Every hard surface in the garden — drive, path, patio, shed base — is only as good as what sits underneath it. The layer you see, whether gravel, pavers or concrete, is the finishing decision. The sub-base is the structural one. Get the layers, the depth and the compaction right and a gravel drive stays flat for decades. Get them wrong and the rutting starts in the first wet winter.
This guide walks the build-up from the ground up: the subgrade–sub-base–surface layer system, typical depths by use, how to compact properly, where a geotextile membrane earns its place, how much fall to build in, and how to turn a compacted volume on paper into the tonnes a supplier actually delivers.
Typical compacted sub-base depths: paths and patios 50–100 mm (2–4 in); domestic driveways 100–150 mm (4–6 in); heavier use or poor soils 150–200 mm or more. Compact in lifts no thicker than about 100 mm with a plate compactor, damp rather than soaked. Loose gravel settles 10–20% when compacted, so order that much extra by loose volume — a 6 × 3 m drive at 100 mm needs about 1.8 m3 compacted, which is roughly 2.1 m3 or 4 tonnes delivered. The Gravel & Sub-Base Calculator converts any area and depth into loose volume and tonnes.
The Layer System: Subgrade, Sub-Base, Surface
Hard landscaping is built as a sandwich, and each layer has one job. The subgrade is the natural ground itself — with all topsoil stripped away, because topsoil is organic, compressible and keeps settling for years. The sub-base is the load-carrying layer of compacted, graded crushed rock. The bedding or surface layer is whatever finish goes on top. Skip a layer or put them in the wrong order and the sandwich fails.
| Layer | What it is | Its one job |
|---|---|---|
| Subgrade | Stripped, compacted natural soil (or engineered fill) | Provide a stable platform — remove topsoil first |
| Geotextile membrane | Non-woven separation fabric (in heavy-use or soft-ground builds) | Stop sub-base and subgrade mixing |
| Sub-base | Graded crushed rock — MOT Type 1, crusher run, road base | Carry and spread the load |
| Bedding / surface | 20–30 mm sharp sand for pavers; 20–50 mm decorative gravel; or a concrete slab | Present the finish and lock it in place |
What you finish with matters for what goes under it. A paver drive ends in a sand bedding course; a gravel drive keeps the graded sub-base as the structure and dresses it with a thinner layer of clean decorative stone; a concrete slab sits straight on the compacted base — the slab thickness guide covers that side of the build-up.
How Deep Does the Sub-Base Need to Be?
Depth is set by two things: what loads the surface, and what the ground underneath is like. A person walking applies a trivial load; a car concentrates several hundred kilograms on each tyre patch; a delivery van doubles that again. Meanwhile clay subgrade holds water and moves with the seasons, while free-draining sand and gravel forgive a thinner layer.
| Use | Typical compacted sub-base depth |
|---|---|
| Garden footpath, pedestrian only | 50–100 mm (2–4 in) |
| Patio or pavers, domestic | 50–100 mm, plus bedding layer |
| Domestic driveway, cars | 100–150 mm (4–6 in) |
| Heavier vehicles, or clay/peat/fill ground | 150–200 mm+, with geotextile — and consider a designed solution |
Read those as typical figures, not a specification — practice varies by region, by material and by what the ground turns out to be once you have dug into it. The consistent rule is that depth is the structural number: if the drive will rut, the fix is almost always more compacted depth, better compaction, or a separation layer — not more decorative gravel on top.
Compaction: Where Driveways Are Won or Lost
Loose tipped gravel is roughly 10–20% fluffier than the same material compacted, and only compaction turns it into a structural layer. Three practices separate a base that lasts from one that settles:
- Compact in lifts. A plate compactor densifies the top of a thick layer and leaves the bottom loose. Keep each lift to about 100 mm of loose material — a 150 mm base goes down as two lifts of about 75–80 mm, each compacted before the next is spread.
- Make several overlapping passes. Four to eight passes of a plate compactor per lift is typical, overlapping each pass by around a third of the plate width, until the surface stops settling under the machine.
- Dampen as you go. Graded material with fines compacts best slightly moist — water lubricates the particles so they pack. Water it lightly during compaction; too dry never locks, too wet starts pumping under the plate.
Compact the subgrade itself before any gravel goes down, once the topsoil is off. And note the exception: clean, rounded decorative gravel cannot be compacted into a lock — it just rolls. That is why it is a wearing surface, never a sub-base.
Geotextile Membrane: The Separation Layer
On soft ground — clay, silt, peat, made-up fill — and under anything that carries regular vehicle loads, a non-woven geotextile between subgrade and sub-base is one of the most effective layers you can install. It prevents two slow failures: gravel punching down into soft soil under load, and wet mud pumping up into the gravel. The base stays a base instead of gradually dissolving into the ground.
Lay it with generous overlaps at the joints (around 300 mm is a common figure), run it up the sides of the excavation, and never drive over exposed fabric. On firm, free-draining subgrade it is often skipped; under a loaded driveway on clay it rarely should be.
Drainage Fall
A slight slope runs through every sound build-up — typically around 1–2% (1:100 to 1:50), always shedding away from the building. On a 6 m drive, a 1.5% fall is 6,000 mm × 0.015 = 90 mm of level difference from one end to the other, set at subgrade level so the whole sandwich drains as one. Gravel surfaces let water through; the fall decides where it goes once it arrives at the bottom.
Loose Volume, Compacted Volume and Tonnes
The volume you design is the compacted volume — the space the finished layer occupies. The volume you order is the loose volume, because material fluffs up when tipped and shrinks under the compactor. Multiply the compacted volume by roughly 1.10–1.20 to get the loose volume to order.
Suppliers mostly sell by mass, so you also need a bulk density. Typical figures:
| Material | Typical bulk density | Working conversion |
|---|---|---|
| Graded sub-base (MOT Type 1, crusher run), loose | ≈ 1.9 t/m3 | 1 t ≈ 0.5 m3 loose |
| Graded sub-base, compacted | ≈ 2.1–2.2 t/m3 | 1 m3 compacted ≈ 2.2 t |
| Clean decorative gravel, loose | ≈ 1.5–1.7 t/m3 | 1 t ≈ 0.6–0.65 m3 |
Densities vary with the rock and the grading, so treat these as planning figures and confirm with your supplier. Selling units vary by country too: the UK trades in tonnes or bulk bags (commonly around 0.85–1 t each), while US suppliers sell by the ton (2,000 lb ≈ 907 kg). Metric volume converts at 1 m3 = 1.31 yd3.
Worked Example: A 6 × 3 m Drive at 100 mm
A small domestic parking bay, 6 m by 3 m, on reasonable ground with a 100 mm compacted sub-base of graded material:
- Area: 6 × 3 = 18 m2.
- Compacted volume: 18 × 0.10 = 1.8 m3 (about 2.35 yd3).
- Loose volume to order: 1.8 × 1.15 ≈ 2.07 — call it 2.1 m3.
- Tonnes: 2.1 m3 loose × 1.9 t/m3 ≈ 4.0 t. Cross-check the other way: 1.8 m3 compacted × 2.2 t/m3 = 3.96 t — the two routes agree.
- Practical order: about 4–4.5 t to absorb uneven subgrade, spillage and edge detailing — five ~0.9 t bulk bags, or one small tipped load.
If the drive finishes in decorative gravel rather than pavers, allow for the wearing layer separately: 18 m2 × 0.04 m ≈ 0.72 m3 of clean stone, roughly 1.1–1.2 t at 1.6 t/m3. It sits on top of the sub-base and never substitutes for it.
Common Mistakes
- Ordering to the decimal. Suppliers deliver whole tonnes, whole bags, whole loads. 1.8 m3 compacted is a 4-tonne order, not 3.96.
- Compacting in one deep lift. The plate only reaches so far; everything beneath stays loose and settles later, unevenly.
- Skipping compaction of the subgrade. The best sub-base on loose ground still follows the ground down.
- Building on topsoil. It compresses and rots. Strip it, however tempting the shortcut.
- Using clean rounded stone as sub-base. It rolls instead of locking. Sub-base wants crushed, graded material with fines.
- No membrane on clay. Under vehicle loads, gravel and mud slowly trade places; the base disappears from the middle outward.
- Dead-flat formation. Without fall, water sits in the base and softens the subgrade — and usually pools against the nearest wall.
Plan It With the Gravel & Sub-Base Calculator
Enter the area and your target compacted depth, and the Gravel & Sub-Base Calculator applies the loose-volume expansion and the bulk density for you — returning loose cubic metres, tonnes and bag counts. Pair it with the Paver Calculator for the surface layer, or the Concrete Slab & Footing Calculator if the base ends under concrete.
Open the Gravel & Sub-Base CalculatorFrequently Asked Questions
How much gravel do I need per square metre of driveway?
At a typical 100 mm compacted depth, one square metre occupies 0.1 m3 compacted — about 0.11–0.12 m3 loose, or roughly 0.22 t of graded sub-base. At 150 mm, allow about a third of a tonne per square metre. Multiply by area and round up to the delivered unit.
Can I compact 150 mm of gravel in one lift?
Not effectively. A plate compactor densifies the top of a thick layer and leaves the bottom loose. Keep lifts to roughly 100 mm of loose material — spread a 150 mm base as two lifts of about 75–80 mm, compacting each in turn.
Do I really need a geotextile membrane under the gravel?
On firm, free-draining subgrade, many builds skip it. On clay, silt or made ground — especially under vehicle loads — it is one of the most worthwhile layers in the sandwich: it stops the sub-base punching into soft ground and mud pumping up into the stone.
How do I convert tonnes of gravel to cubic metres?
Divide the mass by the bulk density. Graded sub-base runs about 1.9 t per loose cubic metre, so 4 t ≈ 4 ÷ 1.9 ≈ 2.1 m3 loose. Clean decorative gravel is lighter, around 1.5–1.7 t/m3, so a tonne goes further by volume.
What is the difference between sub-base gravel and decorative gravel?
Sub-base material is crushed and graded with fines that lock under the compactor — MOT Type 1 in the UK, crusher run, road base or Item 4 elsewhere. Decorative gravel is clean, often rounded stone that stays loose and drains freely: a wearing surface, never a structure.
Related MyHouseLogic Guides
The sub-base is half the build-up; the finish is the other half.
- Concrete slab thickness — the same layering logic from the concrete side, and what the base owes the slab above it.
- Paver quantity guide — counting the surface layer that goes on top of your compacted base and sand bed.
- How much concrete do I need — volume, waste and thickened-edge arithmetic if the base ends under a slab.
- All Concrete & Building guides — sub-bases, slabs, mortar and quantities.
The bottom line: depth by use, compaction in lifts, membrane on soft ground, fall to shed water, and 10–20% extra by loose volume. The surface is the cosmetic decision — the sub-base is the structure, and it is the part nobody sees once the job is done.