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Shed bases · Patios · Fence posts · Gravel and aggregates
These are planning examples, not structural specifications or supplier quotes. Check product instructions, site conditions and professional guidance where necessary.
Shed bases
How much concrete do I need for a shed base?
Measure the footprint, confirm the base size with the shed manufacturer and convert the concrete depth from millimetres to metres. Multiply length × width × depth to estimate cubic metres. A 3 × 2 m base at 100 mm is 0.60 m³ before allowance.
Ordering checks
Bagged concrete quantities depend on finished yield. The weight printed on a bag is not its finished volume. Compare bag costs with a full ready-mix delivery quote, including small-load charges.
Important: Avoid assuming a particular depth or base type suits every shed. Ground, loading and the manufacturer’s instructions matter.
Patios
How many paving slabs will I need?
Start with the finished dimensions, slab dimensions and joint width. A rectangular grid can be estimated by counting how many slab-plus-joint units fit across each direction.
Ordering checks
Allow separately for cuts and breakages. A 4 × 3 m patio using 600 mm square slabs with 10 mm joints has a simple 7 × 5 grid, or 35 slabs before allowance.
Important: The slab count does not determine the correct bedding system, drainage fall or foundation depth.
Fence posts
How many bags of post concrete will I need?
For a cylindrical hole, calculate π × radius² × concrete-fill depth, then subtract the space occupied by the post. Repeat for the number of holes.
Ordering checks
For six 300 mm diameter, 600 mm concrete-fill holes containing 100 × 100 mm posts, the theoretical total is around 0.255 m³ before allowance. Use the product’s actual yield to estimate bags.
Important: Hole size and depth depend on ground, post system, fence height and wind exposure; this calculation is not a footing design.
Gravel and aggregates
How many bulk bags of gravel do I need?
Multiply length × width × depth in metres. Add your chosen extra material allowance, then multiply by the supplier’s bulk density to estimate tonnes.
Ordering checks
For 5 × 3 m at 50 mm depth, 10% allowance and 1.6 t/m³, the result is 1.32 tonnes. That rounds to two 0.8-tonne bags. Compare delivered prices with loose aggregate and check minimum orders.
Important: A tonne is not a cubic metre. Bulk density, bag weights and compaction differ by material.