How the concrete calculator works
This calculator finds the volume of your pour and converts it into the concrete you need to buy:
- Calculate the volume. Slabs and footings use length ร width ร thickness; walls use length ร height ร thickness; columns use the cylinder formula ฯ ร rยฒ ร height.
- Add waste. We add your waste percentage to cover uneven ground and spilled mix.
- Convert to bags or yards. 27 cubic feet = 1 cubic yard. A 60lb bag yields about 0.45 cu ft; an 80lb bag yields about 0.60 cu ft. We round bag counts up.
Tip: a small 4" slab for a shed or patio is a classic first DIY pour. Anything much larger than a cubic yard is usually cheaper and easier to order as ready-mix.
Concrete bag yield reference
| Bag size | Approx. yield | Bags per cubic yard |
|---|---|---|
| 40 lb | 0.30 cu ft | 90 |
| 50 lb | 0.38 cu ft | 71 |
| 60 lb | 0.45 cu ft | 60 |
| 80 lb | 0.60 cu ft | 45 |
How much does a cubic yard cover?
| Slab thickness | Coverage per cubic yard |
|---|---|
| 4 in | 81 sq ft |
| 5 in | 64.8 sq ft |
| 6 in | 54 sq ft |
| 8 in | 40.5 sq ft |
Pro tips
- Buy bags in the same batch. Pre-mixed bag colors can vary slightly between batches.
- Order ready-mix with a margin. Ask the plant for about 1/2 yard over โ running short mid-pour is the worst outcome.
- Add rebar or wire mesh for anything load-bearing, and rent a power mixer if you're doing more than a few bags.
- Wet the subgrade before pouring so dry ground doesn't suck water out of the concrete.
Frequently asked questions
How many 80lb bags of concrete are in a cubic yard?
About 45. One cubic yard is 27 cubic feet, and an 80lb bag yields roughly 0.60 cubic feet. A 60lb bag yields about 0.45 cu ft, or about 60 bags per yard.
How many square feet does a cubic yard of concrete cover?
At 4 inches thick, one yard covers about 81 sq ft. At 6 inches, 54 sq ft; at 8 inches, about 40.5 sq ft. Thicker slabs use more concrete per square foot.
How thick should a concrete slab be?
Patios, walkways and sheds are usually 4 inches. Driveways and garage floors are typically 5โ6 inches. Load-bearing floors should be 6 inches or more, usually with rebar or wire mesh.
Should I buy bags or order ready-mix by the yard?
Under about 1 cubic yard, bags are usually cheaper and easier to manage. Beyond that, ready-mix delivery is less expensive per yard and much less labor.
How much extra concrete should I order?
Add about 10% for waste and uneven ground. For large ready-mix orders, a 1/2 yard overage is common practice so you never run short.
Does this concrete calculator work in metric (metres, centimetres and kilograms)?
Yes. Use the Imperial / Metric toggle at the top of the calculator to enter dimensions in metres and centimetres, and see volume in cubic metres. Bag sizes switch to 20 / 25 / 30 / 40 kg, and ready-mix is shown in cubic metres. Your choice is saved on your device.