Driveway Sub-Base Depth Calculator

Summary

Sub-base depth is the single biggest driver of whether a driveway lasts 25 years or fails within three winters. It is also the layer that's easiest to skimp on, because once it's covered by blocks, tarmac or a concrete slab, nobody can see whether it was done properly — until the driveway starts to rut, sink, or crack under wheel loads.

The confusion tradespeople and homeowners run into is that "sub-base depth" isn't one number. It depends on what's going on top (block paving, tarmac, resin, concrete slab), what the vehicle loading will be (car only vs delivery vans vs occasional HGV), what the subgrade is like underneath (firm sand/gravel vs soft clay), and whether the surface needs to be permeable for SuDS compliance. Getting any of these wrong either wastes money (over-specifying depth on a firm subgrade) or causes early failure (under-specifying on clay).

This calculator gives the reference depths by scenario, the compaction rule that catches out most self-builders and general builders (compacting Type 1 in a single thick layer instead of 100mm lifts), and worked volume/tonnage examples so you can order the right amount of aggregate first time.

Key Facts

• MOT Type 1 — the standard graded, crushed aggregate sub-base material for UK driveways; contains fines that bind and compact hard; specified to SHW (Specification for Highway Works) Series 800
• MOT Type 3 — an open-graded aggregate with fines removed; used for permeable/SuDS paving because it stores water in the voids instead of compacting solid
• Pedestrian/patio sub-base — 100mm compacted MOT Type 1 minimum
• Domestic car driveway sub-base — 150mm compacted MOT Type 1 minimum (BS 7533-3 reference for block paving; widely applied to other surface types too)
• Heavier/shared/occasional HGV access — 200mm or more, sometimes with a geogrid reinforcement layer between sub-base and subgrade
• Compaction rule — compact in layers no thicker than 100mm; a single 200mm pour compacted in one pass leaves the lower half loose, which is the single most common cause of driveway rutting after 6–18 months
• Tarmac driveway — 150mm Type 1 sub-base + 50mm AC20 dense bitumen macadam binder course + 25–30mm AC6 surface course
• Concrete slab driveway — 100–150mm compacted hardcore/Type 1 sub-base + DPM + C32/40 XC4 XF3 concrete slab with A142 mesh reinforcement
• Permeable block paving (SuDS) — Type 3 (no-fines) sub-base, typically 150mm for light domestic use up to 250mm where the sub-base itself is doing significant water attenuation; exact depth depends on soakaway/infiltration design (BRE Digest 365, CIRIA C753)
• Resin bound surfacing — needs a sound existing base (tarmac or concrete); a new-build resin driveway needs the full tarmac or concrete construction beneath it, not a resin-specific sub-base
• Clay or soft subgrade — add 50–100mm to the standard depth and lay a geotextile separation membrane between subgrade and sub-base to stop fines migrating up and contaminating the sub-base
• CBR (California Bearing Ratio) — a soil strength test; subgrades below CBR 2% typically require deeper sub-base or ground improvement (geogrid, additional excavation) — get a site-specific assessment on obviously soft or wet ground
• Falls — minimum 1:60 across the finished surface for drainage runoff (1:80 acceptable on permeable systems where water is meant to infiltrate rather than run off)
• DPC clearance — finished paving level must sit minimum 150mm below the dwelling's damp-proof course
• GPDO SuDS rule — under Schedule 2, Part 1, Class F of the General Permitted Development Order, a new or replacement impermeable driveway over 5m² at the front of a house needs planning permission unless water is directed to a permeable area or a soakaway — this affects material choice, not sub-base depth directly, but is the reason permeable Type 3 builds are so common
• Sub-base density for ordering — MOT Type 1 compacts to approximately 2.0–2.2 tonnes/m³; MOT Type 3 (open-graded, no fines) is lighter, approximately 1.5–1.6 tonnes/m³ — always confirm exact bulk density with your merchant, as it varies by quarry source and moisture content
• Waste/compaction allowance — order 10% extra material on top of the calculated compacted volume to allow for over-excavation, spillage and compaction loss

Quick Reference Table

Detailed Guidance

Working Out How Much Sub-Base You Need

The calculation has three steps: establish the required compacted depth for the driveway type and subgrade, calculate the compacted volume, then convert to a tonnage order (aggregate is sold by weight, not volume).

Step 1 — depth. Pick the depth from the table above based on use (pedestrian, car, heavier) and subgrade condition (firm vs clay/soft).

Step 2 — volume. Volume (m³) = Area (m²) × Depth (m).

Step 3 — tonnage. Tonnage = Volume (m³) × bulk density (tonnes/m³). Use approximately 2.0–2.2 t/m³ for Type 1, 1.5–1.6 t/m³ for Type 3 — but always confirm with the merchant, since actual density varies by source material and moisture content.

Add 10% to the final tonnage for waste, over-dig and compaction variance.

Worked Example 1: Standard Domestic Driveway

A 40m² driveway for a family car, firm sandy subgrade, block paving finish.

• Required depth: 150mm (0.15m) MOT Type 1
• Volume = 40m² × 0.15m = 6m³
• Tonnage = 6m³ × 2.0 t/m³ = 12 tonnes
• Add 10% waste = 13.2 tonnes to order

Compact in two 75mm lifts (or one 100mm + one 50mm lift) with a vibrating roller, not a hand plate — Type 1 needs the mass of a roller to achieve full density at this depth.

Worked Example 2: Driveway on Clay Subgrade

A 60m² driveway, clay subgrade identified during excavation (sticky, plastic when wet), tarmac finish.

• Standard depth for a car driveway: 150mm
• Add 50–100mm for clay subgrade — use 200mm total for this example
• Volume = 60m² × 0.20m = 12m³
• Tonnage = 12m³ × 2.0 t/m³ = 24 tonnes
• Add 10% waste = 26.4 tonnes
• Lay a geotextile separation membrane directly on the clay before placing the sub-base, to stop the sub-base punching into the clay and clay fines migrating upward
• Compact in three layers of approximately 65–70mm each

Worked Example 3: Permeable (SuDS) Block Paving

A 55m² driveway needing SuDS compliance because it's over the 5m² impermeable threshold at the front of the house, with moderate infiltration ground conditions.

• Required sub-base: MOT Type 3 (no fines), 200mm as a working figure pending a specific infiltration/soakaway calculation — refer to suds regulations driveways for the full compliance route decision
• Volume = 55m² × 0.20m = 11m³
• Tonnage = 11m³ × 1.55 t/m³ (Type 3 approximate) = 17.05 tonnes
• Add 10% waste = ~18.8 tonnes
• Note: Type 3 depth is genuinely a design variable, not a fixed number — it depends on how much attenuation storage the sub-base itself needs to provide if there's no separate soakaway. Get the infiltration testing done (BRE Digest 365 method) before ordering on a borderline site.

Excavation Depth — Don't Forget the Full Build-Up

Sub-base depth is only one part of total excavation. Total dig depth = sub-base + laying course (if block paving, typically 30mm sharp sand) + surface thickness (block, slab, or binder/surface course) + 5mm settlement tolerance.

Example: 150mm sub-base + 30mm sand + 80mm blocks = 260mm total excavation for a standard block-paved car driveway. See block paving installation for the full layer-by-layer specification.

When to Get a Site-Specific Assessment Instead of Using Standard Depths

The standard depths in this article assume a reasonably firm subgrade (CBR broadly 2% or above). If any of the following apply, don't rely on the table — get a geotechnical assessment or at minimum a trial pit and CBR estimate before ordering material:

• Visibly soft, wet, or peaty ground
• A driveway that will regularly carry heavier vehicles (delivery vans, small plant, horseboxes)
• Made ground or fill of unknown origin/depth
• A history of subsidence or drainage problems on the plot

Frequently Asked Questions

Can I use less sub-base if I'm only doing a small patio, not a driveway?

Yes — pedestrian-only areas (patios, paths) need only 100mm of compacted Type 1, versus 150mm for a car driveway. The extra depth for driveways exists specifically to spread vehicle wheel loads; foot traffic doesn't generate that loading.

Do I need a different sub-base for resin bound driveways?

Resin bound surfacing itself is only 15–18mm thick and relies entirely on the strength of whatever is beneath it. If you're resurfacing an existing sound tarmac or concrete driveway, no new sub-base is needed. If you're building a new resin driveway from scratch, you need the full tarmac or concrete construction (including its own sub-base) first — resin is a surface finish, not a structural layer.

What happens if I compact a 200mm sub-base in one go instead of two layers?

The vibrating plate or roller can only achieve full compaction density in the top 100–150mm of any single lift. Compact 200mm in one pass and the lower half stays loose. Under repeated vehicle loading, the compacted crust eventually presses down into the loose material beneath, causing visible rutting and depressions — usually appearing within 6–18 months, concentrated under wheel tracks.

Is MOT Type 1 the same everywhere in the UK?

MOT Type 1 is a specification (crushed, well-graded aggregate to SHW Series 800), not a single product from one quarry — the exact stone (limestone, granite, recycled) and therefore the exact bulk density varies by region and supplier. Always confirm bulk density (tonnes per m³) with your specific merchant before finalising a tonnage order, especially on larger jobs where a density error compounds into a meaningful cost difference.

Do I need geotextile membrane under every driveway?

Not for a firm subgrade. Geotextile separation membrane becomes necessary when the subgrade is soft, wet, or fine-grained (clay, silt) — its job is to stop those fines migrating up into the sub-base voids over time, which would otherwise reduce the sub-base's load-bearing capacity. It's also standard practice under permeable (Type 3) sub-bases to keep the open-graded aggregate free of fines that would clog it.

Regulations & Standards

• BS 7533-3:2005+A1:2009 — code of practice for pavements constructed with concrete or clay pavers; sub-base and laying course specification

• SHW Series 800 — Specification for Highway Works; the underlying specification for MOT Type 1 and Type 3 aggregates

• BS EN 1338:2003 — concrete paving blocks specification

• Approved Document H, Section 3 — surface water drainage; applies where the driveway forms part of a new dwelling or extension

• GPDO Schedule 2, Part 1, Class F — the 5m² impermeable-driveway planning permission rule for front gardens

• BRE Digest 365 — soakaway design guidance for permeable/SuDS surfaces

• CIRIA C753 — the SuDS Manual, referenced for infiltration and attenuation design on permeable paving

• Interpave: Guide to the Design, Construction and Maintenance of Concrete Block Permeable Pavements — UK concrete block paving association technical guidance

• BSI — BS 7533 series — primary reference standards for pavement construction

• GOV.UK: Permeable paving and drainage — planning permission for driveways — the 5m² rule explained

• BRE Digest 365: Soakaway Design — infiltration testing and soakaway sizing methodology

• block paving installation — full sub-base, laying course and block specification for block-paved drives

• tarmac driveway installation — tarmac-specific sub-base, binder and surface course build-up

• suds regulations driveways — the 5m² planning rule and permeable compliance routes

• screed depth — companion calculator for internal floor build-ups