Calculate concrete volume for a rectangular slab and the approximate number of bags. A 10 x 10-foot slab at 4 inches is 1.2346 cubic yards before allowance. With the example 10% allowance, it needs 1.3580 cubic yards or 62 of the selected 80 lb bags. Choose the slab thickness from your project design, not from this estimate.
How the concrete calculator works
Net cubic feet = length (feet) x width (feet) x thickness (inches) / 12.
Net cubic yards = cubic feet / 27.
Volume with allowance = net volume x (1 + allowance / 100).
Bags = volume with allowance / yield per bag, rounded up. One cubic yard is 3 x 3 x 3 = 27 cubic feet. The form calculates the unrounded volume before choosing a whole-bag count.
Measure the inside dimensions of the area receiving concrete. Use the concrete thickness only, not the combined concrete-and-gravel depth. The calculator assumes a flat rectangular slab with uniform thickness. It does not select a safe thickness, reinforcement, load capacity or local-code design.
Slab volume chart: 4, 6 and 8 inches
Our calculated table shows net cubic yards before any allowance. The thicknesses are examples, not recommendations. Add your chosen allowance after selecting the designed thickness.
| Slab size | 4 inches | 6 inches | 8 inches |
|---|---|---|---|
| 4 x 4 feet | 0.1975 yd³ | 0.2963 yd³ | 0.3951 yd³ |
| 5 x 10 feet | 0.6173 yd³ | 0.9259 yd³ | 1.2346 yd³ |
| 10 x 10 feet | 1.2346 yd³ | 1.8519 yd³ | 2.4691 yd³ |
| 10 x 20 feet | 2.4691 yd³ | 3.7037 yd³ | 4.9383 yd³ |
| 12 x 12 feet | 1.7778 yd³ | 2.6667 yd³ | 3.5556 yd³ |
| 20 x 20 feet | 4.9383 yd³ | 7.4074 yd³ | 9.8765 yd³ |
| 20 x 30 feet | 7.4074 yd³ | 11.1111 yd³ | 14.8148 yd³ |
Bag yields used by this calculator
The selected bag yields come from the QUIKRETE Concrete Mix 1101 technical sheet: 80 lb = approximately 0.60 cubic foot, 60 lb = 0.45 and 40 lb = 0.30.1 These are product-specific approximate mixed yields, not the volume of the dry bag. QUIKRETE’s own calculator rounds up to a whole bag and warns that its approximate yields do not include uneven substrate or waste.2
| Selected bag | Yield used | Bags for 1 yd³, no allowance |
|---|---|---|
| 80 lb | 0.60 cubic foot | 45 |
| 60 lb | 0.45 cubic foot | 60 |
| 40 lb | 0.30 cubic foot | 90 |
A different product can have a different yield even at the same bag weight. If your bag’s printed yield differs, use the calculator’s cubic-foot result and divide it by your product’s stated yield, then round up. Do not substitute mortar, cement alone or an unrelated mix for concrete without checking the project specification.
Three checked examples
1. A 10 x 10 slab at 4 inches
Net volume: 10 x 10 x (4 / 12) = 33.333333 cubic feet, or 1.234568 cubic yards. With an example 10% allowance: 33.333333 x 1.10 = 36.666667 cubic feet, or 1.358025 cubic yards. At 0.60 cubic foot per 80 lb bag: 36.666667 / 0.60 = 61.111111, rounded up to 62 bags. The form displays 1.3580 cubic yards; that display is not an order increment.
2. A 20 x 20 slab at 6 inches
Net volume: 20 x 20 x (6 / 12) = 200 cubic feet, or 7.407407 cubic yards. An example 5% allowance gives 210 cubic feet, or 7.777778 cubic yards. Dividing by 0.60 gives exactly 350 of the 80 lb bags. This example shows the mixing workload at a larger scale; compare delivery and placement options rather than choosing by bag price alone.
3. A small 4 x 4 pad at 3 inches
This is a volume example only, not a thickness recommendation. Net volume: 4 x 4 x (3 / 12) = 4 cubic feet. With allowance set to zero and a 60 lb bag selected: 4 / 0.45 = 8.888889, rounded up to 9 bags. Their stated combined yield is 9 x 0.45 = 4.05 cubic feet, slightly above the net geometry because bags are whole units.
Allowance, low spots and thickened edges
Our 10% default is an editable planning example, not a sourced universal requirement. QUIKRETE says its basic calculator yields do not account for uneven substrate or waste.2 Confirm the actual base level and the needed margin before ordering. Do not assume an arbitrary percentage covers a large depression or a planned thickened edge.
A 10 x 10-foot area averaging half an inch deeper adds 10 x 10 x (0.5 / 12) = 4.166667 cubic feet, before allowance. Add separate geometric volumes for steps, footings and thickened edges. If an edge is already inside the slab rectangle, add only the extra depth below the slab, not the full edge volume again.
Bags versus ready-mix
The cubic-yard and bag results are alternatives for the same planned volume. Do not order both quantities unless you intentionally split the pour. The calculator does not round ready-mix to a supplier’s ordering increment or minimum. Ask the supplier to quote the mix specification, usable delivery quantity, short-load terms, access limits and any waiting or placement charges.
For more detail, use our concrete bag guide and concrete cost-per-yard guide. Circular forms need a different shape formula: see Sonotube concrete volume and calculate each separate shape before adding volumes.
Metric inputs
The unit switch converts length and width to meters and thickness to centimeters. Equivalent inputs return the same physical volume, with metric results shown first. The bag dropdown still names the US product weight and cubic-foot yield. For example, the 10 x 10-foot by 4-inch default converts to 3.048 x 3.048 meters by 10.16 centimeters; with 10% allowance it is about 1.0383 cubic meters.
FAQ
How many bags of concrete are in one cubic yard?
At the selected QUIKRETE 1101 yields and with no allowance, 27 / 0.60 = 45 of the 80 lb bags, 27 / 0.45 = 60 of the 60 lb bags, or 27 / 0.30 = 90 of the 40 lb bags.1 Add a deliberate allowance separately; check the product yield before buying.
Does the result include gravel, rebar or labor?
No. It estimates concrete volume and bags only. Base material, reinforcement, forms, finishing, curing, labor and delivery are separate. The gravel calculator can estimate a separately specified base depth.
Can I leave the allowance blank?
Yes. Blank means zero allowance. Negative values are rejected instead of silently treated as zero. All three slab dimensions must be positive.
Can this decide whether my slab is safe?
No. It cannot check loads, soil, frost, reinforcement, drainage or code compliance. Use the project design and the exact mix instructions. The QUIKRETE 1101 sheet describes a product for placements at least 2 inches thick; that product limit is not a recommendation that a 2-inch slab suits your use.1
Sources
- QUIKRETE Concrete Mix 1101 technical sheet (approximate bag yields, product limits and safety instructions; revised October 2022)
- QUIKRETE Concrete Calculator (whole-bag rounding and uneven-substrate/waste caveat)
What you’ll need
A plain checklist. Needs vary by project, so follow the instructions on your materials.
- Mixing tub or wheelbarrow and hoe – for mixing bagged concrete by hand.
- Forms (boards) and stakes – they shape the slab and hold wet concrete in.
- Compacted gravel base – a firm base reduces cracking and settling.
- Screed board, float and edger – to level and finish the surface.
- Control joint tool or saw – planned joints give cracks a place to form.
- Impervious gloves such as nitrile, eye protection and the specified dust protection – follow the product Safety Data Sheet. QUIKRETE warns that its mix causes severe skin burns and serious eye damage.1
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Turn concrete quantities into a budget
Use our concrete cost guide to compare ready-mix material quotes with installed slab quotes. This calculator gives volume and selected bag counts, not a finished-project price. Match the quote to the same slab dimensions and scope. Check delivery or minimum-load charges, site preparation, forms, reinforcement, finishing and removal separately. Do not add concrete material cost again when an installed quote already includes it.