Work out the Type 1, cement, sharp sand, slabs and skip for a patio laid on a semi-dry mortar bed. Change any value and the order list updates straight away.
100mm recommended for patios. 75mm only on firm, well-drained ground
Semi-dry mortar bed, cement : sharp sand
Area × Type 1 depth gives the compacted volume. We multiply by 2,100 kg/m³, the typical density of compacted Type 1, add your waste allowance, then round up to whole bulk bags of about 850 kg.
Area × bed depth gives the volume of mortar. Sand and cement lose about a quarter of their volume once mixed with water, so we multiply by 1.3 before splitting by your mix ratio (1:4 is 1 part cement to 4 parts sharp sand). Cement is taken as 1,440 kg/m³ and sharp sand as 1,600 kg/m³.
Each slab covers its own size plus a 10mm joint. The area divided by that gives the count. We then add the laying pattern's cutting waste and your general waste, and round up.
You dig the full depth of every layer: Type 1, bed and slab. Loosened soil bulks up (clay by about 30%), and the skip is chosen by both volume and weight, because soil is heavy and most firms won't take it in anything bigger than an 8-yard skip.
These are estimates for ordering materials, not a structural design. Check site conditions, and anything unusual, with a qualified builder or engineer.
100mm of compacted Type 1 is the usual recommendation for a patio. 75mm can be enough on firm, well-drained ground, while soft or clay ground may need 150mm. Compact it in layers no thicker than about 100mm.
For a 50mm semi-dry bed at 1:4, allow roughly 0.8 of a 25 kg bag per m² including 10% waste. A 20 m² patio needs about 17 bags.
Most UK patios are laid on a semi-dry mix: damp enough to hold its shape when squeezed, but not sloppy. It gives a firm, full bed. Porcelain slabs also need a slurry primer painted on the back so they bond to the bed.
10mm is standard for natural stone and concrete slabs, and the calculator allows for that. Porcelain is often laid with narrower 3–5mm joints, which means slightly fewer slabs.
10% covers breakages, offcuts and small errors on a simple rectangle. Allow 15% or more for curves, lots of cuts or mixed-size patterns.
It isn't always essential, but a geotextile membrane under the Type 1 is cheap and worth it on clay or soft ground. It stops the stone sinking into the soil and helps keep weeds down.