Sub-Base Calculator
Calculate how much MOT Type 1, Type 2, Type 3 or crushed concrete sub-base you need. Get volume, weight and all delivery options compared — compaction factor included.
Type 1: ~2.1 t/m³. Industry standard for patios and driveways. Compacts to a solid load-bearing base.
Area = π × (diameter ÷ 2)²
Footpath: 50mm. Patio: 100mm. Driveway: 150mm. Heavy vehicles: 200mm+.
Type 1 compacts ~20%. Type 3 needs only ~15% — fewer fines means less volume reduction.
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Foundation Layers
Construction Layer Build-Up
A patio or driveway is only as good as its foundation. Understanding all the layers helps you plan the full excavation depth correctly.
Choose Material
Sub-Base Material Comparison
The five common UK sub-base options compared side by side. Type 1 is the default for most domestic projects, but the right choice depends on permeability, budget and load.
Laying Guide
How to Lay MOT Type 1
The 20% Compaction Rule
MOT Type 1 contains fine particles that fill the voids between larger stones when a vibratory wacker plate compresses the material. This void-filling is what gives it structural strength — but it also reduces the volume by around 20%. The loose pile you order will always be larger than the finished depth you need.
Type 3 has fewer fines so it compacts less (around 15%). Crushed concrete behaves like Type 1 at 20–25%. The compaction slider in the calculator adjusts automatically when you select a material.
Step-by-Step Laying
Building Regulations — Part H, SuDS & Highway Standards
Under Building Regulations Approved Document H, any new driveway over 5 m² in England must either use a permeable surface or drain to a soakaway within the property. Draining to the street or public sewer requires planning permission.
For permeable surfaces (resin-bound, permeable block paving), MOT Type 3 is required under BS 7533-103:2026 (formerly BS 7533-13, withdrawn 2026). Its reduced fines allow water to percolate through the sub-base, satisfying Part H without a soakaway. MOT Type 1 creates an impermeable base — only suitable where separate drainage is in place.
Highway standard references: MOT Type 1 (SHW Clause 803) and MOT Type 3 (SHW Clause 805) are specified in the Specification for Highway Works — the UK national standard for road construction. Using SHW-compliant materials ensures your project meets the same engineering standards used in public highway construction.
CBR — California Bearing Ratio & Soft Ground
The California Bearing Ratio (CBR) measures the load-bearing capacity of your subgrade. Most domestic gardens have a CBR of 2–5%. Soft clay, waterlogged ground or recently disturbed fill can fall below 2%, which significantly affects the sub-base depth required.
As a guide: firm ground (CBR ≥ 5%) suits standard 150mm driveway depths. Soft clay (CBR 2–4%) warrants 200–250mm. Very soft ground (CBR < 2%) may need a geogrid reinforcement layer — this distributes load laterally and can reduce the required sub-base depth by up to 30%.
Quick field test: if your foot sinks more than 25mm into the ground when you step on it, your CBR is likely below 2%. Consult a structural engineer before proceeding with a standard sub-base specification on suspect ground.
HIAB & Delivery Access Checklist
Bulk bags and loose tipper deliveries typically require a HIAB (crane-equipped) lorry or 6-wheel tipper. Before ordering, confirm the following:
FAQ
Common Questions
Multiply your area by the depth in metres then add the compaction factor. For a 5m × 4m patio at 100mm: 5 × 4 × 0.1 × 1.2 = 2.4m³, roughly 5 tonnes of Type 1. Our calculator handles this automatically.
150mm is the standard for domestic driveways with car traffic. Pedestrian patios need 100mm. Footpaths on firm ground can use 50mm. For HGV or commercial use, increase to 200–300mm and consult a structural engineer.
Type 1 contains fine particles (dust) that compact into a solid impermeable base — ideal for most driveways and patios. Type 3 has minimal fines, leaving air voids that allow water to drain through. Type 3 is required for SuDS-compliant surfaces under Building Regulations Part H.
Loose MOT Type 1 weighs approximately 2.1 tonnes per m³. After compaction the volume reduces by around 20%, so you always order more loose material than your finished depth requires.
Yes for most domestic projects. A membrane between the subgrade and sub-base prevents the fines migrating down into soft ground over time, which would cause the surface to sink. It is particularly important on clay or silty ground. Less critical on firm chalk or rock.
Ballast (mixed sand and gravel) does not compact to the same structural strength as Type 1 and is not suitable for driveways or load-bearing surfaces. It may work for very light-duty temporary paths, but for any permanent paving use Type 1 or approved crushed concrete.
Recycled crushed concrete performs comparably to Type 1 for most domestic projects and is often 15–20% cheaper. Ensure it is specified as 6F2 or 6F5 graded for structural use — ungraded recycled material can vary too much in particle size to compact reliably.