How to measure a patio and count paving slabs

By Measure & Make · Updated 15 October 2026

Measure the finished area

For a rectangular patio, measure the planned length and width in metres. Use the finished paving outline, not the whole garden. For an L-shaped space, sketch it first and split it into rectangles without overlapping them. The current planner calculates one rectangle at a time; its separate slab counts should not be treated as an optimised combined cutting plan.

Use actual slab sizes and specified joints

Enter the length of a slab along the patio length and its width across the patio. The Rotate slabs button swaps these directions. Use the slab’s actual dimensions and the joint recommended for that particular range. Published pack coverage can include joints, so it is not always the same as the sum of the stone face areas.

Understand the cut positions

A slab at an edge may only need a narrow cut, but still occupies a position in the grid. This planner counts one purchased slab for every full or cut position, then adds the extra percentage you choose. It does not assume offcuts can be reused. A professional cutting layout may therefore use fewer slabs, while damage, awkward edges or a different laying pattern may need more.

Worked example: 4 m by 3 m

With 600 × 600 mm slabs and 5 mm joints, the grid has seven columns and five rows: 35 slab positions. Adding 10% and rounding up gives 39 slabs. A 100 mm compacted sub-base has a volume of 1.2 m³; a 30 mm mortar bed has a volume of 0.36 m³. Those depths are example inputs, not a specification for every patio.

Get useful supplier quotes

1. Ask the supplier how much loose sub-base will produce the compacted volume you need.
2. Obtain a bedding quote for the full mortar volume or appropriate constituent materials.
3. Price the slabs per slab, then enter the complete sub-base and bedding quotes in the budget.
4. Add delivery and any separately priced primer, jointing material or edging under Other costs.

Falls, drainage, ground preparation and movement joints are outside the calculator. Refer to Marshalls’ paving technical guidance and the instructions for the exact slab range.