Concrete Calculator
Calculate the cubic yards of concrete and number of pre-mix bags needed for a slab, footing or column.
| Material / Unit | Quantity Needed |
|---|---|
| Ready-mix concrete | 1.23 cubic yards |
| Cubic feet (raw) | 33.3 cu ft |
| 80 lb pre-mix bags | 56 bags |
| 60 lb pre-mix bags | 75 bags |
Either buy ready-mix by the cubic yard for a delivery truck, or use one of the bag counts if you're mixing bags yourself — don't add both bag rows together, they're two alternatives for the same slab.
- 80 lb Bags56
- 60 lb Bags75
About the Concrete Calculator
This calculator converts a slab's length, width and thickness into the cubic yards of ready-mix concrete you'd order from a truck, plus the equivalent number of pre-mix bags if you're hand-mixing instead. It's built for the moment before a materials run — sizing a patio, footing, shed pad or fence post job before you're standing in the aisle guessing.
Concrete is priced and delivered by the cubic yard for anything of real size, but most DIY jobs (a small pad, a handful of post holes) are more practically done with bagged mix from a hardware store — this tool gives you both units side by side so you can pick whichever matches how you're actually buying.
Thickness is entered in inches because that's how slabs are specified, while length and width stay in feet — the calculator handles the unit conversion so you don't have to convert inches to feet in your head before multiplying.
How it’s calculated
Volume starts as length × width × thickness, with thickness converted from inches to feet first (divided by 12), giving a raw cubic-foot volume. Dividing by 27 converts that to cubic yards, since a cubic yard is 3 ft × 3 ft × 3 ft = 27 cubic feet.
For bagged concrete, an 80 lb bag yields about 0.6 cubic feet of mixed concrete and a 60 lb bag yields about 0.45 cubic feet — those are standard figures from bag manufacturers, so the calculator divides your total cubic footage by the appropriate yield and rounds up, since you can't buy a partial bag.
Frequently asked questions
How many bags of concrete do I need per cubic yard?
About 45 bags of 60 lb mix or roughly 34 bags of 80 lb mix per cubic yard, though it's easier to let the calculator do the exact math for your specific slab size rather than working from a flat per-yard rule.
Should I use 60 lb or 80 lb bags?
80 lb bags deliver more concrete per bag (fewer bags to carry and mix) but are noticeably heavier to lift; 60 lb bags are easier to handle solo. Both produce the same final concrete once mixed.
What thickness should I use for a driveway versus a walkway?
A typical walkway or patio slab is usually 4 inches thick, while a driveway that needs to support vehicle weight is commonly poured at 5 to 6 inches — check local code for anything load-bearing.
Should I order extra concrete beyond the calculated amount?
Most contractors add 5–10% over the calculated volume to cover an uneven subgrade, spillage and minor measurement error — it's cheaper to have a little extra than to run short mid-pour.
Is ready-mix delivery worth it for a small job?
For anything under roughly half a cubic yard, bagged mix is usually more practical since most ready-mix suppliers have delivery minimums; above that, a truck often works out cheaper per yard and saves a lot of manual mixing.
Related calculators
Powered by GetCalculator.online