Key Takeaways
- An 850kg bulk bag of MOT Type 1 covers roughly 4.4m² at 100mm, allowing for compaction.
- Type 1 sub-base compacts down by about 30%, so you will need roughly 1.3 times the compacted depth you are aiming for.
- A driveway wants 150mm of compacted sub-base, a patio 100mm, a garden path 75 to 100mm.
More sub-base jobs go wrong at the ordering stage, with the density off on a calculator site, or a compaction allowance added to a figure that already included it, etc. The result is a big bag of heavy aggregates left over on the drive, or a level 30mm short with the whacker plate due back that day.
So I stopped trusting the 2.1 tonnes per cubic metre every calculator copies from the next one, and measured our own bulk bags: 850kg a bag, 0.57 cubic metres. Every figure here comes from that. The MOT Type 1 sub-base calculator below runs on those measured figures rather than some average density.
How to Calculate How Much MOT Type 1 Sub-Base You Need
MOT Type 1, and most sub-base, is calculated as area times compacted depth, plus roughly 30%, because loose material compacts down once the whacker gets at it:
- Measure the area in square metres (length × width).
- Multiply by the compacted depth in metres for the compacted volume.
- Multiply by 1.3 for the loose volume you actually order.
- Divide by 0.57 for bulk bags, rounded up. Tonnes are bags × 0.85.
But, our calculator does it all for you in an instant.
How Much Does a Bulk Bag of Type 1 Cover?
An 850kg bulk bag of Type 1 covers about 4.4m² at 100mm compacted, 2.9m² at a 150mm driveway depth, and 8.8m² as a 50mm blinding layer.
| Compacted depth | Coverage per 850kg bag |
|---|---|
| 50mm | ~8.8m² |
| 75mm | ~5.8m² |
| 100mm | ~4.4m² |
| 150mm | ~2.9m² |
| 200mm | ~2.2m² |
Coverage is approximate; freshly tipped material fluffs up slightly. And a bulk bag lands on a pallet at the kerb, so the last leg behind the house is you and a wheelbarrow.
How Much Does Type 1 Compact?
MOT Type 1 compacts by 20 to 40% depending on rock type, blend and moisture; around 30% is the figure I plan with.
Now the trap, live on several online calculators. A sum uses either the compacted volume with the compacted density, or the loose volume with the loose density. Some tools multiply a compacted density by a compaction factor as well, counting the same squash twice and over-ordering you by 20 to 30%. Mass does not change when you compact something. Only the depth does.
Compact in layers no thicker than 150mm, eight passes of the plate on each, and always hire a plate for anything a car will sit on. Press your heel in and twist: if the surface marks or shifts, it is not done yet.
What Depth of Sub-Base Do You Need?
Sub-base depth runs from 75mm under a garden path to 150mm under a driveway, and doubt about the ground adds depth, never subtracts. Once you know your quantity, how to lay a sub-base is the build.
| Job | Compacted Type 1 depth |
|---|---|
| Garden path | 75 to 100mm |
| Patio | 100mm |
| Shed base | 100mm |
| Artificial grass | 75 to 100mm, blinded with grano dust |
| Driveway (cars) | 150mm |
| Heavier vehicles or soft ground | 200mm+, built in lifts |
The real question behind the depth is load: a path only carries people, so 75 to 100mm is plenty, but once a vehicle is involved, 150mm is the floor, not the target. The gravel patio and gravel driveway guides cover the full dig. Sound broken concrete, crushed to 40 or 50mm with fines, can stay as lower fill under 100mm of fresh Type 1; anything powdery comes out, because it settles.
A geotextile heavy duty membrane under Type 1 is there for separation, not weeds, and earns its keep on heavy clay or filled ground; the full rundown is in what to put under gravel.
Type 1, Type 2, Type 3, Hardcore and Scalpings: Which Do You Need?
Type 1 is the benchmark sub-base for driveways and patios, Type 3 is the permeable grade, and hardcore is a catch-all name, not a specification:
- MOT Type 1 runs from 40mm stone down to dust, and that spread of sizes is what locks it into a solid layer; the full story is in our MOT sub-base guide.
- MOT Type 2 has more fines, for paths and lighter use.
- MOT Type 3 is open-graded and permeable, the one for permeable paving or ground that already sits wet.
- Hardcore means whatever the seller wants it to mean, from proper Type 1 to a load of old bricks.
- Scalpings are quarry by-product, anywhere from excellent to dreadful depending on the quarry.
- Grano dust is the fine crushed granite for blinding layers and artificial grass bases; we sell grano dust by the bag.
Good Type 1 looks like stone of every size with dust through it; clean, uniform large stone will never lock up, and that is worth a phone call before it goes in the hole. The MOT Type 1 calculator works for all of these, just round up between grades.
Next Steps
We carry MOT Type 1, grano dust and the rest of the aggregates range in 850kg bulk bags, delivered kerbside. We can do artic wheelers if you contact us. The gravel calculator works out the decorative layer on top. Get the boring layer right and everything above it behaves.
FAQs
How do you calculate sub-base?
Sub-base is calculated as area times compacted depth times 1.3 for compaction, giving the loose volume to order. Multiply by 1.49 tonnes per cubic metre for weight.
How much sub-base per square metre?
A square metre needs about 195kg of Type 1 at 100mm compacted depth, and about 290kg at a 150mm driveway depth. Both figures already include the compaction allowance.
How much does a tonne of MOT Type 1 cover?
A tonne of MOT Type 1 covers just over 5m² at 100mm compacted, roughly 3.5m² at 150mm.
Do you need a wacker plate for MOT Type 1?
A wacker plate is worth hiring for any Type 1 that will carry a car, no debate from me. A small pedestrian patch in thin lifts can be hand-tammed at a push.
Can you use gravel instead of MOT Type 1?
Clean gravel cannot replace MOT Type 1 as a sub-base, because without the fines the stones never interlock into a solid layer