Shed Base Calculator — Materials for Concrete, Slabs or Gravel

This calculator works out the materials needed for your shed base. It adds an overhang around the shed (default 75mm each side) to give the base proper weather protection, then calculates materials based on your chosen base type.

Base size formula

Base Length = Shed Length + (2 × Overhang)
Base Width = Shed Width + (2 × Overhang)
Base Area = Base Length × Base Width

Concrete slab

Volume (m³) = Base Area × Thickness
Each 25kg premix bag yields approximately 0.012 m³. For site-mixed concrete (1:2:3 C25 ratio), we calculate ~380kg cement, ~760kg sand, and ~1140kg aggregate per m³ — these weights include the standard 1.54 dry-volume bulking factor, since roughly 1.54 m³ of dry materials is needed to make 1 m³ of finished concrete.

Paving slabs

Slabs needed = Base Area ÷ Individual Slab Area
Laid on a 30mm sharp sand bed over 100mm compacted Type 1 sub-base.

Gravel pad

Gravel (tonnes) = Base Area × Depth × 1.7 t/m³
We use a typical loose decorative-gravel density of ~1.7 t/m³ (this varies roughly 1.4–1.8 t/m³ by stone type, so check your supplier’s figure). Gravel is contained within a timber frame and laid on weed membrane.

All calculations include an editable wastage allowance (default 10%) and all options include a 100mm Type 1 sub-base where appropriate.