Concrete calculator
How much concrete do you need?
Slab, patio, footing pad or round hole: put in the size and get cubic yards for a ready-mix order, or the bag count if you are mixing it yourself.
Feet and inches in, cubic yards out. Nothing you enter is saved or sent.
Mixing from bags instead?
Bag counts use the usual yields for pre-mixed concrete: about 0.60 cubic feet from an 80 lb bag, 0.45 from 60 lb and 0.30 from 40 lb. Check the yield printed on the bag you buy.
The formula
Concrete is sold by volume. Multiply length by width by thickness, all in feet, to get cubic feet, then divide by 27 to get cubic yards. The step people trip on is thickness: a 4-inch slab is 4 ÷ 12 = 0.33 feet, not 4.
Common slabs at 4 inches thick
| Slab | Cubic yards | With 10% waste | 80 lb bags |
|---|---|---|---|
| 8 × 8 ft | 0.79 | 0.87 | 40 |
| 10 × 10 ft | 1.23 | 1.36 | 62 |
| 12 × 12 ft | 1.78 | 1.96 | 88 |
| 10 × 20 ft | 2.47 | 2.72 | 123 |
| 20 × 20 ft | 4.94 | 5.43 | 245 |
Bag counts in the table include the 10% allowance.
Bags or a truck?
Sixty-two bags for a 10 × 10 patio is more than two tons of mixing by hand. As a rough guide, bags make sense for post holes, small pads and repairs; once you are near a cubic yard, price a ready-mix delivery, keeping in mind that many suppliers charge a short-load fee on small orders.
Why add waste
The ground is never perfectly flat, forms bow a little, and some concrete stays in the wheelbarrow. Ten percent covers a well-prepared base. Go higher on rough or uneven subgrade. Running short mid-pour costs far more than a little left over.
The quantity is sorted. Now price the job.
ScopeBid: Estimate & Invoice is an offline estimate and invoice app for independent service professionals, on iPhone and iPad. Its listing says Profit Guard shows gross margin and markup side by side before a quote is sent, and each quote goes out as a PDF.
Get ScopeBid on the App StoreYour first three quotes are free with full PDFs. ScopeBid Pro is an optional auto-renewing subscription for unlimited quoting and the wider job-costing and invoicing workflow. Check the App Store for current terms.
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