How to use this concrete calculator
- Measure the length and width of the area to pour.
- Enter the slab thickness. 4 inches is standard for patios and walkways; driveways are usually 4–6 inches.
- Keep a 5–10% waste allowance for uneven ground and spillage.
- Use cubic yards to order ready-mix, or the bag count for small jobs.
Example
A 12 × 12 ft patio poured 4 inches thick holds 48 cubic feet of concrete. With the default 10% waste allowance that becomes 1.96 cu yd (1.5 m³). If you mix it yourself you would need 89 bags of 80 lb or 118 bags of 60 lb, so for a slab this size a ready-mix delivery is usually faster and cheaper.
What to know before you buy
Bags or ready-mix?
Bagged concrete is ideal for post holes, steps, small pads and repairs up to about one cubic yard. Beyond that, mixing 45 or more heavy bags by hand is slow and it is hard to keep a consistent mix. Ready-mix trucks deliver a uniform product; many suppliers have a minimum order and charge a short-load fee for small quantities, so ask when you call.
Choosing the slab thickness
Four inches is the standard for patios, walkways and shed floors. Driveways for cars are typically 4–5 inches, and 6 inches for heavier trucks or RVs. Thickened edges (8–12 inches) under walls or along driveway edges add strength where loads concentrate.
Concrete for common slab sizes (including 10% waste)
| Slab | Concrete needed | 80 lb bags | 60 lb bags |
|---|---|---|---|
| 10 × 10 ft, 4 in | 1.36 cu yd | 62 | 82 |
| 12 × 12 ft, 4 in | 1.96 cu yd | 89 | 118 |
| 10 × 20 ft, 4 in | 2.72 cu yd | 123 | 163 |
| 16 × 20 ft, 4 in | 4.35 cu yd | 196 | 261 |
| 20 × 20 ft, 4 in | 5.43 cu yd | 245 | 326 |
| 24 × 24 ft, 4 in | 7.82 cu yd | 353 | 470 |
| 20 × 20 ft, 6 in | 8.15 cu yd | 367 | 489 |
| 12 × 40 ft driveway, 5 in | 8.15 cu yd | 367 | 489 |
Formula
Volume = length × width × thickness. One cubic yard is 27 cubic feet. Typical yields: an 80 lb bag makes about 0.60 cu ft, a 60 lb bag about 0.45 cu ft and a 40 lb bag about 0.30 cu ft.
Tips from the pros
- Measure the inside of your forms, not the rough excavation, and check the depth in several places.
- Order ready-mix rounded up to the next quarter yard; running short mid-pour leaves a weak cold joint.
- Compact 4 inches of gravel under the slab. A solid base reduces cracking and the amount of concrete you need.
- Cut control joints at intervals of 2–3 times the slab thickness in feet (8–12 ft for a 4 in slab) so cracks form where you want them.
Common mistakes to avoid
- Assuming an 80 lb bag makes 0.8 cubic feet: it yields about 0.60 cu ft of mixed concrete.
- Skipping the waste allowance. Uneven ground easily adds 5–10% to the real volume.
- Adding extra water to make mixing easier, which weakens the finished concrete.
Frequently asked questions
How many 80 lb bags of concrete make a yard?
About 45 bags, since one cubic yard is 27 cubic feet and each 80 lb bag yields roughly 0.60 cubic feet.
When should I order ready-mix instead of bags?
For more than about 1 cubic yard, ready-mix delivered by truck is usually cheaper and faster than mixing bags by hand.
How thick should a concrete slab be?
4 inches for patios, sidewalks and shed floors; 5–6 inches for driveways that carry heavier vehicles.
How much concrete do I need for a 10x10 slab?
At 4 inches thick, a 10 × 10 ft slab needs about 1.23 cubic yards (33.3 cu ft), or about 1.36 yards with 10% waste.
How long does concrete take to cure?
Concrete can usually be walked on after 24–48 hours and driven on after about 7 days; it reaches most of its design strength after 28 days.