Sub-Base Calculator UK - MOT Type 1 Depth, Volume & Tonnage

How Deep Should a Sub-Base Be?

Sub-base depth is driven by two things: what the finished surface needs to carry, and how firm the ground underneath already is. Here are typical UK depths at standard ground conditions:

Ground condition matters because the sub-base's job is to spread load across a wider area of subgrade than the surface alone would. Soft or clay subgrade has lower bearing capacity, so it needs a thicker layer to spread the same load without the surface flexing or rutting over time.

MOT Type 1 vs Type 3 vs Crusher Run vs Recycled vs Hardcore

Not all sub-base material is the same job. Here is what each is actually for:

• MOT Type 1 (SHW Clause 803) – a well-graded crushed stone, typically limestone or granite, from 40mm down to dust. The fines fill the gaps between larger stones, so it compacts into a dense, strong, load-bearing layer. This is the default choice for driveways, paths and patios, and what most UK merchants mean when they just say "MOT" or "hardcore" informally.
• MOT Type 3 (SHW Clause 805) – the same stone sizes as Type 1, but with the fines screened out. That makes it free-draining rather than dense, so water passes through it instead of sitting on top. Used under permeable block paving and in SuDS (sustainable drainage) schemes where you need water to soak away rather than run off.
• Crusher run – a generic term for crushed stone graded similarly to Type 1, but not necessarily certified to the SHW 803 specification. Often cheaper and fine for garden or low-traffic use; ask your supplier whether theirs meets Type 1 spec if it matters for your job.
• Recycled Type 1 – crushed, screened recycled concrete and masonry, graded to match virgin Type 1. A greener option that performs similarly for most domestic jobs, though quality can vary more between suppliers since the source material isn't uniform.
• Hardcore – broken brick, concrete and stone rubble, less tightly graded than Type 1. Commonly used as a cheaper bulk fill under a Type 1 capping layer on deep digs, rather than as the sole sub-base for anything load-bearing on its own.

Why Compacted and Loose Volumes Differ

When you dig a hole and fill it with sub-base, the finished, compacted layer sits at a higher density than the loose material was when it arrived on your drive. Compacting squeezes air out from between the stones, packing more mass into less space. That has one important consequence, and one non-consequence:

• It affects volume. To end up with, say, 1m³ of compacted sub-base, you need to start with more than 1m³ of loose material, since it will shrink as you compact it. For MOT Type 1, that is roughly 7% more loose volume.
• It does not affect weight. A tonne of MOT Type 1 is a tonne, whether it is sitting loose in a pile or compacted flat in your driveway. If you order by weight (which is how most UK aggregate is sold), you do not need to add anything extra for compaction – the tonnage figure this calculator gives you already accounts for it correctly.

This calculator applies the compaction allowance only to the loose volume figure, never to tonnage, which is why the two outputs don't move by the same multiplier. If you have ever seen a calculator apply a "compaction factor" directly to weight, that is double-counting: it is applying a volume correction to a figure that does not need one.

Compacting in Layers: Lift Depth and Passes

Sub-base should never be dumped in and compacted all at once beyond a certain depth, or the bottom of the layer won't compact properly no matter how many passes you make. The Specification for Highway Works guidance behind MOT Type 1 (Clause 803) specifies compacting in layers of no more than 100mm at a time.

For any sub-base deeper than 100mm, split it into two or more lifts: spread and compact the first 100mm fully, then add and compact the next layer on top. Each layer typically needs several passes of a vibrating plate compactor, working from the edges inward, until the surface stops visibly moving underfoot and no more fines work their way to the surface.

When You Need a Geotextile Membrane

A geotextile membrane is a permeable fabric laid between the subgrade and the sub-base. It doesn't stop water, but it does stop the two layers mixing under load.

On firm, well-drained ground it is optional: a nice-to-have for weed suppression and long-term stability, but not essential. On soft or clay ground it is worth fitting as standard. Without it, sub-base stone can slowly work its way down into soft subgrade under repeated loading (cars, foot traffic, rain), while clay works its way up into the sub-base. Over a year or two that shows up as uneven settling, potholing or a driveway that starts to feel soft in patches, especially after wet weather.

Excavation Depth and Spoil Disposal

Your total dig depth is the sub-base depth plus whatever sits on top of it, since you need to excavate room for both before you start filling back in. For a driveway with 150mm of MOT Type 1 under 50mm of block paving, that's 200mm total.

The soil you dig out takes up more space once it's loose than it did packed into the ground, an effect called bulking. Clay bulks by roughly 25-30% by volume; firmer, sandier ground bulks by around 15%. So a 20m² area excavated to 200mm produces 4m³ of in-situ soil, but that becomes roughly 5m³ of loose spoil once it's dug out and needs to go in a skip or grab lorry. Get a muck away quote based on the loose (bulked) volume or weight, not the in-situ figure, or you will under-order the skip or lorry capacity you need.

Ordering: Loose by the Tonne vs Bulk Bags

For anything beyond a small patio, loose material delivered by tipper and priced per tonne is almost always cheaper than bulk bags once you're ordering more than 3-4 tonnes. Travis Perkins and Jewson both supply MOT Type 1 loose by the tonne for trade and larger domestic jobs, with a minimum load size that varies by branch and postcode. For smaller jobs, or if you don't have vehicle access for a tipper delivery, Wickes and B&Q sell MOT Type 1 in 850kg bulk bags, which a small crane or HIAB can place close to where you need it.

Sub-Base by Area and Depth: Quick Reference

MOT Type 1, standard ground, 10% waste allowance, figures regenerated directly from the calculator above:

Use the calculator above for your exact job, especially on soft or clay ground where depth requirements increase.

Frequently Asked Questions

How deep should a sub-base be?

Depth depends on what's going on top and what it needs to carry. As a general guide: 75mm for a footpath, 100mm for a patio or shed base, 150mm for a domestic driveway, 200mm for a driveway that sees occasional heavier vehicles, and 250mm for heavy or commercial use. Soft or clay ground needs roughly 50mm more than firm ground at the same duty, since the sub-base is doing more work to spread the load evenly.

What is the difference between MOT Type 1 and Type 3?

MOT Type 1 (SHW Clause 803) is a well-graded crushed stone that includes fines (dust), which lock together tightly when compacted, giving strong load-bearing capacity but limited drainage. MOT Type 3 (SHW Clause 805) has the fines screened out, so it is more free-draining but does not compact as densely. Type 1 is the standard choice for driveways and paths. Type 3 is used where water needs to pass through, such as under permeable paving or in SuDS drainage schemes.

Why do I need more loose material than the compacted volume?

Compacting sub-base squeezes air out of the material, packing more mass into a smaller space. That means the loose material you buy takes up more volume than the compacted layer it ends up as, even though the weight does not change. For MOT Type 1, expect to need roughly 7% more loose volume than the compacted depth you are filling. This does not affect tonnage: buying by weight already accounts for it.

How much MOT Type 1 do I need for a driveway?

For a standard domestic driveway at the typical 150mm depth, MOT Type 1 works out to roughly 288kg per m² (0.15m × 1,920kg/m³), plus whatever waste allowance you are using. A 20m² driveway needs around 6.3 tonnes including 10% waste.

Do I need a geotextile membrane under a sub-base?

On firm, well-drained ground it is optional, though it is cheap insurance against weeds and against the sub-base slowly working its way into the subgrade over time. On soft or clay ground, a geotextile membrane is worth fitting: it stops the sub-base and subgrade mixing under load, which is one of the main causes of a driveway or path settling unevenly.

How do I work out muck away or spoil volume?

Multiply your excavation area by the total dig depth (sub-base plus whatever surface layer sits on top) to get the in-situ volume, then add a bulking allowance, since dug soil takes up more space loose than it did in the ground. Clay bulks by roughly 25-30%, firmer or sandier ground by around 15%. A 20m² driveway excavated to 200mm total depth on standard ground produces about 5 m³ of loose spoil to remove, from 4 m³ in-situ. See our excavation and spoil section below.