A 10 × 10-foot slab at 4 inches of concrete thickness has 33.333333 cubic feet of net volume. With our example 10% quantity allowance, that is 36.666667 cubic feet: 62 bags at 0.60 cu ft per bag, 82 at 0.45 or 123 at 0.30. Those yields match the cited QUIKRETE Concrete Mix sheet for 80-, 60- and 40-lb bags.1 Check the exact mix packaging before buying.
These charts estimate a plain rectangular slab of uniform thickness. They do not select slab thickness, reinforcement, base, joints, frost protection or load capacity. Ten percent is an example added-volume allowance, not a universal waste rate or a compaction conversion. Thickened edges, beams, steps and other concrete shapes need separate volume.
The bag-count formula
Net volume (cu ft) = length (ft) × width (ft) × thickness (in) / 12.
Volume with allowance = net volume × (1 + allowance percent / 100).
Cubic yards = cubic feet / 27.
Bags = allowance-inclusive cubic feet / actual mixed yield per bag, rounded up. Use full precision until the final bag count. A rounded cubic-yard display is not the input for bag division.
4-inch slab chart
Our calculated table uses 10% example allowance. The net and allowance-inclusive volumes are separate; bag columns use the allowance-inclusive volume. Yield presets are product-specific examples, not all brands.
| Slab | Net cu yd | Cu yd with 10% | 80 lb / 0.60 | 60 lb / 0.45 | 40 lb / 0.30 |
|---|---|---|---|---|---|
| 4 × 4 ft | 0.198 | 0.217 | 10 | 14 | 20 |
| 4 × 8 ft | 0.395 | 0.435 | 20 | 27 | 40 |
| 8 × 8 ft | 0.790 | 0.869 | 40 | 53 | 79 |
| 10 × 10 ft | 1.235 | 1.358 | 62 | 82 | 123 |
| 12 × 12 ft | 1.778 | 1.956 | 88 | 118 | 176 |
| 10 × 20 ft | 2.469 | 2.716 | 123 | 163 | 245 |
| 20 × 20 ft | 4.938 | 5.432 | 245 | 326 | 489 |
How thickness changes bags for a 10 × 10 slab
Calculated with 10% example allowance and the stated yields. Thickness values are quantity comparisons, not suitable-thickness recommendations.
| Concrete thickness | Net cu ft | Cu ft with 10% | 80 lb / 0.60 | 60 lb / 0.45 |
|---|---|---|---|---|
| 3 in | 25.000000 | 27.500000 | 46 | 62 |
| 4 in | 33.333333 | 36.666667 | 62 | 82 |
| 5 in | 41.666667 | 45.833333 | 77 | 102 |
| 6 in | 50.000000 | 55.000000 | 92 | 123 |
| 8 in | 66.666667 | 73.333333 | 123 | 163 |
Mixed yield is not bag weight
| QUIKRETE Concrete Mix example | Approximate mixed yield |
|---|---|
| 40 lb | 0.30 cu ft |
| 50 lb | 0.375 cu ft |
| 60 lb | 0.45 cu ft |
| 80 lb | 0.60 cu ft |
| 90 lb | 0.675 cu ft |
The cited sheet lists these yields and gives product-specific mixing and placement instructions.1 Other mixes can differ. If your bag yields a different volume, divide the unrounded total by that exact yield. Do not assume a fast-setting or repair mix can follow the same slab-placement procedure simply because it comes in a familiar bag weight.
Three checked examples
1. 10 × 10 ft at 4 inches
Net volume = 10 × 10 × 4 / 12 = 33.333333 cu ft. With 10% example allowance: 36.666667 cu ft, or 1.358025 cu yd. At 0.6 cu ft per bag: 61.111111, rounded up to 62 bags. The allowance is a planning input, not a claim about required waste.
2. 12 × 12 ft at 6 inches
Net volume = 12 × 12 × 6 / 12 = 72.000000 cu ft. With 10% example allowance: 79.200000 cu ft, or 2.933333 cu yd. At 0.45 cu ft per bag: 176.000000, rounded up to 176 bags. The allowance is a planning input, not a claim about required waste.
3. 4 × 8 ft at 4 inches
Net volume = 4 × 8 × 4 / 12 = 10.666667 cu ft. With 0% example allowance: 10.666667 cu ft, or 0.395062 cu yd. At 0.375 cu ft per bag: 28.444444, rounded up to 29 bags. Zero allowance supplies no extra material; this is a comparison scenario, not an ordering recommendation.
Concrete thickness and base thickness are different
Only concrete goes into the slab volume. A gravel base beneath the concrete is a separate material and is not included in the thickness input. A 4-inch concrete layer over a 4-inch base is not an 8-inch concrete slab. Likewise, excavation depth alone does not tell you concrete thickness. Get the section, ground preparation, reinforcement and local requirements checked for the intended use.
Add other concrete shapes without double-counting
Thickened edges or integral beams need additional volume beyond the uniform slab. Calculate only the extra region below or outside the slab already counted, or divide the whole shape into non-overlapping volumes. Steps, piers and a separate footing cannot be inferred from the rectangle. Our tube-volume guide covers plain cylinders; the post calculator subtracts listed post displacement and is not a deck-footing design tool.
Bags versus ready-mix: compare the whole job
No universal volume threshold is used here. Get quotes for the actual quantity, minimum delivery, short-load fees, access, discharge method, waiting time and any pump or placement work. Compare those with bag cost, transport, mixing equipment, labor and the planned placement sequence. The cheaper material subtotal is not necessarily the cheaper completed pour.
A bag count does not show that a large slab can be mixed and placed safely within the product’s working time. The QUIKRETE sheet cautions against partial-depth fills that can create cold joints and gives product-specific placement and curing directions.1 Plan the pour with the installer rather than staging it from arithmetic alone. This chart does not approve traffic or loading time.
Metric formula
Cubic meters = length (m) × width (m) × concrete thickness (m). Convert centimeters or millimeters to meters first, apply any separately selected allowance and divide by a yield in cubic meters. The 10 × 10-foot, 4-inch net example equals about 0.943895 cubic meters, or 1.038284 with 10%. Do not divide cubic meters by a cubic-foot bag yield without conversion.
FAQ
How many 80-lb bags for a 10 × 10 slab?
At 4 inches of concrete and 10% example allowance, 62 bags if the exact mix yields 0.60 cu ft per bag. At 6 inches with the same assumptions, 92 bags. The footprint alone is not enough.
Do these charts include 10%?
The two bag-count charts do. The net-volume columns do not. The worked examples label each allowance separately; Example 3 deliberately shows zero.
Can I use these counts for a patio or driveway?
Only as volume arithmetic after a suitable section has been chosen. This page does not specify the thickness, reinforcement, drainage, base or load rating for either use.
Why round bags up but not cubic yards first?
Bag packaging is discrete. First calculate the unrounded coverage requirement, then round the final package count. Rounding volume up early can change the count without a reason.
Related tools
Rectangular concrete calculator | 20 × 20 slab volume guide | Post concrete guide
Source
- QUIKRETE: Concrete Mix sheet (approximate mixed yields and product-specific placement/curing instructions)