Sub-Base Calculator UK: MOT Type 1 Tonnes, Bulk Bags and Depth
Enter your area and choose a project. This MOT Type 1 calculator then estimates the tonnes, bulk bags and small bags required. Many people search for it as a subbase calculator or an MOT calculator. It also compares a loose tipper delivery, counts the compaction layers and calculates the excavation depth. The second tab answers a different question: how much area will the stone you already have cover? Many builders call Type 1 hardcore or MOT1, so it works as a hardcore calculator too.
More free UK building calculators
A sub-base is the first layer of a bigger job. These calculators cover the surface that goes on top.
Slabs, block paving, sub-base, bedding mortar and patio fall.
MOT Type 1, ballast, sand and gravel in tonnes and bulk bags.
Cubic metres, bags, cement and ballast for slabs and footings.
Sharp, building and kiln-dried sand, plus sand and cement mixes.
Tonnage, coverage and driveway cost per m².
Bricks per m², mortar and cost for any wall.
Concrete, breeze and aircrete blocks, with mortar and cost.
Sand, cement and hydrated lime quantities for brickwork, blockwork and repointing.
Why sub-base orders go wrong
Picture this. Three bulk bags of MOT Type 1 sit on the drive, and the patio area looks comfortably covered. Then the plate compactor passes over the first layer. The stone settles, and the finished base ends up 30 mm short.
Loose aggregate typically settles by around a tenth during compaction. Many estimates also overlook the second layer on a driveway, or the excavation that creates it. As a result, the order runs short, the spoil pile grows, and the project stops for another delivery.
Order for the finished depth: decide the compacted depth you need, then add a compaction allowance. This sub-base calculator handles both calculations before you contact the merchant.
How much MOT Type 1 per m²?
At 100 mm compacted depth, each m² requires approximately 0.22 tonnes, including 10% for compaction. A 200 mm driveway base needs double that quantity.
How much does a bulk bag of Type 1 cover?
A typical 850 kg bag covers approximately 3.9 m² at 100 mm. One tonne covers roughly 4.5 m² at the same compacted depth.
How to use the Type 1 calculator
Complete the form from top to bottom, and the tool updates every quantity together. Most estimates take about a minute.
- Enter the area. Add separate rectangles for L-shapes, or select a circle, a triangle or a known total area.
- Choose the project. The tool then suggests a typical compacted depth and the surface layers above it.
- Describe the ground. Soft clay or old fill generally needs a deeper base and a separating membrane.
- Select the material. MOT Type 1 suits nearly every project, while Type 3 and clean stone suit permeable designs.
- Confirm the bag sizes. Bulk bag weights vary, so check the figure on your merchant’s listing.
- Add prices if available. The calculator then highlights the cheapest delivery option, including or excluding VAT.
Already have some stone? Open the Coverage and tonnes tab and enter tonnes, bags or cubic metres. It then shows the area covered at your chosen depth.
What is a sub-base, and how does it differ from the base?
A sub-base, also written subbase, is the compacted layer of crushed stone under a patio, path or driveway. It spreads the load and keeps the surface level as the seasons change.
| Layer | What it is | Typical depth |
|---|---|---|
| Surface | Slabs, blocks, tarmac or gravel | 20–80 mm |
| Base or laying course | Mortar bed, sharp sand or binder course | 25–60 mm |
| Sub-base | Compacted MOT Type 1 or similar | 75–250 mm |
| Subgrade | The natural ground, compacted after digging | Not ordered |
Road engineers call the sub-base the sub-base course. For a garden patio, it simply means the Type 1 layer.
The subgrade is the soil you dig down to, while the sub-base is the stone you add. A sub-base calculator like this one works out that stone layer. In short, a strong patio foundation is a firm subgrade, a compacted sub-base and a full bed above it.
How deep should a sub-base be?
The correct depth depends on the expected load and the ground conditions. These compacted depths suit firm, well-drained soil, and the Type 1 calculator applies them automatically. Add 50–100 mm on soft clay, peat or made ground.
| Project | Compacted depth | Layers | Type 1 per 10 m² |
|---|---|---|---|
| Garden path or footpath | 75 mm | 1 | 1.65 t |
| Patio, foot traffic | 100 mm | 1 | 2.2 t |
| Shed or greenhouse base | 100 mm | 1 | 2.2 t |
| Artificial grass | 75 mm | 1 | 1.65 t |
| Hot tub base | 150 mm | 2 | 3.3 t |
| Driveway, cars | 150 mm | 2 | 3.3 t |
| Driveway, vans or soft ground | 200 mm | 2 | 4.4 t |
| Heavy vehicles | 250 mm | 3 | 5.5 t |
Tonnes include 10% for compaction. Layers assume 100 mm or less per compacted layer.
In our opinion, 100 mm under a patio and 150 mm under a car driveway are sensible minimums. A thinner base saves a bulk bag or two, but it risks sunken slabs and expensive repairs later. Our guide to why driveways sink shows what a thin base does over time.
Patio and driveway sub-base depth and layers
The sub-base depth sets the excavation depth, so plan every layer together. These build-ups suit firm ground and match the presets in the sub-base calculator.
Patio sub-base and base layers
| Layer | Porcelain or stone patio | Concrete slab patio |
|---|---|---|
| Slabs | 20–25 mm | 35–50 mm |
| Mortar bed | 40 mm | 40 mm |
| MOT Type 1 sub-base | 100 mm | 100 mm |
| Dig depth | about 160 mm | about 190 mm |
Driveway sub-base depth
| Layer | Block paving drive | Gravel drive |
|---|---|---|
| Surface | 60 mm blocks | 40–50 mm gravel |
| Laying course | 30 mm sharp sand | None |
| MOT Type 1 sub-base | 150 mm | 150 mm |
| Dig depth | about 240 mm | about 200 mm |
On soft clay, add 50–100 mm of sub-base and lay a geotextile membrane first.
Gravel and tarmac drives need the same 150 mm sub-base as block paving. The surface changes, but the load on the stone underneath does not.
How to calculate how much sub-base you need
The sub-base calculator multiplies the area by the compacted depth, then by the material density. Compacted Type 1 weighs approximately 2.0 tonnes per m³, and the 10% allowance covers settlement and uneven ground.
MOT Type 1 coverage per tonne and per bag
| Depth | Covered per tonne | Per 850 kg bulk bag | Tonnes per m² |
|---|---|---|---|
| 50 mm | 9.1 m² | 7.7 m² | 0.11 t |
| 75 mm | 6.1 m² | 5.2 m² | 0.17 t |
| 100 mm | 4.5 m² | 3.9 m² | 0.22 t |
| 150 mm | 3.0 m² | 2.6 m² | 0.33 t |
| 200 mm | 2.3 m² | 1.9 m² | 0.44 t |
| 250 mm | 1.8 m² | 1.5 m² | 0.55 t |
MOT Type 1 at 2.0 t/m³ compacted, plus 10%. Calculated with the tool on this page.
Merchant listings often quote 8–10 m² per bag at 50 mm. Our figure is slightly lower because it includes the compaction allowance. That small margin costs considerably less than a second delivery.
Buying small bags instead? One 25 kg bag covers about 0.11 m² at 100 mm, so allow nine bags per m². The Coverage and tonnes tab works as a Type 1 coverage calculator for any bag size.
How much Type 1 sub-base for common areas
| Area | At 100 mm (patio) | At 150 mm (driveway) |
|---|---|---|
| 10 m² | 2.2 t, 3 bulk bags | 3.3 t, 4 bulk bags |
| 20 m² | 4.4 t, 6 bulk bags | 6.6 t, 8 bulk bags |
| 30 m² | 6.6 t, 8 bulk bags | 9.9 t, 12 bulk bags |
| 40 m² | 8.8 t, 11 bulk bags | 13.2 t, 16 bulk bags |
| 50 m² | 11 t, 13 bulk bags | 16.5 t, 20 bulk bags |
MOT Type 1 needed, with 10% for compaction, in 850 kg bulk bags. Calculated with the sub-base calculator on this page.
MOT Type 1 calculator examples for UK projects
We calculated each project with this MOT Type 1 calculator, using a 10% compaction allowance throughout.
Example 1: shed base
A 3 × 2.4 m shed base, 100 mm of Type 1 under a 100 mm concrete slab
Two bulk bags leave a little spare for levelling. Our concrete calculator then works out the slab on top.
Example 2: porcelain patio
A 5 × 3.6 m patio, 100 mm of Type 1 under slabs on a 40 mm mortar bed
These figures match our patio and paving calculator exactly, which also adds the slabs and mortar.
Example 3: block paving driveway
An 8 × 5 m car driveway with 150 mm of Type 1 under block paving
The excavation produces about 17 tonnes of spoil, so book two grab lorry loads. These prices are example inputs, so replace them with your own quotes.
Example 4: artificial grass or turf
A 5 × 5 m lawn area, 75 mm of Type 1 under 25 mm of granite dust
Type 1, Type 3, scalpings or hardcore?
MOT Type 1 consists of crushed limestone or granite, graded from 40 mm down to dust. During compaction, the fines lock the larger stones together. That creates a strong, stable base, but one that water cannot easily pass through.
| Material | Density used | Drains water? | Best for |
|---|---|---|---|
| MOT Type 1 | 2.0 t/m³ | No | Paths, patios, shed bases and driveways |
| Type 1, recycled concrete | 1.9 t/m³ | No | The same jobs, if certified to Clause 803 |
| MOT Type 2 | 2.0 t/m³ | No | Rarely stocked, so check first |
| MOT Type 3 | 1.8 t/m³ | Yes | Permeable driveways and SuDS |
| Clean gravel or 20 mm stone | 1.6 t/m³ | Yes | Permeable block paving bases |
| Ballast, 20 mm all-in | 1.9 t/m³ | Partly | Light paths only |
| Scalpings | 2.0 t/m³ | Partly | Paths and shed bases only |
| Hardcore or crusher run | 1.9 t/m³ | Partly | Bulk fill below Type 1 |
Compacted densities used in the calculator. Check your supplier’s data sheet for its own figure.
Type 1 and Type 3 follow the Specification for Highway Works, Clauses 803 and 805. Type 3 has far fewer fines, so rain drains through, as Holcim explains. Our honest view? Choose Type 1 unless you genuinely need a permeable surface. Type 3 only performs properly with the right paving system and soil.
Can you use gravel or ballast as a sub-base? Clean gravel has no fines, so it never locks into a solid layer. Ballast compacts better, but it still falls short of Type 1 under a drive. Select any of these materials, including Type 2, Type 3 and scalpings. The type 1 sub base calculator then adjusts the tonnage automatically.
Bulk bags or a loose tipper load?
How you buy the stone affects the price more than which merchant supplies it. Once you add your prices, the Type 1 calculator compares all three delivery options automatically.
| Option | Typical size | Best for | Watch out for |
|---|---|---|---|
| Small bags | 20 or 25 kg | Repairs under 1 tonne | Expensive per tonne and slow to spread |
| Bulk bags | 800–1,000 kg | Patios and shed bases | A crane lorry needs firm access |
| Loose tipper load | From 4–5 tonnes | Driveways and big bases | It tips where the lorry stops |
Many quarries set a minimum loose load of 4–5 tonnes.
Bulk bags of MOT Type 1 cost approximately £75–£78 at national merchants in October 2026. Loose prices vary with haulage distance, so always request a delivered price per tonne. In our driveway example, the loose load cost less than half the bulk bag total.
Sub-base cost per m²
| Depth | Bulk bags at £75 | Loose at £35 per tonne |
|---|---|---|
| 100 mm | about £19 per m² | about £8 per m² plus delivery |
| 150 mm | about £29 per m² | about £12 per m² plus delivery |
| 200 mm | about £39 per m² | about £15 per m² plus delivery |
Stone only, using the example prices from our driveway case study. Labour and membrane are extra.
Add both prices to the Type 1 calculator, and it compares the options for your exact area and depth.
How to lay and compact MOT Type 1
A durable sub-base depends on thin layers, careful compaction and patience. Follow these steps for paths, patios and driveways.
- Excavate to the full depth. Remove the topsoil and roots, then compact the exposed ground.
- Lay a geotextile membrane. It prevents the stone sinking into soft soil, so overlap every join by 300 mm.
- Spread the first layer. Keep each compacted layer to 100 mm or less for consistent density.
- Dampen and compact. Run a plate compactor across the whole layer four to six times.
- Repeat and check the levels. A string line and spirit level keep the falls accurate.
- Check the finished surface. A properly compacted base shows no footprints when you walk across it.
Planning a tarmac drive instead? Our tarmac calculator works out the asphalt tonnes for the layer above. For the full detail on depth and quality checks, read our MOT Type 1 depth and compaction guide.
Sub-base mistakes that cost money
Most sunken patios and rutted driveways start with one of these avoidable errors.
- Ordering for the loose depth. Aggregate settles during compaction, so always include an allowance.
- One thick layer. A plate compactor cannot consolidate 150 mm at once, so lay two thinner layers.
- No membrane on clay. The stone gradually pushes into the soil, and the surface dips.
- Forgetting the spoil. Every m² of a 240 mm dig creates over 400 kg of soil to move.
- Using scalpings under a driveway. They break down under car tyres and develop ruts.
Paving a front garden? Hard surfaces over 5 m² need planning permission unless water can drain through, according to GOV.UK guidance. A permeable Type 3 base can help meet that rule.
Sub-base calculator FAQs
How the Type 1 calculator gets its numbers
The Type 1 calculator multiplies your area by the compacted depth to find the volume. It then applies the material density and your compaction allowance, before rounding up to whole bags and tonnes.
MOT Type 1 uses 2.0 t/m³ compacted, plus a default 10% allowance. Spoil uses 1.8 tonnes per m³ in the ground and expands by a quarter once excavated. These figures match our paving and aggregate calculators exactly.
Back to those three bulk bags and a base 30 mm short? Ordering for the compacted depth, plus a sensible allowance, finishes the job in one delivery. Prices checked in October 2026. Treat results as estimates, and follow your installer’s specification.
What are you building on your sub-base, and how did the ground behave when you dug? Tell us through our contact page. Real projects help us improve the calculator.