How to use
- Choose a slab for a rectangular pour or a column for a round form.
- Measure slab length and width in feet and thickness in inches. For columns, enter diameter in inches and height in feet.
- Use quantity for identical shapes. Estimate differently sized pours separately.
- Review the extra allowance, then calculate to compare mixed volume and bag counts.
How the estimate is calculated
A slab uses length × width × thickness ÷ 12. A column uses π × (diameter ÷ 24)² × height. These formulas produce cubic feet; multiplying by quantity gives the combined pour. The order volume adds your selected percentage, and dividing by 27 converts cubic feet to cubic yards.
Example: a 10 ft × 10 ft slab, 4 in thick, has 33.3 cu ft of concrete, or 1.23 cubic yards. With 10% extra, the order estimate is 1.36 cubic yards. Dividing the unrounded order volume by the bag yields gives 123 bags of 40 lb, 82 bags of 60 lb, or 62 bags of 80 lb.
The result uses the mixed concrete yield from Quikrete Concrete Mix 1101. It does not divide the finished concrete weight by the weight printed on a dry bag, because mixing water and product yield are separate considerations.
Quikrete 1101 mixed yield by bag size
| Dry bag | Approximate mixed yield | Bags for one cubic yard, rounded up |
|---|---|---|
| 40 lb | 0.30 cu ft | 90 |
| 50 lb | 0.375 cu ft | 72 |
| 60 lb | 0.45 cu ft | 60 |
| 80 lb | 0.60 cu ft | 45 |
| 90 lb | 0.675 cu ft | 40 |
Source: Quikrete Concrete Mix 1101 data sheet. These are product-specific yields; another concrete mix may produce a different volume. The same sheet gives an approximate mixed density of 140 lb/cu ft, used only for the weight estimate.
Choosing between bags and ready-mix
One cubic yard corresponds to 45 bags of this 80 lb mix before any extra. That comparison describes handling and mixing volume, not a price break-even. For a larger pour, discuss delivery minimums, truck access, discharge time, and the specified mix with a ready-mix supplier. For bagged work, check that your crew and equipment can keep up with the pour.
Common mistakes
- Do not enter a column diameter in feet; the diameter field uses inches in US mode.
- Avoid adding the same allowance twice when your supplier has already included extra. The 10% default is adjustable, not a prescribed standard.
- A volume estimate does not select reinforcement, footing dimensions, or concrete strength. Use the construction plans and engineering requirements for your pour.
Sources
Yield and approximate mixed weight: Quikrete Concrete Mix 1101 data sheet, for the specific mix used in this estimate. Unit arithmetic uses 27 cubic feet per cubic yard.
Related calculators
- Concrete Slab Calculator — plan joints and a stone subbase
- Gravel Calculator — estimate aggregate volume
Frequently asked questions
How many 80 lb bags make a cubic yard?
For Quikrete Concrete Mix 1101, 45 bags at 0.60 cubic foot each make 27 cubic feet. Add your chosen allowance before rounding the purchase count.
Does this calculator include several columns?
Yes. Enter one round column and change quantity to the number of identical columns. Use separate calculations when diameters or heights differ.
Is the 10% extra an official concrete requirement?
No. It is a planning allowance for the estimate. Adjust it to suit the forms, excavation, and supplier’s ordering advice.
Can I use these bag counts for any concrete product?
Only when the label gives the same mixed yield. Fast-setting concrete, mortar, and specialty mixes can have different yields.