Home & Construction⏱ 5 min read
How Much Concrete Do You Need? The Formula for Any Job
Ordering too little means a second delivery and a visible join. Ordering too much wastes money. Here's the precise formula for slabs, fence posts, footings, and steps — plus ready-mix bag calculations.
Concrete is sold by the cubic metre for ready-mix and by the bag for DIY projects. Getting the volume right before you order saves both money and the frustration of running short mid-pour.
The Core Formula
Volume (m³) = Length (m) × Width (m) × Depth (m)
Always convert everything to metres first.
150mm depth = 0.15m | 100mm = 0.10m
Common Shapes and Their Formulas
Rectangular slab (patio, path, floor):
Volume = L × W × D
Example: 4m × 3m patio, 100mm deep
= 4 × 3 × 0.10 = 1.2 m³
Circular slab (base for hot tub, shed):
Volume = π × r² × Depth
Example: 2m diameter (r=1m), 100mm deep
= 3.14159 × 1² × 0.10 = 0.314 m³
Cylindrical fence post hole:
Volume = π × r² × Depth
Example: 200mm diameter hole (r=0.10m), 600mm deep
= 3.14159 × 0.01 × 0.6 = 0.019 m³ per hole
For 10 posts: 0.19 m³ total
Standard Depths for Common Jobs
ApplicationRecommended DepthNotes
Pedestrian patio / path100mm (4 in)Add 50mm hardcore below
Driveway (cars)150mm (6 in)C30 mix minimum
Shed / summer house base100mmOversize base by 50mm each side
Fence post footing1/3 post height undergroundMin 450mm for 1.8m fence
House extension footingsVaries by soil — engineer requiredTypically 600mm–1m
Ready-Mix Bags vs Bulk Delivery
Pre-mixed bags (typically 20–25kg) are convenient for small jobs. At roughly 0.01 m³ per 25kg bag:
Bags needed = Volume (m³) ÷ 0.01
= Volume × 100
Example: 0.3 m³ job
Bags = 0.3 × 100 = 30 bags of 25kg
Above approximately 0.5 m³, bulk ready-mix delivery becomes cheaper per m³ and far less labour-intensive. A standard ready-mix truck carries 6 m³ minimum — smaller "volumetric" mixers can deliver 0.5 m³ and up, mixing to order on site.
Always Add 10% Wastage
Uneven sub-bases, spillage, and the imprecision of hand-mixing mean you should always add 10% to your calculated volume before ordering.
Order quantity = Calculated volume × 1.10
For the 1.2 m³ patio: order 1.32 m³ (round to 1.4 m³)
Mix Ratios for DIY Mixing
ApplicationCement : Sand : AggregateStrength
General purpose (paths, bases)1 : 2 : 4C20
Driveways, structural1 : 1.5 : 3C30
Fence post footings1 : 2 : 3C25