How to measure for a type 1 sub-base
Measure the area the sub-base will go under. For a rectangular patio, path or drive, take the length (A) and the width (B). Split awkward areas into rectangles, triangles, circles and L-shapes, and add each one with Add another area. For a shed, Wickes suggests a base the same size as the shed or slightly larger.
The depth is the compacted thickness of type 1, the figure on your plan or in the paving maker’s instructions. Do not add anything for compaction yourself. The calculator uses the density of type 1 once it has been compacted, typically 2–2.2 t/m³, so the tonnes already include the extra stone that compaction packs in.
Dig out enough for the sub-base plus everything that goes on top of it, such as the laying course and the paving, but enter only the type 1 layer here. If you measure in feet and inches, switch the units at the top: type 1 is still sold by the tonne.
Recommended type 1 depths
Sub-base depth depends on the load. Paths that only take feet need the least; drives that take cars need the most. These are typical compacted depths from Marshalls, Wickes, B&Q and UK quarries. The buttons apply them to the calculator.
| Use | Compacted depth | Apply |
|---|---|---|
| Garden pathFoot traffic only. | 75–100 mm of MOT type 1 sub-base | |
| PatioUnder slabs on a mortar bed. | 100 mm of MOT type 1 sub-base | |
| DrivewayFor ordinary cars. | 100–150 mm of MOT type 1 sub-base | |
| Shed baseUnder a concrete or slab shed base. | 75 mm of MOT type 1 sub-base |
Treat these as starting points, not a design. Soft or wet ground, clay, slopes and heavier vehicles all call for more, and block paving drives are usually built on a deeper sub-base than paths (the block paving calculator has its own figures). Check the depth for your ground with your supplier, the paving manufacturer or a groundworker before you dig.
How much type 1 per m²?
Each square metre at 100 mm deep is 0.1 m³ of compacted sub-base. At 2–2.2 t/m³, that weighs about 200 kg or a little more before any extra. Depth matters as much as area: a drive built half as deep again as a patio needs half as much type 1 again for every square metre.
That is why sub-bases add up faster than people expect. Compacted type 1 is heavier than gravel, because compaction packs the stones and the fine dust tightly together, so a bulk bag covers less ground than the same bag of gravel. If your supplier gives a compacted density for their product, open Density under the depth and type it in.
Loose or compacted?
Type 1 arrives loose. It is spread, raked level and compacted with a plate compactor or roller until it stops moving, and it ends up noticeably thinner than it was spread. Marshalls’ guide allows for this by multiplying up the loose volume, and quarries quote a heavier density for compacted type 1 than for loose.
This calculator does it the other way round: you give the compacted depth and it uses the compacted density, so you get the same answer without a separate compaction step. Spread and compact in layers rather than all at once; the instructions for your plate compactor, or the hire shop, will say how thick each layer can be.
MOT type 1, hardcore and scalpings
MOT type 1 is crushed stone graded from coarse pieces down to fine dust, so it locks together firmly when compacted. Because it is made to a road specification, one load is much like another, and suppliers publish a density for it.
Hardcore is a looser term for broken brick, concrete and stone. It can do a similar job under a path or shed, but it is not graded, it varies from load to load, and we found no UK supplier publishing a density for it. If you have been looking for a hardcore calculator, use this one and type your supplier’s figure into Density.
Scalpings, often made from recycled road planings, are another sub-base material; the aggregate calculator includes them. For a gravel drive, the gravel calculator works out the gravel and the sub-base together, while this MOT type 1 calculator does the sub-base on its own.
Worked example: a driveway sub-base
Here is the full calculation for a single-width driveway using the Driveway preset and the suggested extra allowance. Every line comes from the same code that runs the calculator above. As with any drive, the depth is a guide: check it for your ground before you dig.
- Area 9 m × 3 m = 27.00 m²
- Volume of MOT type 1 sub-base 27.00 m² × 0.15 m (150 mm) = 4.05 m³
- Weight of MOT type 1 sub-base 4.05 m³ × 2.1 t/m³ = 8.505 t (8,505 kg)
- Plus extra 8,505 kg + 10% = 9,356 kg (9.36 t)
- In bulk bags 9,356 kg ÷ 850 kg = 11.01, rounded up = 12 bulk bags
On the list:
- 12bulk bags of MOT type 1 sub-base (approx. 850 kg each)9.36 t needed
Notice how many bulk bags even a modest drive takes: type 1 is heavy, and the depth for cars is the deepest in the table. Look at the bulk-bag line too. When the division goes only just over a whole number, the last bag is mostly the extra allowance. Keep it unless you are sure of your measurements: running short halfway through compacting means a second delivery and a sub-base that is harder to get even. For larger jobs, ask your merchant whether type 1 can be delivered loose.
UK type 1 bulk bag sizes
MOT type 1 is sold by weight, almost always in bulk bags of 800 kg or 850 kg. We found no reliable source for small bags of type 1, so the calculator offers bulk bags only; for a very small job, ask your merchant what they stock.
| Product | Size | Good for |
|---|---|---|
| Bulk bag, 850 kg | 850 kg | The most common merchant bulk bag. About 10 m² of gravel at 50 mm. |
| Bulk bag, 800 kg | 800 kg | Sold by several DIY chains and merchants; weight can vary by about 5% with moisture. |
Bag weights are nominal and can vary by a few per cent with moisture. A full bulk bag can’t be moved by hand once it is down, so agree the drop point with the supplier and keep it close to where you are working, on firm ground.
Common mistakes when ordering type 1
- Adding compaction twice. The depth here is the compacted depth and the density is for compacted type 1. Adding your own allowance for compaction on top orders far too much.
- Measuring the dig, not the layer. The hole holds the paving, the laying course and the sub-base. Only the sub-base layer goes in this calculator.
- Skimping on a drive. Cars need the deepest sub-base in the table, and soft ground needs more. Check the depth with your supplier or a groundworker.
- Compacting it in one thick layer. A plate compactor only reaches so deep. Spread and compact in layers, as its instructions say.
- Stopping exactly at the edge. A sub-base that ends at the paving line leaves the outer slabs or blocks with nothing under their edges. Agree any extra width with your supplier or installer.
- Ordering hardcore by eye. Mixed rubble varies a lot. If you use it, ask the supplier for a density and type it in.
What else you'll need
- Slabs and a mortar bed for a patio: the patio calculator covers the paving.
- Blocks, laying course sand and jointing sand for block paving: use the block paving calculator.
- Gravel on top for a gravel drive or path: the gravel calculator works out both layers.
- Concrete or slabs for a shed base: the shed base calculator does the whole base.
- A plate compactor, usually hired, plus a rake, a shovel, and string lines or a long spirit level to set the levels and falls.
Last checked 28 October 2026