Ordering concrete is a one shot job. Concrete cannot be returned, so the measuring matters. Here is a simple way to get the number right.
Step 1: Mark the outline
Use stakes and string to mark the finished edges of the slab. Measure the length and width between the strings, not the size on your drawing, because forms take up space.
Step 2: Check the depth
Four inches is common for patios and walkways. Driveways and garage floors are often 5 or 6 inches. Measure the depth of the excavated area, because a low spot adds more concrete than the flat math predicts.
Step 3: Do the math
Multiply length, width, and depth in feet, then divide by 27. For a 12 by 12 foot slab at 4 inches thick:
- Area is 144 square feet
- Volume is 144 times 0.333, which is 48 cubic feet
- Divide by 27 to get 1.78 cubic yards
Step 4: Add extra
Add 5 to 10 percent for uneven ground and spillage. The 12 by 12 slab becomes about 1.96 cubic yards with 10 percent extra.
Step 5: Check the order with a supplier
Ask about minimum orders and short load fees. Many plants have a minimum, so a small slab may be cheaper in bags. See how many bags make a cubic yard for the numbers.
Common mistakes
- Forgetting that thickness is in inches, not feet
- Measuring the plan instead of the staked area
- Ordering the exact number with no extra
- Skipping the base layer, which adds its own material order
Try the concrete calculator or look up your size on the slab size pages.