Slab Calculator

How to Calculate Slab Requirements

Concrete Slab Calculations

Calculating concrete volume requires multiplying the length, width, and depth of your slab. The basic formula is:

For example, a 6m × 4m patio with 150mm thickness requires: 6 × 4 × 0.15 = 3.6m³ of concrete.

Standard Concrete Slab Thicknesses

• 100mm: Garden paths and light-use patios
• 125mm: Standard residential driveways
• 150mm: Heavy-duty driveways and garage floors
• 175mm: Commercial applications and heavy traffic areas
• 200mm+: Industrial use and heavy machinery areas

Paving Slab Calculations

To calculate the number of paving slabs needed, divide the total area by the area of each individual slab:

Popular Paving Slab Sizes in the UK

• 600mm × 600mm: Most popular size for patios and gardens
• 450mm × 450mm: Smaller areas and intricate designs
• 900mm × 600mm: Modern rectangular format for contemporary looks
• 400mm × 400mm: Traditional size for pathways

Material Requirements Guide

Sub-Base Materials

Both concrete and paving installations require proper sub-base preparation:

• MOT Type 1: 100-150mm depth for most applications
• Sharp Sand: 50mm bedding layer for paving slabs
• Membrane: Damp-proof membrane for concrete slabs

Concrete Mix Ratios

Standard concrete mix for slabs uses a 1:2:4 ratio:

• 1 part cement
• 2 parts sharp sand
• 4 parts aggregate (gravel)

Mortar Mix for Paving

Use a 5:1 mix of sharp sand to ordinary Portland cement for bedding mortar. This provides excellent strength and bonding for paving slabs.

Installation Guidelines

Concrete Slab Installation Steps

• Excavate to required depth (slab thickness + 100mm sub-base)
• Install and compact MOT Type 1 sub-base
• Lay damp-proof membrane with edges turned up
• Set up formwork using 25mm timber planks
• Pour concrete and level using screed boards
• Allow 28 days for full curing strength

Paving Slab Installation Process

• Excavate to 200mm depth below finished level
• Install 150mm compacted sub-base
• Apply 50mm mortar bedding layer
• Lay slabs with 10-15mm joints
• Check levels regularly with string line
• Point joints with suitable mortar or compound

Quality Control Measures

Professional installation requires attention to several critical factors:

• Drainage: Include 1:60 fall (50mm per 3 metres) for water runoff
• Levels: Use laser level or string line for consistent heights
• Expansion Joints: Include joints every 5-6 metres in concrete slabs
• Weather: Avoid concreting in freezing conditions or extreme heat

Frequently Asked Questions

How thick should my concrete slab be?

Thickness depends on intended use. Patios and paths need 100mm, standard driveways require 125-150mm, and commercial applications need 175mm minimum. Always include reinforcement for spans over 3 metres.

What’s the difference between sharp sand and builder’s sand?

Sharp sand has angular particles providing better strength and drainage, making it ideal for concrete and mortar bedding. Builder’s sand is softer and used for bricklaying mortar.

How much wastage should I allow for paving slabs?

Standard allowance is 10% for regular rectangular areas. Complex shapes or patterns may require 15% wastage. Simple rectangular areas might only need 5% extra.

Can I lay paving slabs directly on sand?

Whilst possible for light pedestrian use, mortar bedding provides much better stability and longevity. Sand-only installation is prone to movement and settling over time.

How long does concrete take to cure?

Concrete reaches handling strength in 24-48 hours but continues hardening for weeks. Allow 7 days before light traffic and 28 days for full strength development.

What tools do I need for slab installation?

Essential tools include spade, rake, wheelbarrow, spirit level, rubber mallet, string line, measuring tape, and plate compactor for sub-base preparation.