A 10 × 10 ft shed pad, 4 in thick.
10 × 10 × (4 ÷ 12) = 33.33 ft³, or 1.235 yd³. With no waste that's 112 bags of 40 lb, 75 of 60 lb or 56 of 80 lb. Add the default 10% and it becomes 1.358 yd³: 123 × 40 lb, 82 × 60 lb or 62 × 80 lb.
Concrete Calculator
Work out cubic yards and how many 40, 60 or 80 lb bags you need for a slab, round pad or post holes.
Results
- Cubic feet
- 33.33 ft³
- Cubic yards
- 1.235 yd³
- Cubic feet with waste
- 36.67 ft³
- Cubic yards with waste
- 1.358 yd³
- 40 lb bags
- 123
- 60 lb bags
- 82
- 80 lb bags
- 62
Ready-mix is usually cheaper once you're past about 1 cubic yard. This is guidance, not a price quote. Ask the supplier about short-load fees.
Follow the water amount printed on the bag or the supplier's instructions. Extra water makes concrete easier to pour but weaker.
Cost comparison
Enter the shape and size of your pour to get cubic feet, cubic yards and bag counts for 40, 60 and 80 lb premix. It handles square slabs, round pads and post holes with the post subtracted, and adds a waste allowance so you don't run short halfway through.
How to use it
- Choose the shape: slab, round slab, or post hole/column.
- Enter the size. Slabs take length and width in feet and inches plus thickness in inches. Round shapes take a diameter and depth.
- For post holes, enter how many holes and the post size. A 4×4 is really 3.5 × 3.5 in and a 6×6 is 5.5 × 5.5 in, and the tool uses those actual sizes.
- Set waste. The default is 10%.
- Read the cubic yards (what a ready-mix supplier asks for) and the bag counts (what you'd buy at the store). Add prices if you want a bags vs. truck cost comparison.
Measuring tips
- Measure the inside of your forms, not the outside of the boards.
- Check thickness in several spots. Ground is rarely flat, and a slab that's 4 in at one corner and 5 in at another can use noticeably more concrete than you planned. Use the deepest typical measurement, or the average if the base is well graded.
- For post holes, measure the actual hole you dug. Diggers often make holes wider at the top than the bit size.
- The calculator warns you below 2 in, because standard concrete mix is meant for pours 2 in thick or more (per the QUIKRETE Concrete Mix data sheet). Thinner jobs usually call for a topping or resurfacing product instead.
How it works
The calculator finds the volume in cubic feet, adds waste, then divides by 27 for cubic yards and by each bag's yield for a bag count. For post holes, it subtracts the space the post takes up.
- Slab = length × width × (thickness in inches ÷ 12). Round slab or hole = π × (diameter ÷ 2)² × depth.
- Bags = volume with waste ÷ bag yield, rounded up. Yields from the QUIKRETE Concrete Mix data sheet: 0.30 ft³ (40 lb), 0.45 ft³ (60 lb), 0.60 ft³ (80 lb). Other brands differ, so the yield is editable.
Quick check: 27 ft³ ÷ 0.60 ft³ = 45 bags of 80 lb mix per cubic yard, the same figure Home Depot lists on its 80 lb bags.
Worked examples
Ten fence posts.
Holes 10 in across and 36 in deep, each holding a 4×4. Each hole is π × 5² × 36 = 2,827.4 in³, minus the post's 3.5 × 3.5 × 36 = 441 in³. Across 10 holes that's 13.81 ft³ (0.511 yd³), so 47 × 40 lb, 31 × 60 lb or 24 × 80 lb bags with no waste.
A two-car driveway.
24 × 20 ft at 5 in with 5% waste is 200 ft³ (7.407 yd³), or 7.778 yd³ with waste. That's 350 bags of 80 lb mix, which is a lot of mixing. This is the kind of job where the calculator suggests pricing ready-mix.
A round patio.
12 ft across and 4 in thick: π × 6² × (4 ÷ 12) = 37.70 ft³, or 1.396 yd³. With 0% waste, that's 63 bags of 80 lb mix. With the default 10% waste it's 41.47 ft³ (1.536 yd³): 139 × 40 lb, 93 × 60 lb or 70 × 80 lb.
Quick reference
| Thickness | Area 1 yd³ covers | 80 lb bags per yd³ |
|---|---|---|
| 3.5 in | 92.6 ft² | 45 |
| 4 in | 81.0 ft² | 45 |
| 5 in | 64.8 ft² | 45 |
| 6 in | 54.0 ft² | 45 |
| 8 in | 40.5 ft² | 45 |
Bags per cubic yard by size: 90 × 40 lb, 60 × 60 lb, 45 × 80 lb. One 80 lb bag covers about 1.8 ft² at 4 in thick.
Bags or a truck?
As a rough guide, ready-mix usually works out cheaper once you're past about 1 cubic yard. Below that, bags are easier to handle and you don't pay for a minimum load. Treat this as a rule of thumb, not a price quote. Enter your local bag and per-yard prices in the tool to compare them for your job, and ask the supplier about short-load fees. Whichever you choose, follow the water amount printed on the bag or the supplier's instructions. Extra water makes concrete easier to pour but weaker.
FAQ
How many bags of concrete do I need for a 10 × 10 slab?
At 4 in thick, a 10 × 10 slab is 1.235 yd³. With 10% waste, buy 62 bags of 80 lb mix, 82 of 60 lb or 123 of 40 lb.
How many 80 lb bags of concrete are in a yard?
45. A cubic yard is 27 ft³ and each 80 lb bag yields 0.60 ft³.
How do I convert square feet of concrete to cubic yards?
Multiply the square footage by the thickness in feet (inches ÷ 12), then divide by 27. For 100 ft² at 4 in: 100 × (4 ÷ 12) ÷ 27 = 1.235 yd³.
How much concrete do I need for a post hole?
Work out the hole's volume, π × radius² × depth, and subtract the part the post takes up. A 10 in hole 36 in deep around a 4×4 takes about 1.38 ft³. That's about 2.3 bags of 80 lb mix (2.5 with the default 10% waste), so you'd buy 3.
How much extra concrete should I order?
10% is the default and covers spills, uneven ground and form bowing on most small jobs. Go higher if your base isn't level.
What does concrete cost for my project?
Prices vary a lot by region and brand. Enter your price per bag and per cubic yard, and the calculator totals both options for you.