What this calculator works out
This calculator works out the patio area, the number of slabs needed at your slab size including 8% waste, and approximate volumes for the sub-base and bedding sand.
The sub-base is the part most often skimped and the part that decides whether the patio is still level in five years. It is quietly the largest material item in the job.
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Before you order
- Slab dimensions are in millimetres. A common paving slab is 600 by 600 mm; 450 by 450 and 900 by 600 are also widely sold.
- The sub-base figure assumes 100 mm of compacted MOT Type 1, which is a reasonable minimum for a patio.
- The sand figure assumes a 50 mm bedding layer.
- Plan a fall of about 1 in 80 away from the house, and keep the finished surface at least 150 mm below any damp-proof course.
Laying slabs on five dabs of mortar leaves voids underneath. Water collects in them, freezes, and the slab cracks or rocks. A full bed supports the whole slab and is the difference between a patio that lasts and one that needs relaying.
How this calculator works
An area, a slab count and two bulk volumes:
Area = length × width
Slab area = (slab length ÷ 1,000) × (slab width ÷ 1,000)
Slabs = area ÷ slab area × 1.08, rounded up
Sub-base ≈ area × 0.1 m³ · Bedding sand ≈ area × 0.05 m³The 8% waste allowance is fixed. That is adequate for a simple rectangular patio and low for anything with curves, angles or a border course, where 10 to 15% is more realistic.
Worked example: a 4 m by 3 m patio
Using the default figures — 4 m by 3 m in 600 by 600 mm slabs:
- Area: 12 m²
- Slabs including 8% waste: 37
- Sub-base: about 1.2 m³
- Sharp sand: about 0.6 m³
The slab count is the small number here. 1.2 m³ of MOT Type 1 is around two tonnes, and 0.6 m³ of sand is another tonne — three tonnes of material to move, spread and compact before a single slab is laid. Anyone budgeting a weekend for a patio this size on the basis of 37 slabs has budgeted for the wrong part of the job.
Common mistakes
- Skimping the sub-base. It is the single largest cause of patios settling and rocking.
- Laying level with no fall. Water must run away from the house.
- Bringing the patio above the damp-proof course. Keep it at least 150 mm below.
- Laying slabs on mortar dabs. Use a full bed.
- Cutting slabs dry. Cutting paving releases silica dust; use water suppression and a mask.
Frequently asked questions
How deep should the sub-base be?
100 mm of compacted MOT Type 1 is a reasonable minimum for a foot-traffic patio on stable ground. Soft or clay ground, or anything taking vehicle loads, needs 150 mm or more. Compaction matters as much as depth — it should be laid and whacked in layers of no more than 50 to 75 mm rather than all at once.
Do I need planning permission for a patio?
Usually not for a patio at ground level in a garden. Permeable surfacing rules apply to front gardens over 5 m² — if the surface is not permeable and does not drain to a soakaway or lawn, planning permission may be required. Listed buildings and conservation areas have additional restrictions.
How do I get the fall right?
Aim for roughly 1 in 80, which is about 12 mm of drop per metre, running away from the building. Set string lines to the finished levels before laying anything and check every slab against them. It is far easier to build the fall in than to discover afterwards that water pools against the wall.
What should I point the joints with?
Options are a dry mortar mix brushed in, a wet mortar pointed by hand, or a resin-based jointing compound. Resin compounds are more expensive, considerably faster and more tolerant of damp conditions. Whichever you use, joints need to be a consistent width, which means using spacers rather than eye.
Is what I enter stored?
No. Measurements are processed in your browser and never transmitted or retained.
Related tools
References
- GOV.UK — permeable paving rules for front gardens and planning guidance
- Health and Safety Executive — controlling silica dust when cutting paving, and manual handling
Sources are checked at publication and can change — how I choose and check references.