How the MOT Type 1 calculator works
MOT Type 1 is sold by weight but laid by depth. Once compacted with a vibrating plate it weighs about 2.1 tonnes per m³. Because this compacted density already accounts for the material settling (loose MOT is about 20–25% bulkier), you only need to add a small allowance for an uneven dig.
Bulk bag coverage
| Compacted depth | Tonnes per m² | m² per 850kg bag |
|---|---|---|
| 50mm | 0.105 | 8.1 m² |
| 75mm | 0.158 | 5.4 m² |
| 100mm | 0.21 | 4.0 m² |
| 150mm | 0.315 | 2.7 m² |
Laying a sub-base
- Dig out to allow for the sub-base, bedding layer and paving. For a 100mm sub-base under a 40mm bed and 20mm slabs, that's about 160mm below finished level.
- Compact the soil, lay a woven geotextile membrane with 300mm overlaps, and set out edge restraints.
- Spread MOT Type 1 in 50–75mm layers and compact each with a wacker plate (at least 3–4 passes).
- Check levels and falls (about 1:60–1:80 away from buildings) before bedding your patio or paving.
Worked examples
A 5m × 4m patio at 100mm: 20m² × 0.1m = 2m³ compacted. At 2.1t/m³ plus 5% that's 4.41 tonnes, or 6 bulk bags of 850kg (5.2 rounded up). Compact it in two layers. You'll also need about 23m² of geotextile membrane, allowing for overlaps.
A 3m × 2m shed base at 100mm: 6m² → 0.6m³ → about 1.3 tonnes, so 2 bulk bags. The shed base planner adds the dig-out, skip and concrete slab with the same measurements.
A 6m × 5m driveway at 150mm: 30m² → 4.5m³ → about 9.9 tonnes. That's 12 bulk bags, or a single loose tipper load, which is often cheaper if you have somewhere to tip it.
Type 1, recycled Type 1 or Type 3?
- MOT Type 1 (virgin): crushed limestone or granite graded from 40mm down to dust. It compacts very hard and is the standard choice for patios, paths and drives.
- Recycled Type 1: crushed concrete and brick graded to the same size range. It's usually cheaper and fine for domestic work if it comes from a reputable supplier. Check it's free of plaster, wood and plastic.
- Type 3 (open-graded): has the fine dust removed, so water drains through it. It's used under permeable paving and SuDS driveways but is harder to get a firm finish on.
Getting the compaction right
Hire a vibrating plate compactor ("wacker plate") for the day. A hand tamper won't compact a sub-base properly. Lay MOT in layers of no more than about 75mm, dampen it slightly if it's very dry, and make three or four passes over each layer, overlapping each run. A well-compacted surface is hard enough that a boot heel barely marks it. On soft clay or made-up ground, dig deeper, lay geotextile and increase the sub-base to 150–200mm.
Common mistakes
- Ordering by loose volume. MOT shrinks by 20–25% when compacted. This calculator uses compacted density, so the tonnage is what actually ends up in the ground.
- Thick single layers. 150mm in one go won't compact at the bottom. Two or three thinner layers give a far firmer base.
- Forgetting the dig. Every 100mm of sub-base is 100mm of soil to remove first. Use the excavation calculator for the spoil.
Read our sub-base depth guide for the right depth for each job. Then work out the surface with the patio, block paving or gravel calculator.