Concrete Calculator: How Much Concrete Do I Need?
Enter the slab dimensions and how thick you want it. You will get the volume in cubic yards, the number of 60 and 80 lb bags it works out to, and a clear steer on whether this is a bag job or a ready-mix delivery.
Slab measurements
Measure the inside of the forms, not the excavation.
Concrete is one of the few materials you cannot top up once the pour has started, which is why the default allowance is higher than the landscape calculators. This estimate covers material only — not reinforcement, forms or a sub-base.
How much concrete do I need?
Multiply the slab area by its thickness and convert to cubic yards. The number itself is simple; the decision that follows is not, because below about a yard you are mixing bags by hand and above it you are booking a truck.
A short load cannot be fixed halfway through a pour: the cold joint where the first batch set is a permanent weakness. That is why the allowance here defaults higher than for mulch or gravel, and why erring up is the right direction.
How thick should the slab be?
| Thickness | Typical use |
|---|---|
| 4 inches | Patios, walkways, shed bases |
| 5 inches | Slabs carrying occasional vehicle weight |
| 6 inches | Driveways and heavier loads |
These are common residential thicknesses, not an engineering recommendation. Anything structural, load-bearing or subject to local code needs a professional, not a calculator.
Bags or ready-mix?
| Bag | Yield | Bags per cubic yard |
|---|---|---|
| 60 lb | 0.45 cu ft | 60 |
| 80 lb | 0.6 cu ft | 45 |
A cubic yard is 27 cubic feet, so it takes 45 eighty-pound bags — around 1.8 tons of material to carry, mix and place before any of it sets. Bags make sense for post holes, small pads and repairs. Past about a yard, a delivery is cheaper, faster and produces a better slab because it all goes in at once.
How to measure a pour
The concrete fills the form, not the excavation around it.
Add each as its own pour. They are summed before rounding, so splitting never changes the order.
Measure from the top of the sub-base to the finished surface, not the depth you dug out.
Common mistakes
- Ordering exactly the calculated volume with no allowance for an uneven sub-base.
- Measuring the excavation instead of the inside of the forms.
- Underestimating how much work 45 bags per cubic yard actually is.
- Treating a material estimate as a structural specification.
Building the base underneath?
Most slabs sit on compacted gravel — work out that layer first.
Open the Gravel Calculator →