Quick answer: Multiply length × width × thickness in feet to get cubic feet, add 5 to 10% for waste, then divide by 27 for cubic yards. A 10 × 10 ft slab 4 inches thick is 10 × 10 × 0.333 = 33.3 cu ft, or 1.36 cubic yards with 10% waste. That is about 62 bags of 80 lb concrete, because each 80 lb bag yields about 0.60 cu ft.
The concrete volume formula
cubic yards = length (ft) × width (ft) × thickness (in ÷ 12) ÷ 27One cubic yard is 27 cubic feet (3 ft × 3 ft × 3 ft).- Measure in feet. Convert inches: 6 inches is 0.5 ft, 4 inches is 0.333 ft.
- Multiply the three dimensions for cubic feet.
- Add waste: 5% for simple formed slabs, 10% for uneven ground or DIY pours, 15% for irregular shapes.
- Divide by 27 for cubic yards, or by the bag yield for a bag count.
Calculate a slab
Concrete to order
1.36cu yd
For footings, columns and multiple pours, the full concrete calculator handles each shape, and the concrete yardage calculator sizes a ready-mix order.
Cubic yards per 100 square feet
A quick way to budget: find your thickness, then scale by area. The figures below exclude waste.
| Thickness | Typical use | Cu yd per 100 sq ft | 80 lb bags per 100 sq ft |
|---|---|---|---|
| 4 in | Patios, walkways, shed bases | 1.23 | 56 |
| 5 in | Driveways for cars | 1.54 | 70 |
| 6 in | Driveways, garage floors, heavier loads | 1.85 | 84 |
| 8 in | Heavy vehicle pads | 2.47 | 112 |
How many bags of concrete per yard
| Bag size | Yield per bag | Bags per cubic yard | Bags per 10 cu ft |
|---|---|---|---|
| 80 lb | 0.60 cu ft | 45 | 17 |
| 60 lb | 0.45 cu ft | 60 | 23 |
| 40 lb | 0.30 cu ft | 90 | 34 |
Yields are the manufacturer figures printed on standard concrete mix bags such as Quikrete and Sakrete. High-strength and fast-setting mixes can yield slightly less, so check the bag.
Footings and post holes
A continuous footing
- 20 ft long, 12 in wide, 8 in deep.
- 20 × 1 × 0.667 = 13.3 cu ft.
- Plus 10% waste: 14.7 cu ft, or 0.54 cu yd.
- 14.7 ÷ 0.60 = 24.4, so 25 bags of 80 lb.
Result: 0.54 cu yd, 25 × 80 lb bags
Three fence post holes
- Each hole is 10 in wide and 3 ft deep: π × (0.417 ft)² × 3 = 1.64 cu ft.
- A 4×4 post (3.5 in actual) takes up 0.29 × 0.29 × 3 = 0.26 cu ft, leaving 1.38 cu ft.
- Three holes: 4.14 cu ft ÷ 0.60 = 6.9 bags.
Result: 7 bags of 80 lb
Bags or ready-mix?
| Volume | Best option | Why |
|---|---|---|
| Under 1 cu yd | Bagged concrete | About 45 × 80 lb bags or fewer; mix by hand or with a rented mixer |
| 1 to 2 cu yd | Either, or a trailer load | Bag count and labor climb fast; small-load fees apply to trucks |
| Over 2 cu yd | Ready-mix truck | Consistent strength, one continuous pour, far less labor |
Many ready-mix suppliers charge a short-load fee for small orders, so compare the total delivered price with the cost of bags plus mixer rental.
Mistakes that leave you short
- Forgetting to convert inches. 4 inches is 0.333 ft, not 4 ft.
- No waste allowance. Uneven subgrade can swallow 5 to 10% more.
- Measuring the form, not the hole. Over-dug footings need more concrete than the drawing.
- Using cubic feet as yards. Divide by 27.
- Ordering exact. Running out mid-pour creates a cold joint.
Frequently asked questions
How many bags of concrete do I need for a 10×10 slab?
A 10 × 10 ft slab 4 inches thick is 33.3 cu ft. With 10% waste that is 36.7 cu ft, or 62 bags of 80 lb concrete.
How many 80 lb bags make a yard of concrete?
About 45, because each 80 lb bag yields about 0.60 cu ft and a cubic yard is 27 cu ft.
How thick should a concrete slab be?
4 inches for patios and walkways, 5 to 6 inches for driveways and garage floors, and 6 inches or more with reinforcement for heavy vehicles.
How much concrete per fence post?
For a 10 inch wide, 3 ft deep hole with a 4×4 post, about 1.4 cu ft, or 2 to 3 bags of 80 lb concrete.
How much extra concrete should I order?
Add 5 to 10%, and up to 15% for irregular shapes or uneven ground.
How do I convert cubic feet to cubic yards?
Divide by 27. 54 cu ft is 2 cu yd.
Keep going
Quantities are estimates. Structural slabs, footings and anything carrying significant loads should follow local building codes and, where required, an engineer’s specification.
Content Lead, Anchor AI Tools, California, USA. Spotted an error? Email hello@anchoraitools.com and it will be checked and corrected. See our editorial standards.