always add extra for these reasons:
- Spillage — concrete is heavy and awkward to pour precisely
- Uneven subgrade — low spots in the ground use more concrete than expected
- Form blowout — if a form shifts or leaks, you need backup material on hand
- Mixing waste — residue left in bags and mixers that cannot be used
Standard waste factor: 10% for most projects. Add 15% for irregular shapes or first-time DIYers.
Pro Tips Before You Buy
- Check the bag weight for your situation. 80lb bags are most economical but heavy. If you are working alone, 60lb bags are easier to manage without risking back injury.
- Buy from one batch. Concrete mix properties can vary slightly between manufacturing dates. Buy all bags at once so every batch mixes consistently.
- Store bags off the ground. Moisture from concrete floors or soil can cause premature hardening inside unopened bags. Stack on wooden pallets.
- Work in the right temperature. Do not pour concrete below 40°F or above 90°F without special additives. Ideal range is 50–75°F.
- Have water on site before you start. Concrete mixing requires a precise water-to-cement ratio. Too much water weakens the final slab.
If you are also planning a driveway, see how concrete compares in our asphalt vs concrete driveway guide. For roofing or flooring projects, check our full range of free home improvement calculators.
Frequently Asked Questions
How many 80lb bags of concrete make a cubic yard?
It takes 45 bags of 80lb concrete to fill one cubic yard. For 60lb bags, you need 60 bags, and for 40lb bags, 90 bags per cubic yard.
How many bags of concrete do I need for a 10×10 slab?
A 10×10 ft slab at 4 inches deep requires 1.23 cubic yards, which equals approximately 56 bags of 80lb concrete before waste. Add 10% to get 62 bags total to purchase.
Is it cheaper to buy bags or ready-mix concrete?
For projects under 1 cubic yard, bags are cost-effective. Above 1 cubic yard, ready-mix is almost always cheaper at $125–$165 per yard, saves labor, and delivers more consistent quality.
Can I use one 80lb bag for a fence post?
Yes. Most standard fence posts in a 10-inch diameter hole at 2 feet deep need approximately 3–4 bags of 80lb fast-setting concrete. Always check the hole size and post specifications.
Calculate Your Exact Bag Count — Free
Enter your project dimensions into our free Concrete Calculator and get an instant result for 40lb, 60lb, and 80lb bags — plus cubic yards, cubic feet, and cost estimates.