Shed Base Slab Calculator

How it is calculated

The slab and the sub-base are both treated as flat boxes with the same footprint. area = length × width in m², with length and width entered in metres. concrete = area × thickness / 1000 and hardcore = area × sub-base thickness / 1000; both thicknesses are entered in millimetres and divided by 1000 to give metres, so the results are in m³. A second line adds 10% waste to the concrete: concrete × 1.10. No allowance is added to the hardcore. Length, width and slab thickness must be above zero; a hardcore thickness of 0 is accepted and gives 0.00. Results are rounded to two decimals, so a small slab can show 0.00. Edge thickening, reinforcement and ground levels are not modelled.

Common mistakes

• Typing the thickness in centimetres or metres. Both thicknesses are in millimetres, so 10 cm must be entered as 100; entering 0.1 gives a slab of 0.1 mm.
• Entering length and width in millimetres or feet. They are in metres, and 3600 instead of 3.6 inflates the volume a thousandfold.
• Using the shed's floor size instead of the base size. If the base is larger than the shed to give an overhang or a path edge, enter the base.
• Ordering exactly the first figure. The extra line already includes 10% waste for the concrete, but the hardcore has no allowance, so round that one up yourself.
• Treating the hardcore volume as the amount to buy. Suppliers often sell by weight and the material compacts when laid, so ask the supplier to convert the volume.