Concrete Slab Volume and Bag Count
Concrete Slab Volume and Bag Count
Pouring a concrete slab — a patio, a shed foundation, a walkway — starts with a simple volume calculation: length times width times thickness. Getting this right before ordering ready-mix concrete or buying bagged mix avoids the common and costly mistake of running short mid-pour, when a construction joint in an unplanned location can weaken the finished slab.For smaller DIY pours, pre-mixed bags are more practical than ordering a ready-mix truck, but each bag only yields a small, fixed volume once mixed with water — so converting the total slab volume into a bag count is an essential step before a trip to the hardware store.
The slab volume is V = L·W·t, and the number of bags needed is bags = V/V_bag, the total volume divided by the yield of one bag. where L_slab, W_slab, and t_slab are the slab's length, width, and thickness, V_slab is the resulting concrete volume, V_bag is the mixed yield of one bag, and N_bags is the number of bags needed.
Inputs
L_slab := 6 m=6 m
W_slab := 4 m=4 m
t_slab := 0.1 m=100 mm
Multiply length, width, and thickness together to get the total volume of concrete the slab requires.
V_slab := L_slab·W_slab·t_slab=2.4 m^3
Set the known mixed yield of one bag of pre-mixed concrete.
V_bag := 0.017 m^3=0.02 m^3
Divide the total slab volume by the yield of one bag to find how many bags to buy — this should be rounded up to the next whole bag in practice.
N_bags := V_slab/V_bag=141.18
Results
A 6 m by 4 m slab at 10 cm thick needs 2.4 m³ of concrete, which works out to roughly 140 bags of pre-mixed concrete at this yield per bag — enough bags that a ready-mix truck delivery would likely be more economical and far less labor-intensive than hand-mixing that many bags. Always round the bag count up and add a small margin (5-10%) for waste, spillage, and uneven subgrade, since running short partway through a pour is far worse than having a few bags left over.
Assumptions & limitations
- Slab is a simple rectangular prism of uniform thickness.
- No allowance for waste, spillage, or subgrade irregularity (add a margin in practice).
- Bag yield assumes full water-to-mix ratio per manufacturer instructions.
- Result should be rounded up to a whole number of bags.