Enter the area you are laying and the compacted depth, and this MOT Type 1 calculator works out the tonnes of sub base you need and how many bulk bags that is. It adds a compaction allowance so you order enough to finish the job in one go.
Type 1 sub base calculator
The compaction allowance matters because loose Type 1 settles when you whacker it down, so a tonne of stone covers less finished area than the loose volume suggests. This tool adds 10% on top of the bare figure so you are not left a few barrows short at the end.
How it is calculated
The calculation is the same one a groundworker does on the back of a delivery note. You work out the volume of the hole or sub base in cubic metres, convert that volume to weight using the density of the stone, then add a margin for compaction.
The formula is:
tonnes = length (m) × width (m) × (depth in mm ÷ 1000) × 2.1
The numbers behind it:
- Density 2.1 t/m³: compacted MOT Type 1 sits at roughly 2.0 to 2.1 tonnes per cubic metre. We use 2.1 so you do not under-order. Loose, uncompacted Type 1 is a touch lighter, around 1.8 to 2.0 t/m³.
- Compaction allowance 10%: Type 1 is laid loose, then compacted in layers with a wacker plate or roller. It loses height as the angular stone and fines lock together, so you need more material than the finished compacted volume implies. A 10% allowance covers this plus normal spillage.
- Bulk bag 0.8 t: a standard builder's bulk bag (often called a tonne bag) of Type 1 holds roughly 0.8 tonnes. They vary by supplier from about 0.75 to 0.9 t, so treat the bag count as a guide and check the weight when you order.
Worked example
Say you are laying a sub base for a 10 m by 3 m driveway at 100 mm compacted depth:
- Area: 10 × 3 = 30 m²
- Volume: 30 × (100 ÷ 1000) = 30 × 0.1 = 3 m³
- Weight: 3 × 2.1 = 6.3 tonnes
- With 10% allowance: 6.3 × 1.10 = 6.93 tonnes
- Bulk bags: 6.93 ÷ 0.8 = 8.66, rounded up to 9 bulk bags
So for that driveway you would order roughly 7 tonnes loose, or 9 bulk bags. Loose tipped is usually the cheaper option at that quantity if you have space for the lorry to tip.
Frequently asked questions
How do I calculate how much Type 1 I need?
Multiply length by width to get the area in square metres, multiply that by the compacted depth in metres (depth in mm divided by 1000), then multiply by 2.1 to convert the volume to tonnes. Add about 10% for compaction. The calculator above does all of this for you, including the bulk bag count.
How many tonnes of Type 1 are in a bulk bag?
A standard bulk bag, sometimes sold as a tonne bag, holds roughly 0.8 tonnes of MOT Type 1, though it can range from about 0.75 to 0.9 tonnes depending on the supplier and how dry the stone is. The type 1 calculator above assumes 0.8 t per bag, so check the actual bag weight on your supplier's product page before ordering.
What is the density of MOT Type 1 sub base?
Compacted MOT Type 1 weighs about 2.0 to 2.1 tonnes per cubic metre. This type 1 aggregate calculator uses 2.1 t/m³ so the figure leans slightly on the safe side and you are unlikely to run short. Loose, uncompacted material is lighter, around 1.8 to 2.0 t/m³.
How deep should a Type 1 sub base be?
For a footpath or patio sub base, 100 mm of compacted Type 1 is usually enough. For a domestic driveway taking cars, aim for 150 mm. For driveways that take vans or the odd heavy delivery, 150 to 200 mm is sensible. Lay and compact it in layers of no more than 100 mm at a time so the wacker plate can consolidate it properly.
Is Type 1 the same as hardcore?
Not quite. Hardcore is a general term for crushed rubble and broken material used as fill. MOT Type 1 is a specific, graded crushed stone (often limestone or granite) made to the Department for Transport specification, with a controlled mix of stone sizes down to fines so it compacts to a dense, stable surface. When people use a type 1 hardcore calculator they almost always mean graded MOT Type 1, which is what this tool is built for.
How do I calculate Type 1 in square metres rather than length and width?
If you already know the area, you can work it out directly: area in m² multiplied by the compacted depth in metres gives the volume in m³, then multiply by 2.1 for tonnes and add 10%. For example, 30 m² at 100 mm is 30 × 0.1 × 2.1 = 6.3 tonnes before the allowance. Enter any length and width that multiply to your area in the calculator and you will get the same answer.
Should I order loose Type 1 or bulk bags?
For anything over a couple of tonnes, loose tipped from a lorry is usually cheaper per tonne than bulk bags, provided the lorry can reach a spot to tip. Bulk bags suit smaller jobs, tight access, or where you want to move the material by hand a bag at a time. The type 1 sub base calculator above gives you both the tonnage and the bag count so you can compare prices either way.
Does the calculator include wastage?
Yes. The result already includes a 10% allowance on top of the bare volume to cover compaction and normal spillage, so the tonnes and bulk bags shown are what you should actually order. On awkward sites with uneven ground or a lot of edges, consider rounding up by another bag to be safe.