How this concrete calculator works
Concrete is sold by volume, so every estimate comes down to one number: how many cubic feet or cubic yards fill your form. The calculator multiplies your dimensions together, converts everything to the same unit, adds your waste allowance, and then works out how many premixed bags that volume equals.
Slab volume (cu ft) = length (ft) × width (ft) × thickness (in) ÷ 12
Cubic yards = cubic feet ÷ 27
For round columns, sonotubes and post holes, the area of the circle replaces length times width:
Column volume (cu ft) = 3.1416 × (diameter in inches ÷ 24)² × height (ft)
Example: a 10 × 10 ft patio poured 4 inches thick is 10 × 10 × 4 ÷ 12 = 33.3 cubic feet, or about 1.23 cubic yards. With 10% extra that becomes about 1.36 cubic yards.
How much concrete is in a bag?
Bag yields vary slightly by brand and mix, but these are the standard figures printed on most US premixed concrete bags:
| Bag size | Yield per bag | Bags per cubic yard |
|---|---|---|
| 40 lb | 0.30 cu ft | 90 |
| 60 lb | 0.45 cu ft | 60 |
| 80 lb | 0.60 cu ft | 45 |
Always check the yield on the bag you actually buy. Fast-setting post mixes, for example, often yield less per pound than standard concrete.
Typical slab thickness by project
| Project | Common thickness |
|---|---|
| Walkway or garden path | 4 in |
| Patio | 4 in |
| Shed or hot tub pad | 4–6 in |
| Residential driveway | 5–6 in |
| Garage floor | 4–6 in |
These are common starting points, not engineering advice. Soil conditions, reinforcement and local code all affect what your slab should be.
Tips before you pour
- Measure the inside of your forms, not the outside, and measure thickness in a few spots. A slab that averages half an inch deeper than planned uses 12% more concrete at 4 inches thick.
- Compact your gravel base well. Soft spots fill with concrete you didn't plan for.
- For more than a cubic yard, compare the cost of bags against a ready-mix delivery. Ask about short-load fees on small orders.
- Have every bag on site before you start. Mixing stops while someone drives to the store are how cold joints happen.
Frequently asked questions
How many 80 lb bags of concrete make a cubic yard?
An 80 lb bag yields about 0.6 cubic feet of concrete, so a full cubic yard (27 cubic feet) takes about 45 bags. With 60 lb bags it's about 60 bags, and with 40 lb bags about 90.
How thick should a concrete slab be?
For patios, walkways and shed floors, 4 inches is the usual thickness. Driveways that carry cars are commonly poured 5 to 6 inches thick. Check your local building code for anything structural.
Should I mix bags or order ready-mix concrete?
Bags make sense for small jobs like setting posts or a small pad. Once you need more than about a cubic yard, ready-mix delivered by truck is usually cheaper per yard and far less physical work. Many suppliers have a minimum order, so call ahead.
How much extra concrete should I order?
Add 5 to 10 percent for spillage, uneven ground and forms that bow slightly. On uneven or dug-out ground, lean toward 10 percent. Running short in the middle of a pour causes weak joints, so a little extra is cheap insurance.
How do I calculate concrete for a circular slab or column?
Use the column option above. It uses the area of a circle (pi times the radius squared) multiplied by the height. For a round pad, enter the pad's thickness as the height.