Home & Construction⏱ 4 min read
How to Calculate Concrete Volume for a Slab, Path or Driveway
Getting the volume right means ordering the exact amount of ready-mix or buying the right number of bags. Here is the calculation for rectangular, L-shaped, and circular slabs.
Ordering too little concrete means a cold joint — a visible seam where fresh and hardened concrete meet that is a structural and cosmetic weakness. Too much wastes money. The calculation takes two minutes.
Rectangular Slab
Volume (m3) = Length (m) x Width (m) x Depth (m)
Garden path: 10m x 0.9m x 0.1m (100mm thick)
Volume = 10 x 0.9 x 0.1 = 0.9 m3
Add 10% for waste and slight over-excavation:
0.9 x 1.10 = 0.99 m3 -- order 1.0 m3 ready-mix
Driveway: 6m x 4m x 0.15m (150mm for vehicle use)
Volume = 6 x 4 x 0.15 = 3.6 m3 + 10% = 3.96 m3 -- order 4.0 m3
L-Shaped Slab
Split into two rectangles:
Shape: 8m x 4m total, but with a 3m x 2m notch cut out of one corner.
Rectangle 1: 8m x 2m = 16 m2
Rectangle 2: 5m x 2m = 10 m2
(8m total length minus 3m notch = 5m; 4m width minus 2m notch = 2m)
Total area: 26 m2
At 100mm depth: 26 x 0.1 = 2.6 m3 + 10% = 2.86 m3 -- order 3.0 m3
Circular Slab
Volume = π x radius^2 x depth
Area = π x r^2
Circular patio, diameter 3.6m (radius 1.8m), 100mm thick:
Area = 3.14159 x 1.8^2 = 3.14159 x 3.24 = 10.18 m2
Volume = 10.18 x 0.1 = 1.018 m3 + 10% = 1.12 m3 -- order 1.2 m3
Bags vs Ready-Mix Decision
Bagged concrete (e.g. Postcrete, Rapid Set):
25kg bag yields approximately 0.012 m3 of mixed concrete
For 1.0 m3: 1.0 / 0.012 = 83 bags -- impractical
Ballpark rule: use bagged concrete for pours below 0.3 m3.
Above 0.3 m3 (approx 3 bags of ready-mix minimum order): order ready-mix.
Ready-mix minimum order: typically 0.5-1.0 m3 (call local supplier)
Small pour surcharge: often £50-£100 for orders under 3 m3
Wait charge: £1-£3/minute if truck waits longer than 30 minutes to unload