Concrete Calculator — Free Online Tool
Concrete is ordered by the cubic yard but mixed by the bag — and the two have nothing to do with each other until you do the math. This concrete calculator gives you both: yards for ready-mix trucks, bags for DIY pours.
What is the Concrete Calculator?
Volume is length × width × thickness (in feet) divided by 27. A 10×10 slab at 4 inches thick is 1.23 cubic yards — small enough to bag-mix, barely. The rule of thumb: under 1 yard, bags are practical; over 2 yards, call a ready-mix truck; between is judgment (and back pain).
One 80-lb bag yields about 0.6 cubic feet, so it takes 45 bags per cubic yard — over a ton of lifting per yard, which is why even “small” slabs exhaust DIYers. Order 5–10% extra; a short load costs more in a second delivery than the extra concrete. This concrete calculator shows both figures so you can price bags versus a truck.
How to use this calculator
- Measure the slab's length and width in feet.
- Enter the thickness in inches (4" for patios, 6"+ for driveways).
- Click Calculate Concrete and compare bags vs truck.
- Order 5–10% extra to avoid a short load.
Formula
Frequently asked questions
How many 80-lb bags of concrete per yard?
45 bags per cubic yard — which is why this concrete calculator also shows yards for truck delivery.
Is there a minimum for ready-mix delivery?
Often 1–2 yards plus a short-load fee — call local plants for their policy.
How thick should a concrete slab be?
4 inches for walks and patios; 6 inches or more for driveways and heavy loads.
How long does concrete take to cure?
Foot traffic in 24–48 hours, but full strength takes about 28 days — keep it moist while curing.