The chart uses one narrow slab model
Every row below models a flat rectangular slab exactly 4 inches thick. Four inches equals one-third of a foot (4/12 ft), so cubic feet equal length times width divided by 3. Divide that volume by the printed bag yield and round up to whole bags.
This chart uses standard QUIKRETE Concrete Mix approximate yields: 0.45 cu ft for a 60 lb bag and 0.60 cu ft for an 80 lb bag. Always check the printed yield on the exact bag you buy because lightweight or fast-setting mixes differ.
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.
Concrete bag chart for a 4-inch slab
| Slab size | Exact volume | 60 lb bags | 80 lb bags | Allowance included? |
|---|---|---|---|---|
| 4 × 4 ft | 5.33 cu ft | 12 | 9 | No |
| 6 × 6 ft | 12.00 cu ft | 27 | 20 | No |
| 8 × 8 ft | 21.33 cu ft | 48 | 36 | No |
| 10 × 10 ft | 33.33 cu ft | 75 | 56 | No |
| 12 × 12 ft | 48.00 cu ft | 107 | 80 | No |
| 12 × 20 ft | 80.00 cu ft | 178 | 134 | No |
| 20 × 20 ft | 133.33 cu ft | 297 | 223 | No |
Worked example: 10 ft by 10 ft by 4 in
Start with volume. Multiply 10 × 10 × 4/12 to get 33.33 cu ft. Divide 33.33 by 0.60 cu ft per 80 lb bag and you get 55.56, which rounds up to 56 whole 80 lb bags before waste; if you switch to 60 lb bags at 0.45 cu ft per bag, the same slab takes 75 bags.
Mixing 56 or 75 bags by hand is a workout. Compare total pallet weight, mixer batch capacity, water control, crew size, and ready-mix minimum load fees before you buy.
Measure before adding a percentage
- 01
Check inside the forms
Use the actual concrete dimensions, not an unchecked drawing or outside form dimensions.
- 02
Probe depth in several places
An uneven subbase changes average thickness and volume.
- 03
Separate thickened areas
Calculate edges, footings and grade beams as additional shapes.
- 04
Confirm exact product yield
Bag weight alone does not guarantee finished volume.
- 05
Add allowance visibly
Record exact volume, chosen allowance and final whole-bag order as separate values.
When a bag chart stops being practical
- The slab is large enough that mixing and placement continuity become difficult.
- Access, working time or crew capacity cannot support the calculated number of batches.
- The base varies or the design includes thickened structural sections.
- The product yield differs from the chart assumptions.
- Project specifications require a particular ready-mix design, testing or delivery record.
- The work is structural, permitted or otherwise requires professional design and inspection.