I have spoken to several people who ordered exactly the volume of MOT Type 1 they calculated, watched it arrive, spread it across their excavated area - and then looked at it with a sinking feeling as they realised it was nowhere near enough. The pile that looked generous coming off the tipper truck somehow disappeared to less than half the depth they needed after spreading.
That is the compaction factor at work. It is one of those things that seems obvious once you understand it, but nobody tells you about it in advance.
What actually happens when you compact MOT Type 1
MOT Type 1 arrives loose. The particles - from fine dust up to 40mm angular pieces - are in a random arrangement with significant air gaps between them. Think of it like a bucket of irregular stones: the air gaps between the pieces take up a large proportion of the total volume.
When you run a plate compactor over the material, two things happen:
- The angular particles interlock and nest together more tightly
- The fine material (dust and grit) fills the remaining air gaps
The result is a much denser, more stable layer - but a noticeably shallower one. The depth reduces by around 20-30%.
The standard figure used for planning is a compaction factor of 1.3. Every cubic metre of compacted sub-base you want to end up with requires 1.3 m3 of loose material to be ordered.
The maths
Step 1: Calculate your compacted volume (Length x Width x Compacted depth, all in metres)
Step 2: Multiply by 1.3 to get loose volume
Step 3: Multiply by 2.1 t/m3 to get weight in tonnes (2.1 t/m3 is the standard bulk density of MOT Type 1)
Step 4: Add 10-15% wastage
Example: patio sub-base
Sub-base for a 5m x 4m patio at 100mm compacted depth:
- Compacted volume = 5 x 4 x 0.1 = 2.0 m3
- Loose volume needed = 2.0 x 1.3 = 2.6 m3
- Weight = 2.6 x 2.1 = 5.46 tonnes
- Add 15% wastage = 6.28 tonnes to order
Without the compaction factor: 4.2 tonnes. That is 2 tonnes short on a relatively modest patio project - roughly 2-3 bulk bags worth of material.
Example: driveway sub-base
Sub-base for a 10m x 5m driveway at 150mm compacted depth:
- Compacted volume = 10 x 5 x 0.15 = 7.5 m3
- Loose volume = 7.5 x 1.3 = 9.75 m3
- Weight = 9.75 x 2.1 = 20.5 tonnes
- Add 15% wastage = 23.6 tonnes to order
Without compaction factor: 18.2 tonnes. The difference is 5.4 tonnes - a significant extra cost if you have to order it as a second delivery.
Compacted depth vs loose depth: a key distinction
This is where most of the confusion comes from. When someone says they need a "150mm sub-base," they mean 150mm after compaction - the finished depth.
To achieve 150mm compacted depth, you need to start with approximately 195mm of loose material (150 x 1.3 = 195mm).
When you are setting out your excavation depth and calculating how much material to order, always work from the compacted depth requirement and apply the factor. The calculator does this automatically.
Does every aggregate have a compaction factor?
Different materials compress by different amounts:
| Material | Approximate compaction factor |
|---|---|
| MOT Type 1 | 1.3 |
| Hardcore (mixed) | 1.25-1.35 |
| Crushed concrete | 1.25 |
| Crushed stone | 1.3 |
| Sharp sand (as bedding) | 1.1-1.15 |
| Decorative gravel (surface layer) | 1.0 (no compaction) |
Decorative gravel used as a surface layer is not compacted, so you calculate quantity based on the layer depth directly with no correction factor.
How much compaction actually occurs depends on your equipment
The 1.3 factor assumes proper compaction with a plate compactor. If you are using:
Plate compactor - Compacts to about 20-25% reduction. The 1.3 factor is appropriate.
Wacker plate (vibratory plate) - Same as above, varies by machine size. The 1.3 factor is appropriate.
Hand tamper - Does not achieve anything like proper compaction. The layer will not reduce to 1.3x - but it will also not be properly stable and will continue to settle under vehicle weight. Do not use a hand tamper for a driveway sub-base.
This is worth knowing because if you compact poorly, your finished depth might appear correct but the material is not properly dense. Vehicles will cause the sub-base to settle further over time, creating ruts and dips.
My tips on compaction
A few things learned from too many trips back to the builder's merchant for extra material:
Always apply the 1.3 factor. Even experienced people forget it on unfamiliar projects. The calculator applies it for you - just enter the compacted depth you need.
Compact in layers of no more than 100mm. Running a plate compactor over 200mm of loose material does not give you 150mm of well-compacted sub-base - it gives you the top 100mm compacted well and the bottom 100mm poorly compacted. Lay and compact in two passes.
Do not underestimate the wastage on top of the compaction factor. Even after accounting for compaction, material is lost to overspill at edges, slight over-excavation and variation in the sub-grade level. 15% wastage is not excessive - it is realistic.
Weigh the cost of getting it right against the cost of getting it wrong. Doing a second loose load delivery costs both money (the delivery charge plus any small-load surcharge) and time. Getting the first order right is nearly always cheaper.
Use the MOT Type 1 Calculator which applies the 1.3 compaction factor and wastage automatically.