Paving Slab Calculator

Paving Installation Guide

What should I know about Sub-Base Requirements?

A solid, well-draining sub-base is the foundation of any long-lasting paved area. Pedestrian patio: 75-100mm compacted Type 1 hardcore (crushed concrete or MOT stone). Driveway: 100-150mm compacted Type 1 hardcore. Without adequate sub-base, slabs will rock, crack, and sink within a few years. The sub-base must be compacted with a plate compactor (hire from a tool hire shop, approximately £60/day) — using a hand rammer is insufficient for larger areas. For clay or soft soils: increase the sub-base depth beyond the standard recommendation, since these ground types are more prone to movement and settling over time.

What should I know about Laying Methods?

Dry mortar method (patio): lay slabs on 5 spots of firm mortar (one in each corner and one in centre), tap level with a rubber mallet, grout with dry sand or kiln-dried sand pressed into joints. Wet bed method (professional): spread a full mortar bed (4:1 sharp sand:cement, semi-dry), set slabs in one operation. This is better for large format slabs and driveway use. For driveways: use a full wet bed, and ensure the mortar strength is appropriate for vehicle loading. Indian sandstone and natural stone generally need a full wet mortar bed rather than a dry-laid approach, given their weight and the need for a stable, even base.

What do I need to know about Drainage Requirements?

From November 2008, replacing or extending front garden paving with impermeable materials requires planning permission in England unless a drainage system is incorporated. The permitted development right requires either using permeable paving (block paving with open joints, gravel, resin-bound) or directing water runoff onto a lawn or planted area. For rear gardens, there is no equivalent restriction. Newly paved areas should have a minimum fall of 1:80 (approximately 12mm per metre) away from the house, to prevent water pooling against the walls or finding its way toward the damp-proof course.

What's the key thing to understand about Jointing and Sealing?

Joint width: typically 10-15mm for standard paving, 3-5mm for precision-cut porcelain. Narrow joints require more accurate cutting. Jointing materials: kiln-dried sand (brush in, compact, repeat until full — budget option). Polymeric sand (activates with water to form a semi-rigid joint — resists weeds and ants effectively, £15-25 per 15kg bag). Mortar pointing (durable but cracks if sub-base settles — can look untidy if not done carefully). Sealing: porcelain requires no sealing. Sandstone benefits from sealing to reduce staining and make cleaning easier, though this needs periodic reapplication every few years.

What should I know about Calculating Slab Quantities and Cuts?

Slab quantity calculations should use the slab's full module size (slab dimension plus the joint width, commonly 10-15mm) rather than just the slab's own size, since joints add up across a large area — for 600×600mm slabs with 10mm joints, the effective module is 610×610mm, meaning a 6m×6m patio needs (6000/610)≈9.8, rounded up to 10 slabs per row, not the 10 you'd get ignoring joints entirely (which happens to match here, but diverges noticeably on larger areas or wider joints). Edge rows almost always need cut slabs to fit the remaining space, and it's standard practice to add 10% to the calculated slab count specifically to cover cutting waste and breakages, on top of using the correct module size for the main calculation — skipping either step is a common reason DIY paving projects run short partway through.

What do I need to know about Slab Size and Pattern Choice?

Larger format slabs (600×600mm, 600×900mm, or larger porcelain/natural stone up to 1200mm+) need a flatter, more carefully prepared sub-base and bedding layer than smaller slabs, because any dip or high spot shows more obviously across a bigger surface and larger slabs are more prone to cracking if not fully and evenly supported (a technique called 'full bedding' — mortar spread across the entire slab back, not just dabbed at the corners, which is a common shortcut that causes larger slabs to crack under point loads like a chair leg or dropped object). Mixed-size random patterns (using 2-4 different slab sizes in a repeating module) look more informal and forgiving of minor cutting inaccuracies than a single uniform grid pattern, but require more careful upfront planning of the laying sequence to avoid running into an awkward final row that doesn't fit the pattern's repeat.

What do I need to know about Patio Falls for Rainwater Drainage?

Every patio needs a slight fall (slope) away from the house to shed rainwater, and getting this wrong is one of the most common and expensive patio mistakes, since a flat or badly sloped patio can direct water back towards the building and cause damp. The standard minimum fall is around 1:60 to 1:80 (roughly 15-17mm per metre), falling away from the house towards a lawn, drain, soakaway, or permeable boundary — never towards a neighbouring property or a sealed boundary where water has nowhere to go. This fall is built into the sub-base and bedding layer during laying, not corrected afterwards, so it needs to be set out with string lines and a spirit level (or laser level) before any slabs go down. Where a patio meets the house, the finished paving level must also sit at least 150mm below the damp-proof course, and the fall should be checked doesn't bring water back up towards the wall at the low end if the patio is long. On larger patios, a shallow channel drain or gully set into the lowest point, connected to a soakaway or the surface water system, is often needed in addition to the fall alone.

What do I need to know about Slabs Needed per Square Metre by Size?

Slab size has a big effect on how many units you need to order and how much cutting waste to expect — larger slabs cover more area per unit but leave bigger offcuts at awkward edges, while smaller slabs waste less material but mean more joints and more labour per square metre.

These counts assume slabs are laid edge to edge with negligible joint width; in practice, standard 8-10 mm pointing joints add a small amount to the effective area each slab occupies, which is why real orders typically round up slightly even before adding any cutting-waste allowance.

What do I need to know about a Worked Example — Slabs and Waste for a Patio?

Worked example: a patio measuring 5 m × 4 m (20 m² total) is being laid with 600 × 600 mm slabs (0.36 m² per slab). Step 1 — minimum whole slabs needed, ignoring cuts: 20 ÷ 0.36 = 55.6 → 56 slabs Step 2 — add a 10% allowance for cuts around the edges and any breakages: 56 × 1.10 = 61.6 → 62 slabs A 10% waste allowance is a common starting point for a simple rectangular patio laid in a straightforward grid pattern, but irregular patio shapes, diagonal or herringbone-style laying patterns, or a patio with lots of curved edges or obstacles typically need a larger allowance — sometimes 15-20% — because more of the slabs touching an edge or obstacle need a custom cut rather than being used whole. Ordering too few slabs part-way through a job risks a batch or colour mismatch if more have to be ordered later, since natural stone and even some concrete paving products can show visible shade variation between production batches. Jointing material needs a similar waste-aware calculation: for 10 mm joints around 600 × 600 mm slabs, each slab's share of joint length is roughly 2.4 m, so 62 slabs need very roughly 62 × 2.4 ≈ 149 m of joint length to fill — a figure worth passing to the jointing compound manufacturer's own coverage-per-bag guidance, since joint depth (which depends on slab thickness) affects compound usage far more than joint length alone. Slab thickness also affects the practical side of this calculation, since thicker slabs (typically used for driveways or areas taking vehicle weight) weigh considerably more per unit and are usually supplied on heavier pallets with lower per-pallet slab counts, which can change the delivery logistics and minimum order quantity even when the total area and slab size are identical to a lighter patio-grade installation.