How much concrete do you actually need?
Concrete is sold by volume, but most DIYers think in bags — which is where mistakes happen. A small footing might only need a handful of 60 lb bags, while a full driveway slab needs volume calculated first, then converted into a bag count (or, past a certain size, ready-mix delivered by the truckload instead).
Volume first, bags second
The calculator above works in two steps: it finds the cubic footage of your pour (length × width × thickness), then divides that by your bag’s yield to get a bag count. This order matters — if you only think in bags from the start, it’s easy to misjudge a slab that’s thicker or larger than it looks.
How to Calculate Bags of Concrete Needed
You can work out the exact same result the calculator gives you by hand, in three steps.
Step One: Find the Volume in Cubic Feet
Multiply the length and width of your pour (in feet) by the thickness (converted from inches to feet by dividing by 12).
volume (cu ft) = length × width × (thickness ÷ 12)
For a 10 ft × 5 ft slab at 4 inches thick:
10 × 5 × (4 ÷ 12) = 16.67 cubic feet
Step Two: Convert to Cubic Yards (Optional)
If you’re comparing against a ready-mix truck quote (which is priced by the cubic yard), divide the cubic footage by 27.
volume (cu yd) = volume (cu ft) ÷ 27
16.67 ÷ 27 ≈ 0.62 cubic yards
Step Three: Divide by Your Bag’s Yield
Each bag size has a published yield in cubic feet (0.30 for a 40 lb bag, 0.45 for a 60 lb bag, 0.60 for an 80 lb bag). Divide your total volume by that yield, then round up — a partial bag still means buying a whole one.
bags needed = round up(volume (cu ft) ÷ bag yield)
Using 60 lb bags:
16.67 ÷ 0.45 ≈ 37.05 → rounds up to 38 bags
When to switch to ready-mix
Once you’re past roughly 1 cubic yard (about 45 sixty-pound bags), ready-mix delivery usually becomes cheaper and far less labor-intensive than mixing bags by hand. Use this calculator to find your volume first, then decide which route makes sense for your project size.
A note on curing
Bag count gets you the pour started, but strength depends on proper curing — keeping the slab moist for the first several days, especially in hot or windy weather. A fast cure in harsh conditions is a common cause of surface cracking.