How the concrete calculator works
Concrete is ordered in cubic yards, so the calculator works out the volume of your forms and divides by 27:
cubic yards = length (ft) × width (ft) × thickness (ft) ÷ 27 × (1 + waste %)
Then it adds:
- Bags: cubic feet ÷ 0.60 for 80 lb bags and ÷ 0.45 for 60 lb bags (typical manufacturer yields), rounded up to whole bags.
- Weight: about 4,050 lb per cubic yard (150 lb per cubic foot), shown in US tons.
- Cost: cubic yards × the price per yard you enter from your quote.
Use Circle for round columns and piers, Triangle for odd corners, How many for repeated pieces, and Add another section for anything made of several parts, such as a slab plus a thickened edge or a patio plus a step.
Worked example
A 12 × 16 ft patio, 4 in thick, with 10 percent extra:
- 4 in ÷ 12 = 0.333 ft
- 12 × 16 × 0.333 = 64 cubic feet
- 64 × 1.10 = 70.4 cubic feet
- 70.4 ÷ 27 = 2.61 cubic yards
- Bags: 70.4 ÷ 0.60 = 117.3, so 118 bags of 80 lb; or 70.4 ÷ 0.45 = 156.4, so 157 bags of 60 lb
At that size, most people would order about 2¾ cubic yards of ready-mix rather than mix 118 bags by hand.
Typical thickness by project
| Project | Common thickness | Cubic yards per 100 sq ft |
|---|---|---|
| Walkway or sidewalk | 4 in | 1.23 |
| Patio | 4 in | 1.23 |
| Shed or hot tub pad | 4 to 6 in | 1.23 to 1.85 |
| Driveway | 4 to 6 in | 1.23 to 1.85 |
| Garage floor | 4 to 6 in | 1.23 to 1.85 |
| Footings, foundations | Set by local code | See the footing calculator |
These are common practice, not rules. Heavy vehicles, poor soil and freezing climates call for more; anything structural should follow your local building code or an engineer’s drawing.
Cubic yards for common slab sizes
| Slab size | 4 in thick | 5 in thick | 6 in thick |
|---|---|---|---|
| 10 × 10 ft | 1.23 | 1.54 | 1.85 |
| 12 × 12 ft | 1.78 | 2.22 | 2.67 |
| 10 × 20 ft | 2.47 | 3.09 | 3.70 |
| 16 × 16 ft | 3.16 | 3.95 | 4.74 |
| 20 × 20 ft | 4.94 | 6.17 | 7.41 |
| 24 × 24 ft | 7.11 | 8.89 | 10.67 |
Exact volumes before waste. The concrete slab calculator has more sizes and bag counts for each.
Bags or a truck?
| Bagged mix | Ready-mix delivery | |
|---|---|---|
| Sold by | the bag (40, 60 or 80 lb) | the cubic yard |
| One cubic yard is | about 45 × 80 lb or 60 × 60 lb bags | 1 yard |
| Good for | post holes, small pads, steps, repairs | slabs, driveways, footings, anything over about a yard |
| Watch for | mixing time, consistency between batches | minimum orders and short-load fees, truck access |
Remember the weight: 45 bags of 80 lb is 3,600 lb of dry mix to lift, mix and place for a single cubic yard. The concrete bag calculator covers bag yields, and concrete cost per yard explains how to compare the two on price.
Before you order
- Measure inside the forms, at several points, and use the actual thickness after the base is compacted, not the nominal one.
- Account for extras: thickened edges, footings under a slab, and steps all add volume. Enter them as extra sections.
- Round to the plant’s increment. Many ready-mix plants sell in quarter or half cubic yards and have a minimum load. Ask when you call.
- Running short is the expensive mistake. A second truck brings its own delivery charges, and a cold joint in the middle of a slab is a weak spot.
Bag yields vary slightly between products and mix types. Check the yield printed on the bag (in cubic feet) and, for ready-mix, confirm the quantity with the plant before the pour.
Questions people ask
- How do I figure concrete yardage?
- Multiply length × width × thickness in feet to get cubic feet, then divide by 27. A 10 × 10 ft slab 4 in thick is 10 × 10 × 0.333 = 33.3 cubic feet, or 1.23 cubic yards. Add 5 to 10 percent before ordering.
- How many bags of concrete make a yard?
- About 45 bags of 80 lb (0.60 cu ft each) or 60 bags of 60 lb (0.45 cu ft each). Check the yield printed on the bag you buy.
- How much extra concrete should I order?
- 5 to 10 percent is a common allowance for uneven subgrade, forms that bow, spillage and what stays in the truck or mixer. The calculator adds 10 percent by default; change it in the waste box.
- How thick should a concrete slab be?
- 4 inches is common for patios, walkways and shed floors. Driveways and garage floors are often 4 to 6 inches. Footings and anything structural follow local building code.
- Should I use bags or order ready-mix?
- Bags make sense for small jobs like post holes, steps and small pads. For larger pours, a ready-mix truck saves a lot of mixing and gives a consistent pour. Compare bag count × bag price against a delivered quote.
- How much does a yard of concrete weigh?
- About 3,900 to 4,050 lb, roughly 2 tons, for normal-weight concrete.