MaterialScopeHow many bags of concrete do I need?
A repeatable method for converting slab volume into a whole-bag shopping quantity.
Start with the volume
For a rectangular slab, multiply length and width in feet by thickness in feet. Convert inches to feet by dividing by 12. If you already have cubic yards, multiply by 27 to get cubic feet.
Example: a small 4 × 4-foot pad
At 4 inches thick, the pad has 5.3333 cubic feet of net volume. With a 10% allowance it needs 5.8667 cubic feet. At 0.60 cubic feet per bag, 9.7778 rounds up to 10 bags. Keep the intermediate values unrounded.
Quick reference: square pads at 4 inches
The table applies the same method to common small pads. Each count includes a 10% allowance and rounds up once.
| Pad size | Net volume | 60-lb bags (0.45 ft³) | 80-lb bags (0.60 ft³) |
|---|---|---|---|
| 4 × 4 ft | 5.33 ft³ | 14 | 10 |
| 6 × 6 ft | 12.00 ft³ | 30 | 22 |
| 8 × 8 ft | 21.33 ft³ | 53 | 40 |
| 10 × 10 ft | 33.33 ft³ | 82 | 62 |
Example: a round column
For a 12-inch-diameter column 4 feet tall, volume is π × 0.5² × 4 = 3.1416 cubic feet. With 10% extra, that is 3.4558 cubic feet, or 5.76 bags at 0.60 ft³, rounded up to 6. The concrete calculator handles round columns directly: choose the round shape, enter the diameter in feet and the height in inches.
Do not count the allowance twice
If a supplier or another calculator already gave you a volume with extra material, set the bag calculator’s allowance to zero. For several small sections using the same mix, add the net volumes before calculating whole bags.
Before putting the bags in your cart
Confirm the mix is appropriate for the job, then check yield, storage conditions and the package directions. Consider how the bags will be transported and moved. This quantity estimate does not select the mix strength or project design.
Compare 60-lb and 80-lb bag options when choosing between products.
Product reference: QUIKRETE concrete mix technical data. Confirm the current label for your chosen product.
Change the measurements and allowance to match your plan.
Open Concrete Bag CalculatorMaterialScope editorial · Updated September 29, 2026 · Calculation methodology