Bag count starts with cubic feet
Find the slab volume in cubic feet first. Then divide by the printed bag yield. This calculator uses standard QUIKRETE Concrete Mix yields: 0.30 cu ft for 40 lb, 0.375 cu ft for 50 lb, 0.45 cu ft for 60 lb, and 0.60 cu ft for 80 lb bags.
Lightweight, rapid-setting, or countertop mixes often cover a different volume at the same bag weight, so always read the manufacturer's data sheet before buying.
How do you calculate bags of concrete by bag weight?
Pre-blended concrete bags are sold by dry weight (40 lb, 50 lb, 60 lb, and 80 lb), and each size yields a specific volume of mixed concrete in cubic feet. Divide your slab's cubic footage by the bag yield and round up to the next whole bag:
where:
- Yield_bag – Published approximate yield per bag (0.60 ft³ for 80 lb, 0.45 ft³ for 60 lb, 0.375 ft³ for 50 lb, and 0.30 ft³ for 40 lb); and
- N_bags – Whole bags required before jobsite spillage or uneven subgrade allowance. For ready-mix truck yardage, use the concrete calculator, or for fence posts use the post hole concrete calculator.
A reliable comparison process
- 01
Calculate exact geometric volume
Measure length, width and thickness, converting thickness from inches to feet.
- 02
Select the actual bag product
Match the bag weight and published yield rather than choosing a generic size.
- 03
Round the mathematical count up
A fractional bag still represents concrete the project needs.
- 04
Add project allowance visibly
Keep uneven base, spillage and handling allowance separate from the exact bag calculation.
- 05
Check mixing and placement capacity
A lower bag price is not useful if the crew cannot mix and place the planned volume correctly.
Worked example: 10 ft by 10 ft by 4 in
Four inches equals 0.333 ft, giving 33.3 cu ft of concrete. Divide by 0.60 cu ft per 80 lb bag to get 55.6, which rounds up to 56 bags before waste.
Switching to 60 lb bags (0.45 cu ft yield) takes 75 bags for the exact same pad. Compare total pallet weight, batch count, mixer size, and crew fatigue alongside the per-bag price before you load the truck.
| Bag weight | Yield per bag | Exact count (rounded up) |
|---|---|---|
| 40 lb | 0.30 cu ft | 112 bags |
| 50 lb | 0.375 cu ft | 89 bags |
| 60 lb | 0.45 cu ft | 75 bags |
| 80 lb | 0.60 cu ft | 56 bags |
Do not compare bags by count alone
- Confirm the product yield printed in the current data sheet or bag instructions.
- Compare total volume and total dry weight, not only the number of bags.
- Follow the manufacturer's water and mixing instructions; extra water weakens concrete.
- Plan lifting, storage, weather protection and batch consistency.
- For large or time-sensitive placements, compare ready-mix delivery and access requirements.
- Structural mix selection, reinforcement, curing and slab design remain outside a bag calculator.
Record the product used
Write down the mix brand, bag weight, printed yield, cubic feet, whole-bag count, and waste allowance on your takeoff sheet. That stops a crew from grabbing a different bag size at the supply yard without adjusting the count.
If your trench or subbase depth varies across the form, remeasure average depth before ordering.