Concrete volume and bag count for slabs, pads and footings
Calculate net concrete volume in cubic meters, add a waste allowance and convert to bags with worked examples for a pad, a walkway slab and strip footings.
Concrete volume for a rectangular element is count times length times width times thickness, in meters, giving cubic meters. Add a waste allowance, then divide by the yield of one bag if you are mixing from bags rather than ordering a truck.
How do you calculate slab volume?
For a fictional equipment pad, 2 pads each 5 m by 4 m and 0.15 m thick:
=B11*C11*D11*E11 gives 2*5*4*0.15 = 6.0 cubic meters.
With a 10 percent allowance, the order volume is 6.0*(1+0.10) = 6.6 cubic meters.
How many bags is that?
If one pre-mixed bag yields 0.02 cubic meters, =ROUNDUP(I11/G11,0) gives 6.6/0.02 = 330 bags. Check the yield printed on the bag, because it varies by product and bag size.
| Element | Net m³ | Waste | Order m³ | Bags |
|---|---|---|---|---|
| Equipment pad (2) | 6.00 | 10% | 6.60 | 330 |
| Walkway slab | 1.80 | 8% | 1.94 | 98 |
| Footing strip (4) | 4.32 | 10% | 4.75 | 238 |
Walkway: 1*12*1.5*0.1 = 1.8, times 1.08 is 1.944, divided by 0.02 is 97.2, rounded up to 98. Footing: 4*6*0.6*0.3 = 4.32, times 1.1 is 4.752, divided by 0.02 is 237.6, rounded up to 238.
What does the volume leave out?
- Rebar displacement is small and normally ignored, while form deflection and uneven subgrade can add real volume.
- Ready-mix suppliers set minimum loads and often sell by whole or half cubic meters.
- Cost needs a price per cubic meter or per bag, plus forming, reinforcement, finishing and labor.
The concrete volume estimator calculates each row. For other finishes, the material quantity takeoff uses the same count, length, width and waste pattern, and the construction cost estimate shows where concrete sits in a MasterFormat breakdown.