Concrete & Masonry
concrete bagsConcrete Bags
Estimate how many pre-mix bags you need for a slab, footing, or post hole, based on volume and standard bag yields.
How to Use the Concrete Bag Calculator
Enter the length and width of the area you're pouring in feet, and the slab thickness in inches. Pick your bag size, then read off the number of bags needed. The calculator recalculates instantly as you change any field, so it's easy to compare 40 lb, 60 lb, and 80 lb bags for the same pour.
How the Calculation Works
The calculator first finds the total volume of concrete needed in cubic feet: length × width × (thickness ÷ 12), since thickness is entered in inches but length and width are in feet. That volume is converted to cubic yards by dividing by 27 (there are 27 cubic feet in a cubic yard). Finally, it divides by the yield of your chosen bag size and rounds up, since you can't buy a partial bag.
Standard bag yields, per the mix manufacturers' published specs:
- 40 lb bag — yields about 0.011 cubic yards (0.30 cubic feet)
- 60 lb bag — yields about 0.017 cubic yards (0.45 cubic feet)
- 80 lb bag — yields about 0.022 cubic yards (0.60 cubic feet)
Example Calculation
Say you're pouring a 10×10 ft patio slab at 4 inches thick. Volume = 10 × 10 × (4 ÷ 12) = 33.3 cubic feet, or 1.23 cubic yards. At 60 lb bags (0.017 yd³ each), that's 1.23 ÷ 0.017 = 72.5, rounded up to 73 bags.
Frequently Asked Questions
Should I add extra for waste?
This calculator gives you the exact volume needed with no waste buffer. Most contractors add 5–10% extra for spillage, uneven subgrade, or forms that aren't perfectly level — for the example above, that would mean rounding up to roughly 76–80 bags to be safe.
Why does bag size matter so much?
Larger bags are more material-efficient (less bag material per pound of concrete) and often slightly cheaper per pound, but they're also heavier to carry — an 80 lb bag is a two-person lift for most people. For small jobs like fence post footings, 40 lb or 60 lb bags are usually easier to manage.
What thickness should I use for a driveway vs. a walkway?
A pedestrian walkway is typically poured at 4 inches. A driveway that will see vehicle traffic is usually 5–6 inches, and heavier vehicles (RVs, trucks) may call for 6+ inches with reinforcement. Check your local code for exact requirements.