How this calculator works
The sub-base is the hidden engine of a driveway: tarmac is a flexible wearing surface, but the weight of a parked car is carried almost entirely by the compacted stone beneath it. This tool takes your area and final compacted depth, works out the volume, and converts it to tonnes of MOT Type 1 — then adds a compaction allowance so you order enough loose material. Size the tarmac that goes on top with the tarmac quantity calculator, and for non-tarmac projects the general MOT Type 1 calculator covers patios, paths and shed bases.
The formula
Volume (m³) = Area (m²) × Compacted depth (m)
Tonnes = Volume (m³) × Density (≈2.0–2.2 t/m³)
Loose to order = Tonnes × (1 + compaction allowance)
Density depends on the stone: crushed limestone and granite sit near the top of the 2.0–2.2 t/m³ range, recycled crushed concrete a little lower. The compaction allowance (about 20–25%) covers the drop in level as the roller drives the air out of the loose material.
Worked example
Example — a 40 m² driveway sub-base at 150 mm
- Area:
- 40 m²
- Compacted depth:
- 150 mm = 0.15 m
- Density:
- ≈ 2.1 t/m³ (MOT Type 1)
- Compacted volume: 40 m² × 0.15 m = 6.0 m³
- Tonnage: 6.0 m³ × 2.1 t/m³ ≈ 12.6 t
- Range across 2.0–2.2: 12.0 t – 13.2 t
MOT Type 1 to order: ≈ 12–13 tonnes (plus compaction allowance)
Order to the top of the range and round up: it is far cheaper to have a little spare stone than to stop a compaction run short of the finished level.
Reference data
| Compacted depth | Volume per m² | Tonnes per m² (≈2.0–2.2 t/m³) |
|---|---|---|
| 75 mm (path) | 0.075 m³ | 0.15–0.17 t |
| 100 mm (patio / shed base) | 0.100 m³ | 0.20–0.22 t |
| 150 mm (driveway) | 0.150 m³ | 0.30–0.33 t |
| 200 mm (heavy use / soft ground) | 0.200 m³ | 0.40–0.44 t |
Key facts
- MOT Type 1 is a graded crushed aggregate from 40mm down to dust; compacted, the dust binds the stone into a strong load-bearing sub-base.
- A domestic tarmac driveway needs at least 150mm of compacted MOT Type 1; soft ground or heavy vehicles call for 200mm or more.
- MOT Type 1 is commonly estimated at 2.0–2.2 tonnes per cubic metre, so a 40 m² drive at 150mm needs roughly 12–13 tonnes.
- Loose MOT Type 1 drops about 20–25% in level as it is compacted, so order extra loose material to reach the finished depth.
Who should use this and when
Anyone building the foundation for a drive, patio or path needs this before the aggregate order goes in. It matters most on soft ground, where skimping on sub-base depth is the single most common cause of a drive that ruts and cracks within a year or two. Lay and compact the material in thin layers of up to 75mm, not all at once, and consider a woven geotextile membrane over the soil first to stop the stone sinking. For the surface course above, use the driveway cost calculator to budget the whole build.
Tips and common pitfalls
- Compact in layers (lifts) of 75mm max. A whacker plate cannot drive compaction energy through 150mm of loose stone in one go.
- Lay a geotextile membrane first. It stops the sub-base sinking into soft soil and adds tensile strength.
- Order for compaction, not the compacted volume. Add 20–25% because loose material drops as it is rolled.
- Never substitute clean gravel. Without fines it will not bind, and the tarmac above it will fail.
Frequently asked questions
What is MOT Type 1?+
MOT Type 1 is a graded crushed aggregate — usually limestone, granite or crushed concrete — running from 40mm stone down to dust. When heavily compacted, the dust binds the larger stones into a strong, interlocking, load-bearing layer. It is the standard structural sub-base beneath tarmac and block paving.
How thick should the sub-base be for a tarmac driveway?+
For standard domestic traffic — cars and light vans — aim for at least 150mm of MOT Type 1 once fully compacted. On soft ground such as clay, or for heavy delivery vehicles, 200mm to 225mm is safer. Paths and patios need less, typically 75mm to 100mm.
How do I work out how much MOT Type 1 I need?+
Multiply the area by the compacted depth for the volume, then multiply by density. MOT Type 1 is commonly estimated at about 2.0 to 2.2 tonnes per cubic metre, so a 40 m² drive at 150mm is 40 × 0.15 × 2.1 ≈ 12.6 tonnes. Add a little extra for compaction settlement.
Why do I need to allow for compaction shrinkage?+
Loose MOT Type 1 is full of air. A vibrating roller drives the stones together, so the loose level drops by roughly 20–25% as it compacts. You must order enough loose material to reach your final compacted depth, which is why the calculator adds a compaction allowance.
Can I use gravel or shingle instead of MOT Type 1?+
No. Clean gravel and shingle have no fines to bind them, so they roll against each other and never compact into a solid base. Tarmac laid over loose gravel will flex and fail quickly. A graded material with dust, like MOT Type 1, is essential.
How many bulk bags of MOT Type 1 is a tonne?+
A standard bulk bag holds roughly 800–900kg, so about one tonne per bag — meaning a bit more than one bag per tonne of material required. Loose tipped deliveries are usually cheaper per tonne for larger jobs.
Related Tarmac Tools
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Estimates & assumptions
- Volume is exact (area × compacted depth); tonnage uses an estimating density of about 2.0–2.2 t/m³, which varies with the stone and its moisture content.
- Recommended sub-base depths (150mm domestic, 200mm+ heavy or soft ground) are common practice, not a substitute for a ground assessment on difficult sites.
- The tool does not carry out geotechnical design or account for drainage, membranes or edge restraints.
- Confirm the exact density and bag weight with your aggregate supplier for a precise order.
Estimates are for material planning. On soft or made-up ground, seek professional advice on the required sub-base depth.