How this calculator works
Every shape becomes a volume in cubic feet, and then cubic yards, which is how ready-mix is sold. We add the extra you choose (10% by default). Then we divide by the yield printed on standard concrete-mix bags to get the bag counts.
| 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 |
Bags or a truck?
Most calculators stop at "you need 1.4 yards." The real question is how to buy it. The truck price includes the plant's short-load fee, because nearly every small residential pour falls under the plant's minimum. Our default is $100 under 6 yards. Change it to match your plant's quote.
- Under 1 yard: bags. A truck's fees eat any savings.
- 1–2.5 yards: it depends. Compare the costs. Then remember that 60+ bags means about 5,000 lb of lifting and a rented mixer.
- Over 2.5 yards: a truck. Mixing in batches by hand also leaves cold joints, which are weak spots where one batch has started to set before the next one meets it.
Typical slab thicknesses
| Project | Thickness | Yards per 100 sq ft |
|---|---|---|
| Walkway, patio, shed floor | 4" | 1.23 |
| Hot tub pad, light vehicle use | 5" | 1.54 |
| Driveway, garage floor | 6" | 1.85 |
These yard figures are exact volume. Add your extra on top.
Common questions
How do I calculate how much concrete I need?
Multiply length × width × thickness, all in feet, to get cubic feet. Divide by 27 to get cubic yards. A 10 × 10 ft slab 4" thick is 10 × 10 × 0.333 = 33.3 cu ft, or 1.23 yd³. Then add 5–10% for spillage, a base that isn't perfectly level, and forms that bow.
How many bags of concrete are in a yard?
One cubic yard is 27 cubic feet. That takes 45 × 80 lb bags (0.60 cu ft each), 60 × 60 lb bags (0.45 cu ft), or 90 × 40 lb bags (0.30 cu ft).
When is a ready-mix truck cheaper than bags?
Usually somewhere between 1 and 2 yards, depending on your plant's short-load fee. Below a yard, bags almost always win. Above about 2 yards, a truck is usually cheaper, and it saves you mixing thousands of pounds by hand.
What is a short-load fee?
Ready-mix plants charge extra for orders smaller than a set minimum, often somewhere from 4 to 8 yards. It may be a flat fee (commonly about $60–$150) or a charge for each yard you're short of the minimum. Ask when you call for a quote.
How much extra concrete should I order?
We default to 10%. Use 5% only when the forms are rigid and the base is laser-flat. Use 15% on uneven ground or for your first pour. Running short mid-pour leaves a cold joint and a second delivery charge, and that costs far more than a little leftover.
How thick should a concrete slab be?
Use 4" for patios, walks and shed floors, and 5–6" for driveways and garage floors that carry vehicles. Thicker slabs, or slabs on poor soil, need an engineer's or local building department's input.
Prices last checked October 2026. This calculator estimates material quantities. It is not structural or engineering advice. Follow your local building code for footings, frost depth and reinforcement.