How to use it
- Enter the slab length and width in feet.
- Enter the slab thickness in inches.
- Click calculate to see cubic yards, bags, and cubic feet.
- Use the bag count to plan your order and budget.
This calculator helps you determine exactly how much concrete you need for a 20x30 slab. Enter your length, width, and thickness, and it instantly returns the volume in cubic yards, the number of 80-lb bags, and cubic feet. The bag count matters for DIY projects—each standard 80-lb bag yields about 0.6 cubic feet. Cubic yards matter if you're ordering ready-mix concrete, which is typically sold by the yard. Cubic feet gives you a raw volume reference for comparing. By using these three numbers together, you can confidently order the right amount: enough to finish the job without paying for extra material you don't need. Avoid last-minute runs to the store or costly overages. Use the result to plan your purchase, schedule, and budget. Whether you're setting a patio, shed base, or walkway, knowing the exact volume up front makes your pour smoother and your project more successful.
Pro tips
- Add 10% waste allowance to account for spillage and uneven subgrades.
- Always round your bag count up—partial bags aren't sold.
- For slabs over 2 cubic yards, consider ordering ready-mix instead of bags.
- Double-check thickness before ordering; an extra inch can add 10% more concrete.
Real-life examples
- Pouring a 20x30 patio slab for backyard entertaining.
- Building a 20x30 shed foundation for a workshop.
- Creating a 20x30 walkway or pathway across your lawn.
Case study
Mike planned a 20x30 slab for his shed. He guessed he'd need 300 bags, but the calculator showed 334 bags for a 4-inch slab. He ordered the right amount, finished the pour in one day, and had zero leftover concrete. No wasted money, no last-minute rush.