Free construction calculator
Concrete calculator
Slabs, footings, walls and round columns: the cubic yards to order, with your extra for waste, and how many 60 or 80 lb bags it would take.
Result
| Concrete to order | 8.25 yd³With the extra, rounded up to the next ¼ yard |
|---|---|
| Exact volume | 7.41 yd³200 ft³ |
| With the extra | 8.15 yd³ |
| 80 lb bags | 3670.6 ft³ each |
| 60 lb bags | 4890.45 ft³ each |
- Bag yields are the usual numbers printed on premixed bags; check yours. Past a yard or so, ready-mix is usually the better buy.
- Forms that bow, a sub-base that isn’t flat, and thickened edges all take more concrete than the drawing.
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How it works
Volume in cubic feet is length × width × thickness, with every number in feet: ft³ = L × W × (thickness in inches ÷ 12). A round column is π × (diameter ÷ 2)² × height.
There are 27 cubic feet in a cubic yard: yd³ = ft³ ÷ 27. Then add your extra for waste and spillage (5 to 10% is usual) and round up to the next quarter yard, which is how ready-mix is ordered.
Bags: an 80 lb bag of premixed concrete makes about 0.60 ft³, a 60 lb bag about 0.45 ft³ and a 40 lb bag about 0.30 ft³. Check the bag you buy.
Worked example
A 20 × 30 ft slab, 4" thick. 20 × 30 × 0.333 = 200 ft³. 200 ÷ 27 = 7.41 yd³.
With 10% extra that is 8.15 yd³, so you order 8¼ yards.
In bags it would be 367 bags of 80 lb, which is why a slab that size is a ready-mix pour.
Questions people ask
How many yards of concrete for a 20x30 slab 4 inches thick?
7.41 cubic yards exactly; about 8¼ yards to order with 10% extra.
How many 80 lb bags make a yard of concrete?
About 45 (27 ft³ ÷ 0.60 ft³ per bag). With 60 lb bags it is about 60.
How much extra concrete should I order?
5 to 10% for most pours. Go higher when the sub-base is uneven, the forms are long, or the edges are thickened.
How thick should a slab be?
4" is common for patios, walks and garage floors on good ground; 5 to 6" for driveways that take heavier trucks. Follow your drawings and local code.
How do I figure a footing?
Choose "Footing or wall" and enter its total length, its width and its depth. For a 100 ft run of 20" × 10" footing: 100 × 1.667 × 0.833 = 138.9 ft³, or 5.14 yd³.
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Built by Northdesk
These calculators are made by Northdesk, construction business software with Dex, an AI assistant, built in.
- Concrete contractorsPours that move with the weather, footings measured off the plans, finishers on the right site at 6 AM, and an inspector who signs in before you pour.
- Masonry contractorsBlock, brick, stone and mortar on the right site, the saw and the mixer where they should be, and a crew that speaks two languages on the same page.
- AI estimating and takeoffDex reads your plans for quantities with the AI takeoff add-on, then drafts the estimate from your own prices, sections and wording.
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