Get precise estimates for slabs, columns, and footings. Avoid overspending on ready-mix or shorting your project with our professional concrete estimator.
Last updated: August 2026
Concrete is one of the more expensive and time-sensitive materials on a job site, so getting the volume right matters in both directions. Order too little and you risk a cold joint — where a second, later batch fails to properly bond to concrete that's already started setting, weakening the pour at that seam. Order too much and you're paying for (and disposing of) material you never needed. The calculator above handles slabs, square or circular columns, and footings, with a spillage buffer built in so you're not doing that math by hand.
Length × width × thickness gives the volume, but the unit mismatch trips people up: length and width are usually in feet while thickness is measured in inches. Divide the thickness by 12 to get everything into feet, then divide the total cubic feet by 27 to convert to cubic yards, which is how ready-mix is priced and delivered.
Cylindrical pours — deck posts, porch supports, anything poured into a Sonotube — use π × radius² × height. Because the radius is squared, a small error in the diameter measurement compounds fast across multiple columns, so it's worth double-checking that number specifically.
Footings follow the same length × width × depth math as a slab, but they typically sit deeper and carry rebar to distribute the structure's load into the soil below. How well the subgrade is compacted underneath has as much to do with long-term performance as the concrete volume itself.
Wet concrete settles into an uneven subgrade and can bulge forms slightly under its own weight, so the theoretical volume from the formula almost always comes up a bit short in practice. A 10% allowance covers that "fluff factor" along with what's left stuck in the mixer or pump line — cheap insurance against a mid-pour scramble for a second batch.
Strength requirements are a separate question from volume, and worth confirming with your supplier before you order: a driveway generally calls for around 4,000 PSI concrete, while a basic sidewalk can get by on 2,500–3,000 PSI.
For smaller pours mixed by hand rather than delivered, here's how the common bag sizes translate to volume:
| Bag Size | Yield | Bags per Cubic Yard |
|---|---|---|
| 80 lb | 0.60 cu ft | ~45 bags |
| 60 lb | 0.45 cu ft | ~60 bags |
| 40 lb | 0.30 cu ft | ~90 bags |
| 25 kg | ~0.44 cu ft | ~61 bags |
Past roughly a cubic yard, ready-mix delivery is usually the more practical (and often cheaper, once labor is counted) option over hand-mixing dozens of bags.
Q: How many bags of concrete make a cubic yard?
A: About 45 bags of 80lb mix or 60 bags of 60lb mix.
Q: What is the standard thickness for a concrete driveway?
A: 4 inches for standard residential use; 5–6 inches with rebar reinforcement for heavier vehicles.
Q: Should I use gravel under my concrete slab?
A: Yes — a 4-inch compactable gravel layer improves drainage and helps prevent frost heave.
Q: How much extra concrete should I order?
A: Around 10% more than the calculated volume, to cover spillage, form deflection, and uneven grade.
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